Animal Physiology

Vertebrate endocrine systems

In contrast to the invertebrate endocrine system, the emphasis in the vertebrate endocrine system is on classical endocrine organs with many physiological processes controlled by these organs. However, the nervous system still exerts an influence over the endocrine system since some of the peripheral endocrine organs are under the control of the anterior pituitary, which will be described later. During vertebrate evolution, there has been much conservation in terms of endocrine function. This means that some hormones have found new roles ― for example, the hormone thyroxine controls metabolic rate in mammals, but in amphibians it is essential for the metamorphosis from tadpole to adult frog. In addition to this, as the vertebrates have evolved, new endocrine organs have emerged, such as the parathyroid glands that control Caz+ levels which first appeared in the teleosts {bony fish). The typical vertebrate endocrine system is seen to consist of three principal glands or groups of glands:

  • • the hypothalamus;
  • • the pituitary gland;
  • • peripheral endocrine glands.

The hypothalamus and pituitary gland

The hypothalamus is part of the vertebrate brain and sits beneath the thalamus. Its main function is as an interface between the nervous and endocrine systems. A major role of the hypothalamus is to control the pituitary gland ― the so-called master gland. The secretions of the hypothalamus are transported to the pituitary gland. There are two types of secretions ― those that are released into the posterior pituitary gland and those released into the anterior pituitary gland. Hormones secreted by the hypothalamus travel down axons extending from the hypothalamus to the posterior pituitary gland (the neurohypophysis). This region has a typical neuroendocrine role in that hormones are released from the posterior pituitary gland directly into the circulation. In mammals, the hormones released from die posterior pituitary gland are antidiuretic hormone {also known as vasopressin), which controls water absorption in the kidney, and oxytocin, which stimulates uterine smooth muscle contraction and milk ejection from the mammary glands. Both of these substances are peptides. Peptides with different amino acid compositions, but which have a similar biological role to either antidiuretic hormone or oxytocin, are found in all vertebrates. Another important group of secretions produced by the hypothalamus are the releasing hormones. These substances are released from axon terminals into capillaries which then pass to the anterior pituitary gland (adenohypophysis). Releasing hormones are thus delivered to the anterior pituitary gland indirectly via the blood system, rather than by direct release from axon terminals. The function of the releasing hormones, as the name suggests, is to influence the release of hormones from the anterior pituitary. The hormones released from the anterior pituitary then influence the secretions from other structures. Alternatively, release of hormones may be inhibited by release-inhibiting hormones secreted by the hypothalamus. The hypo-thalamic releasing and release-inhibiting hormones and the hormones whose release they control are shown in Table Hypothalamic hormones and the anterior pituitary hormones they influence. It is essential that the plasma concentrations of all the secretions in this system are maintained at acceptable levels.

Table Hypothalamic hormones and the anterior pituitary hormones they influence

Hypothalamic hormone Anterior pituitary hormone influenced
Releasing hormones
Growth hormone-releasing hormone (GHRH) Growth hormone
Thyrotropin-releasing hormone (TRH) Thyrotropin-stimulating hormone (TSH)
Protactin-releasing hormone (PRH) Prolactin
Luteinizing hormone-re I easing hormone (LHRH) Leutenizing hormone
Fotticle stimulating hormone-releasing hormone (FSHRH)a Follicle-stimulating hormone
Melanocyte-stimulating hormone releasing hormone (MSHRH) Melanocyte-stimulating hormone
Corticotropin-releasing hormone (CRH) Corticotropin (adrenocorticotropic hormone, ACTH)
Release-inhibiting hormones
Growth hormone release-inhibiting hormone (GHRIH, somatostatin) Growth hormone
Prolactin release-inhibiting hormone (PRIH) Prolactin
Melanocyte-stimulating hormone release-inhibiting hormone (MSHRIH) Melanocyte-stimutating hormone

a FSHRH and LHRH may be identical substances and are sometimes referred to as gonadotropin releasing hormone (GnRH)

Growth hormone promotes growth in all vertebrates. It has effects on carbohydrate, lipid and protein metabolism. It also induces the liver to release a compound called somatomedin which stimulates mitosis in bone tissue. Thyrotropin releasing hormone stimulates the thyroid gland to secrete thyroxine and triiodothyronine. The secretions of the thyroid gland have a variety of effects ― the control of metabolic rate in mammals and the control of metamorphosis in amphibians. Prolactin is a hormone which is well known for its effects on reproductive tissue, and its stimulatory effect on milk production. However, it also has other effects, influencing water and Na+ exchanges in amphibians. Follicle-stimulating and luteinizing hormones (FSH and LH) affect the gonads. FSH promotes gamete (i.e. egg and sperm) development, whilst LH, amongst many other functions, promotes steroid production. Melanocyte-stimulating hormone is involved in physiological color change in some of the lower vertebrates, e.g. amphibians and fish, whilst in some higher vertebrates it may be involved in osmotic and ionic regulatory processes. Adrenocorticotropic hormone stimulates the adrenal cortex to release the hormones produced there (i.e. the miner-alocorticoids, such as cortisol).

Peripheral endocrine organs

Within the vertebrate phyla, there are a vast array of organs which have an established endocrine function. The list is ever increasing; it is now known that the heart produces a hormone (atrial naturetic peptide, ANP), which is involved in the renal regulation of Na+. Table Some established endocrine organs and the functions of the hormones they produce shows a few examples of some of the more established endocrine organs and their secretions in the vertebrates. It should not be considered a complete list and the reader is directed to more comprehensive endocrinology texts for further information.

Table Some established endocrine organs and the functions of the hormones they produce

Endocrine organ Hormone released Hormone function
Parathyroid gland3 Parathormone/calcitonin Increase/decrease of blood Ca2+ levels
Stomachb Gastrin Regulation of acid secretion
Adrenal medullac Adrenaline Short term response to stress, e.g. increases blood sugar levels, increases cardiac output
Adrenal cortex Glucocorticoids, e.g. corticosterone Regulation of metabolism
Mineralocorticoids, e.g. aldosterone Regulation of electrolyte levels
Ovary Estrogens Initiates proliferation of endometrium
Progestogens Supports thickened endometrium
Testis Androgens, e.g. testosterone Maintains production of gametes and is involved in the development of secondary sexual characteristics

aExcept tish, there is no secretion of parathormone and a calcitonin-like substance called hypocalcin is secreted by the corpuscles of Stannius
bGastrin is not produced in the lower vertebrates, e.g. fish and amphibians
cln higher vertebrates the adrenal medulla and cortex constitute a single gland, in lower vertebrates they are separate


Aglepristone (Alizin, Alizine)

Injectable Progesterone Blocker

Highlights Of Prescribing Information

Injectable progesterone blocker indicated for pregnancy termination in bitches; may also be of benefit in inducing parturition or in treating pyometra complex in dogs & progesterone-dependent mammary hyperplasia in cats

Not currently available in USA; marketed for use in dogs in Europe, South America, etc.

Localized injection site reactions are most commonly noted adverse effect; other adverse effects reported in >5% of patients include: anorexia (25%), excitation (23%), depression (21%), & diarrhea (13%)

What Is Aglepristone Used For?

Aglepristone is labeled (in the U.K. and elsewhere) for pregnancy termination in bitches up to 45 days after mating.

In dogs, aglepristone may prove useful in inducing parturition or treating pyometra complex (often in combination with a prostaglandin F analog such as cloprostenol).

In cats, it may be of benefit for pregnancy termination (one study documented 87% efficacy when administered at the recommended dog dose at day 25) or in treating mammary hyperplasias or pyometras.


Aglepristone is a synthetic steroid that binds to the progesterone (P4) receptors thereby preventing biological effects from progesterone. It has an affinity for uterine progesterone receptors approximately three times that of progesterone. As progesterone is necessary for maintaining pregnancy, pregnancy can be terminated or parturition induced. Abortion occurs within 7 days of administration.

Benign feline mammary hyperplasias (fibroadenomatous hyperplasia; FAHs) are usually under the influence of progesterone and aglepristone can be used to medically treat this condition.

When used for treating pyometra in dogs, aglepristone can cause opening of the cervix and resumption of miometral contractility.

Within 24 hours of administration, aglepristone does not appreciably affect circulating plasma levels of progesterone, cortisol, prostaglandins or oxytocin. Plasma levels of prolactin are increased within 12 hours when used in dogs during mid-pregnancy which is probably the cause of mammary gland congestion often seen in these dogs.

Aglepristone also binds to glucocorticoid receptors but has no glucocorticoid activity; it can prevent endogenous or exogenously administered glucocorticoids from binding and acting at these sites.


In dogs, after injecting two doses of 10 mg/kg 24 hours apart, peak serum levels occur about 2.5 days later and mean residence time is about 6 days. The majority (90%) of the drug is excreted via the feces.

Before you take Aglepristone

Contraindications / Precautions / Warnings

Aglepristone is contraindicated in patients who have documented hypersensitivity to it and during pregnancy, unless used for pregnancy termination or inducing parturition.

Because of its antagonistic effects on glucocorticoid receptors, the drug should not be used in patients with hypoadrenocorticism or in dogs with a genetic predisposition to hypoadrenocorticism.

The manufacturer does not recommend using the product in patients in poor health, with diabetes, or with impaired hepatic or renal function as there is no data documenting its safety with these conditions.

Adverse Effects

As the product is in an oil-alcohol base, localized pain and inflammatory reactions (edema, skin thickening, ulceration, and localized lymph node enlargement) can be noted at the injection site. Resolution of pain generally occurs shortly after injection; other injection site reactions usually resolve within 2-4 weeks. The manufacturer recommends light massage of the injection site after administration. Larger dogs should not receive more than 5 mL at any one subcutaneous injection site. One source states that severe injection reactions can be avoided if the drug is administered into the scruff of the neck.

Systemic adverse effects reported from field trials include: anorexia (25%), excitation (23%), depression (21%), vomiting (2%), diarrhea (13%) and uterine infections (3.4%). Transient changes in hematologic (RBC, WBC indices) or biochemical (BUN, creatinine, chloride, potassium, sodium, liver enzymes) laboratory parameters were seen in <5% of dogs treated.

When used for pregnancy termination, a brown mucoid vaginal discharge can be seen approximately 24 hours before fetal expulsion. This discharge can persist for an additional 3-5 days. If used in bitches after the 20th day of gestation, abortion maybe accompanied with other signs associated with parturition (e.g., inappetance, restlessness, mammary congestion).

Bitches may return to estrus in as little as 45 days after pregnancy termination.

Overdosage / Acute Toxicity

When administered at 3X (30mg/kg) recommended doses, bitches demonstrated no untoward systemic effects. Localized reactions were noted at the injection site, presumably due to the larger volumes injected.

How to use Aglepristone

WARNING: As accidental injection of this product can induce abortion; it should not be administered or handled by pregnant women. Accidental injection can also cause severe pain, intense swelling and ischemic necrosis that can lead to serious sequelae, including loss of a digit. In cases of accidental injection, prompt medical attention must be sought.

Aglepristone dosage for dogs:

To terminate pregnancy (up to day 45):

a) 10 mg/kg (0.33 mL/kg) subcutaneous injection only. Repeat one time, 24 hours after the first injection. A maximum of 5 mL should be injected at any one site. Light massage of the injection site is recommended after administration. (Label information; Alizin — Virbac U.K.)

To induce parturition:

a) After day 58 of pregnancy: 15 mg/kg subcutaneously one time. 24 hours after aglepristone injection, give oxytocin 0.15 Units/kg every 2 hours until the end of parturition. ()

b) On or after day 58 of pregnancy: 15 mg/kg subcutaneously; repeat in 9 hours. In treated group, expulsion of first pup occurred between 32 and 56 hours after treatment. Use standard protocols to assist with birth (including oxytocin to assist in pup expulsion if necessary) or to intervene if parturition does not proceed. ()

As an adjunct to treating pyometra/metritis:

a) For closed cervix: 6 mg/kg twice daily on the first day followed by the same dose once daily on days 2, 3, and 4. Some prefer using larger doses (10 mg/kg) once daily on days 1, 3,and 8, then follow up also on days 15 and 28 depending on the bitch’s condition. ()

b) For metritis: 10 mg/kg subcutaneously once daily on days 1,2 and 8.

For open or closed pyometra: aglepristone 10 mg/kg subcutaneously once daily on days 1,2 and 8 and cloprostenol 1 meg/ kg subcutaneously on days 3 to 7. Bitches with closed pyometra or with elevated temperature or dehydration should also receive intravenous fluids and antibiotics (e.g., amoxicillin/clavulanate at 24 mg/kg/day on days 1 – 5). If pyometra has not resolved, additional aglepristone doses should be given on days 14 and 28. ()

Aglepristone dosage for cats:

For treating mammary fibroadenomatous hyperplasia: a) 20 mg/kg aglepristone subcutaneously once weekly until resolution of signs. Cats who present with heart rates greater than 200 BPM should receive atenolol at 6.25 mg (total dose) until heart rate is less than 200 BPM with regression in size of the mammary glands. ()


■ Clinical efficacy

■ For pregnancy termination: ultrasound 10 days after treatment and at least 30 days after mating

■ Adverse effects (see above)

Client Information

■ Only veterinary professionals should handle and administer this product

■ When used for pregnancy termination in the bitch, clients should understand that aglepristone might only be 95% effective in terminating pregnancy when used between days 26-45

■ A brown mucoid vaginal discharge can be seen approximately 24 hours before fetal expulsion

■ Bitch may exhibit the following after treatment: lack of appetite, excitement, restlessness or depression, vomiting, or diarrhea

■ Clients should be instructed to contact veterinarian if bitch exhibits a purulent or hemorrhagic discharge after treatment or if vaginal discharge persists 3 weeks after treatment

Chemistry / Synonyms

Aglepristone is a synthetic steroid. The manufactured injectable dosage form is in a clear, yellow, oily, non-aqueous vehicle that contains arachis oil and ethanol. No additional antimicrobial agent is added to the injection.

Aglepristone may also be known as RU-534, Alizine, or Alizin.

Storage / Stability/Compatibility

Aglepristone injection should be stored below 25°C and protected from light. The manufacturer recommends using the product within 28 days of withdrawing the first dose.

Although no incompatibilities have been reported, due to the product’s oil/alcohol vehicle formulation it should not be mixed with any other medication.

Dosage Forms / Regulatory Status

Veterinary-Labeled Products:

Note: Not presently available or approved for use in the USA. In several countries:

Aglepristone 30 mg/mL in 5 mL and 10 mL vials; Alizine or Alizin (Virbac); (Rx)

The FDA may allow legal importation of this medication for compassionate use in animals; for more information, see the Instructions for Legally Importing Drugs for Compassionate Use in the USA found in the appendix.

Human-Labeled Products: None


Treatment of RFM

Although many mares with RFM do not become clinically ill, early prophylactic intervention is widely practiced because the complications associated with RFM may be severe and potentially life threatening. Many farm managers and horse owners with a veterinary client-patient relationship may be instructed to begin intramuscular (IM) injections of oxytocin 2 to 4 hours postpartum if the fetal membranes have not been passed. The membranes should be tied up above the hocks to prevent soiling and tearing. Tying a weight (e.g., a brick) to the membranes is not recommended because it may predispose the mare to development of a uterine horn intussusception. Injections of oxytocin should be given every hour for at least 6 treatments. The half-life of oxytocin in the mare is brief (12 min).

The initial starting dose of oxytocin should be on the low side (10-20IU/500 kg) because sensitivity to oxytocin varies widely. The dose of oxytocin can then be tailored to each individual mare. A positive response will result in passage of uterine fluid from the vagina. Mares should be monitored following injection because any obvious cramping will begin within 10 minutes of IM injection. If a 10- to 20-IU oxytocin treatment does not result in an outward manifestation of discomfort by the mare, such as sweating and restlessness, then the dose can be increased in 10- to 20-IU increments until an effect is noticed. The dose should only be high enough to elicit mild colic signs. Mares with uterine inertia because of dystocia may be initially very resistant to the effect of oxytocin and may become more sensitive in the subsequent hours. If cramping and rolling result then the dose should be reduced. Some mares become inattentive mothers during the time when they are distracted by RFM or uncomfortable from the oxytocin-induced cramping. Thus the foal should be kept in a safe place when the mare is in pain. Nursing should be encouraged to stimulate the natural release of oxytocin associated with milk letdown.

If the mare fails to respond to six oxytocin injections or if she is clinically ill, a thorough veterinary examination is indicated. One option is to start an intravenous (IV) drip of oxytocin at 0.1 IU/ml of saline (i.e., 100 IU oxytocin per 1 L saline). The IV flow rate should be set so that the mare has visible signs of contractions every S to 10 minutes. The oxytocin drip treatment protocol will, in effect, revert the mare back into labor for about 1 hour.

The technique described by Burns and colleagues () works best when the membranes are fresh. Some clinicians perform the procedure prophylacti-cally after a dystocia to reduce the likelihood of membrane retention. The clinician should wear waterproof clothing and a sterile surgical glove over a clean rectal sleeve. The perineum of the mare and external portion of the membranes are washed thoroughly. The opening at the cervical star, which leads into the allantoic cavity, is identified. A clean large-bore stomach tube is introduced, and the membranes are gathered around the tube. In addition, 4 L or more of a warm 1% povidone iodine solution is pumped or gravity fed into the chorioallantois until the fluid overflows. The tube is withdrawn as the RFM are tied shut with umbilical tape. Oxytocin may then be administered so that the uterus contracts against the distended membranes. This technique distends the endometrial crypts and often permits release of the microcotyledons. If the procedure is unsuccessful then it may be repeated several hours later. However, the retained membranes soon become autolytic and tend to tear as soon as distention starts.

If partial retention of the membranes is diagnosed, or if the membranes are badly torn, the uterus may be distended with 1% povidone iodine solution as described previously. The fluid distention and uterine contractions may help loosen the membranes. If the piece of membrane can be reached, it may be gently teased off the en-dometrium and removed. However, if the membrane tag is firmly adhered then continued traction is contraindicated. Once or twice daily flushing and the process of au-tolysis will eventually loosen the membranes. This procedure also may be carefully performed in mares that retain the membranes after a cesarean section. However, it is important to use a lower volume of infusate so that the uterine closure and fibrin seal are not disrupted.

Toxemic mares that are clinically ill and are passing a fetid uterine discharge may require systemic support with IV fluids, frequent IV treatments with oxytocin, and twice daily high-volume uterine lavage. Gentle manual removal of the fetid membranes may be necessary in these mares. Back and forth uterine lavage is performed with a clean stomach tube, bilge, or stomach pump. A dilute (1%) povidone iodine solution or sterile fluids are used to remove bacteria and inflammatory debris from the uterus. The clinician should hold the end of the tube cupped in the hand within the uterine cavity to prevent the tube from forcefully sucking against the wall when the fluid is being siphoned back. During the first few lavage procedures, persistence and patience in obtaining a clean return from the uterus is often rewarded with rapid clinical improvement and uterine involution. Lavage should be repeated once or twice daily until all debris is removed, the lavage is clear, and the uterus is well involuted.

Prophylactic administration of antibiotic and antiinflammatory medication is often prescribed early in the course of RFM in an attempt to prevent complications. Common antimicrobial choices are trimethoprim sulfa (30 mg/kg, q24h PO), or procaine penicillin G (22,000 IU/kg ql2h IM) for a minimum of 3 to 5 days. If the mare is systemically ill then broad-spectrum medications such as penicillin-aminoglycoside combinations are recommended. The formulations or derivatives of penicillin include the following: procaine penicillin (22,000 IU/kg ql2h IM), sodium and potassium penicillin (22,000 IU/kg q6h IV), ampicillin (50 mg/kg q8h IV), or ticarcillin (44 mg/kg q8h IM) for resistant cases. Aminoglycosides such as gentamicin (6 mg/kg q24h IM or IV) or amikacin (6.6 mg/kg ql2h IV or IM) are used for mixed and gram-negative infections or resistant cases. Appropriate antibiotic use is confirmed by uterine culture and sensitivity results.

The mostly commonly used antiinflammatory medication for endotoxemic mares is flunixin meglumine, 1.1 mg/kg IV. In milder cases, flunixin meglumine (0.25-0.5 mg/kg q8h IV), ketoprofen (2 mg/kg ql2h IV), vedaprofen (2 mg/kg ql2h PO), or phenylbutazone (4 mg/kg IV or PO) are used. Hyperimmune plasma is administered if it is available.

Laminitis in mares with RFM is a serious complication. Lateral radiographs of the distal phalanx will help establish the degree of rotation, and the prognosis. Symptomatic care such as hosing the hooves with cold water, or application of foam pads or special shoes to the hooves can provide extra support and promote comfort. Phenylbutazone at 2 g every 24 hours by mouth is sometimes used prophylactically.

Mares with lactation failure should be treated with domperidone at 1.1 mg/kg orally every 12 hours to encourage lactation.


Postpartum Hemorrhage: Physical Examination

Unremitting signs of colic in the postfoaling mare warrant a thorough examination by a veterinarian. The restlessness may be caused by uterine contractions, especially if oxytocin has been administered to promote passage of the fetal membranes. If the discomfort continues, however, the possibility of a dissecting hematoma in the broad ligament must be considered. If at any time the pressure causes the ligament to tear, blood can flow freely into the abdominal cavity.

Clinical signs associated with hemorrhage can vary greatly from mare to mare (Table Clinical Signs of Postpartum Hemorrhage). Most will exhibit colicky behavior to varying degrees, but some will be stoic. The unpredictable and dangerous behavior of many of these mares, along with the fact that most have a viable and newly ambulating foal, make it imperative that at least one, if not more, competent handlers be present. If manpower permits, a very small enclosure can be constructed of straw bales outside the stall door allowing safety of the foal and comfort to an anxious mare.

Table Clinical Signs of Postpartum Hemorrhage

Clinical Signs Comments
Mental status Agitated
Pain and associated signs Rolling or curling of upper lip
Frequent standing up and lying down
“Stomping” at abdomen with hindlegs
In author’s experience, less pawing and rolling than is seen with bowel problems such as large colon torsion
Mucous membranes Gums often severely blanched
Capillary refill undetectable
Skin Coldness
Clammy feel
Jugular distensibility Increased jugular fill time (may be difficult to place an IV catheter)
Pulse character May not even be detectable at facial artery
Heart rate Usually tachycardic in the range of 80-100+ bpm
Possibly normal
Rectal temperature Frequently <95° F
Respiration Usually increased and labored; a reflection of metabolic acidosis

IV, Intravenous; bpm, beats per minute.

It bears restating that treatment of a mare in hemorrhagic shock involves expedience and a concert of activities going on at once. Thus, diagnosis and treatment usually occur simultaneously. After the initial physical examination, if hemorrhage is presumed, vascular access needs to be established and preliminary fluids started. Handlers can usually take care of the latter while the veterinarian continues with further diagnostics, such as ultrasound. In this day of evaluation of ovarian follicular growth and 14-day pregnancies, many practitioners are privileged to have at their disposal ultrasound machines with a 5.0-MHz linear rectal probe. Although a 3.0-MHz probe is preferable, the 5.0 rectal probe can be used to evaluate a mare’s abdomen for signs of hemoperitoneum. Blood will appear hypoechoic and swirling of the cellular elements may be seen. Palpation per rectum and an internal ultrasound exam are discouraged because manipulation in this area can cause further problems. Application of a twitch to these mares can cause even more agitation, not to mention that placing an arm in a distressed mare is even more dangerous than bending down to scan an abdomen. If no fluid is seen with the transabdominal examination, it can be assumed that the hemorrhage is contained within the broad ligament or uterine wall. Symptotic treatment for these mares should continue. If fluid is seen, abdominocentesis is highly recommended to differentiate between other possible diagnoses including colon rupture, uterine tear or rupture, or mesenteric tear with hemorrhage and possible spillage of gut contents.


Postpartum Prolapse and Genital Tract Lacerations

Uterine Prolapse

The veterinarian can be presented with a case of uterine prolapse under two conditions. The first is the prolapse that occasionally occurs under general anesthesia when the mare is in dorsal recumbency. The second is the prolapse that occurs during natural foaling. The former type of prolapse usually is associated with delivery of the fetus; it is secondary to the abortion or delivery and probably occurs because of the positioning and the relaxation of anesthesia. The uterus is usually undamaged by the prolapse and is easily replaced. The surgery table should be tipped to aid in the replacement of the uterus and towel clamps are often placed in the vulva. Once the mare recovers, reprolapse is usually not a problem; however, the uterus should be lavaged with sterile fluid to allow complete reduction of any horn eversion. Oxytocin should be given intramuscularly to assist in uterine involution.

Prolapse also occurs during foaling. Uterine prolapse associated with foaling is an emergency. At a minimum, the uterus can be severely damaged with negative effects on future fertility. It is not uncommon for a mare to hemorrhage sufficiently, usually from a uterine artery rupture, to die from hypovolemia. The prolapsed uterus should be protected, and suspended if possible, until it can be replaced. This author prefers general anesthesia for uterine replacement because it provides optimal control and relaxation of the mare, and it is a safer procedure for the personnel involved. Although anesthesia is a greater risk for the mare, this author believes that the benefits outweigh the risks.

The uterus must be cleaned and then replaced through the cervix. Topical treatments to remove uterine edema have not been useful in this author’s experience. Lubrication of the uterine tissues and manual compression seem to provide the most benefit. Obstetricians should be aware that during replacement it can often seem as though not much progress is being made, and then the uterus will seem to fall in place all at once. Persistence is the key to uterine prolapse replacement. Lavage and oxytocin should be provided as indicated above once the mare recovers from anesthesia.

Rectal Prolapse

Irritation to the anus that can result in straining and subsequent rectal prolapse is unusual after foaling but can occur. Most rectal prolapses are relatively short and result in a rather gruesome, doughnut-shaped mass of bloody tissue that protrudes from the anus. If the feces are soft enough, most mares can pass some fecal material through the prolapse. However, the appearance of the prolapse, and the associated straining, may necessitate surgical treatment under epidural anesthesia.

The clinician removes the edematous and hemorrhagic tissues by sharp dissection, leaving enough mucosa and submucosa to circumferentially suture without tension. Because the cranial rectum pulls orally once the edematous tissue is removed, the prolapse should be dissected free and then closed in quarters. This method facilitates exposure of the anastomosis site for the first three quarters of the circumference. Oral laxatives such as mineral oil and a nonsteroidal antiinflammatory will facilitate fecal passage. Short-term use of antimicrobials is at the surgeon’s discretion.

Rupture Of The Small Colon Mesentery

Bladder Eversion

Bladder eversion also occurs as a result of straining of the urogenital tract. The everted surface of the bladder is covered with urothelium and is textured. If enough bladder is everted, the openings of the ureters can be seen at the cranial aspect of the everted tissue. To replace it, the bladder must be manually inverted back through the external urethral sphincter. The bladder is first cleansed and then is compressed between the surgeon’s hands and slowly pushed through the sphincter. Water-soluble sterile lubricants are very helpful, as is massage of the bladder to remove edema. If patient massage is unsuccessful in replacing the bladder, the external urethral sphincter can be transected dorsally to allow replacement. The incision must be closed to help keep the bladder from re-everting. The bladder should be lavaged with sterile fluid to ensure it is fully replaced and administration of antimicrobials with urinary excretion and a nonsteroidal antiinflammatory drug is recommended for several days.

Bladder Prolapse

Intact bladders prolapse as a result of a defect in the peritoneum and vaginal wall. This injury is the result of severe vaginal trauma. The bladder surface will be shiny, smooth, and white with visible subperitoneal vessels. The prolapsed bladder should be cleaned, and the bladder can be drained of urine by aspiration before replacement in the abdomen. A Foley catheter is then placed to keep the bladder decompressed. Urine drainage from the catheter will also reduce urine spillage into the vagina and subsequent contamination of the peritoneal cavity through the rent in the vaginal wall. Treatment of the vaginal wound involves reduction of further peritoneal contamination and prevention of eventration. The vaginal wound heals best by second intention. A cross tie can be used to prevent the mare from lying down; the possibility of eventration is thus reduced. Systemic antimicrobials should be administered.

Bladder Rupture

Bladder rupture can occur either before, or more commonly after, foaling. The resulting uroperitoneum results in elevated serum levels of creatinine, blood urea nitrogen, and potassium, and lowered concentrations of sodium and chloride. Confirmation of urine in the abdomen makes the diagnosis, and the rent can be observed by an endoscopic examination of the bladder. Medical therapy requires urine drainage and correction of electrolyte abnormalities. Surgical access to the tear is impossible through a celiotomy, so reports of access for surgical repair of bladder ruptures are limited to either a colpotomy approach or to eversion of the bladder through the urethral sphincter. Both surgical procedures are technically demanding, as is a laparoscopic approach, which is unreported but may be possible in some instances. Conservative therapy for bladder rupture is successfully used in many species and may be of value in the mare. Medical therapy must be instituted, and the bladder is kept decompressed by the use of a self-retaining catheter such as a Foley.

Vaginal Lacerations

Vaginal lacerations associated with eutocia are rare. When present, these lacerations are usually linear defects in the vaginal mucosa that heal by second intention and require no treatment. Occasionally an older mare will rupture a vaginal varicocele and cause hemorrhage. This injury is usually self-limiting, and treatment is unnecessary. Dystocias can cause severe vaginal lacerations, and their extent is dependent on the amount of vaginal trauma. The most common severe vaginal laceration is seen in primiparous mares when damage has occurred to the transverse urethral fold. The entire fold can be torn off or undergo necrosis, resulting in an incompetent external urethral sphincter. This laceration can result in urinary incontinence, scalding, and urine pooling. Immediate treatment for vaginal lacerations is gentle cleansing of the wounds, the use of a nonsteroidal, antiinflammatory drug, and topical emollients to reduce scarring and adhesion formation. Corticosteroids can be added to the topical medication to also reduce inflammation. Systemic antibiotics are indicated for mares with multiple or deep lacerations. Wounds are allowed to heal by second intention.

In mares with extensive vaginal adhesions or incontinence, surgical therapy is indicated after the wounds have healed. Both conditions have a guarded prognosis. The difficulty with treatment of vaginal adhesions is that they frequently reform. These adhesions seem to be most common in Miniature Horse mares, possibly because of the small pelvic canal in this breed, and the domed head of the fetus predisposes Miniature Horse mares to vaginal trauma. Sharp resection of adhesions followed by topical ointments with steroids may be beneficial. A vaginal vaultsized, soft tampon that is coated with ointment can be used to keep transvaginal adhesions from reforming.

Incontinent mares should have their caudal urethral sphincter pressure augmented. Because these horses are incontinent because of scarring and tissue loss, this procedure can be very challenging. Surgery should be considered only after all inflammation has subsided. The goal is usually to recreate a transverse urethral fold, as well as to extend the urethra in a similar manner to that performed to correct urine pooling.


Induction of Parturition

Elective, attended foalings are advantageous to monitor mares that have experienced dystocia or premature placental separation in previous deliveries. Mares with gestational abnormalities such as rupture of the prepubic tendon or hydroallantois may require assistance during delivery. However, induction of parturition in itself can be associated with side effects such as dystocia, premature placental separation, fetal hypoxia, and dysmaturity. Thus careful case selection before induction of parturition is critical for successful delivery.

Criteria For Induction Of Parturition

Methods Of Induction

Oxytocin is generally considered to be the drug of choice for induction of parturition in the mare. Oxytocin has a rapid effect and results in delivery within 15 to 90 minutes after administration. The progress of parturition is consistent with oxytocin and few adverse effects are noted in the term foal. Various methods and doses of oxytocin induction have been described including a bolus injection of 2.5 to 120 units oxytocin, via the intramuscular (IM) or intravenous (IV) route; IM or subcutaneous (SQ) injection of 2.5 to 20 units oxytocin at 15 minute intervals; and IV administration of 60 to 120 units oxytocin in 1 L of saline delivered at a rate of 1 unit/minute. Method of delivery of oxytocin (bolus injection, repeated incremental injections or IV drip) does not appear to affect neonatal outcome in induced deliveries. Logistically, administration of oxytocin by injection allows the mare to move about freely in a stall or paddock without human intervention. When administering oxytocin through an IV drip an individual must stand at the head of the mare at all times. Alternatively, an elaborate tubing system must be constructed to prevent the mare from becoming tangled in the drip lines as she lays down to roll or push.

The dose of oxytocin is an important consideration when parturition is induced in the mare. Initial reported doses of oxytocin ranged between 75 and 120IU. Few untoward effects on the mare or foal were reported with these high doses of oxytocin; however, one must consider the possibility of uterine hyperstimulation with higher doses. More recently, doses of 15 to 20 units oxytocin (IV, IM, or SQ) have been used. Injections are administered at 15- to 30-minute intervals until the chorioallantois ruptures. Before further injections, a vaginal examination and evaluation of the progress and position of the fetus are critical. First-stage labor is abbreviated in induced parturition, so a higher likelihood exists that the fetus will be abnormally positioned/postured. Correction of fetal limbs should be made before administering additional oxytocin, because the expulsive effects of the mare may make it difficult to perform manipulations on the fetus later on. In all cases of induced foaling, the veterinarian should remain present through the delivery of the foal.

Recently, it has been shown that doses of oxytocin as low as 2.5 IU IV are effective in inducing the term mare. In one study, mares that had 8 mmol/L Ca++ in mammary secretions were considered to be near foaling, and 2.5 IU oxytocin were administered. Mares that did not foal within 1 hour of oxytocin administration were judged not ready to foal, and they received a second dose of oxytocin (2.5 IU, IV), daily, until foaling occurred. Fourteen of 17 mares (14/17, 82%) foaled after the first treatment; one mare foaled after oxytocin administration on day 2, and 2 mares foaled after oxytocin on day 3. The investigators concluded that a single, low-dose injection of oxytocin (2.5 IU, IV) was effective to induce parturition in mares.

Furthermore, the researchers suggested that this induction scheme would work only in mares with a mature fetus, and that mares foaling on days subsequent to the initial treatment did not respond to oxytocin because the fetus was not fully mature. This method of induction appears to be a safer but more labor-intensive approach for a field practitioner.


In summary, several factors affect the success of induced parturition in the mare. Fetal readiness for birth is paramount to survival of the foal after birth. Critical evaluation of mammary secretion electrolytes, cervical relaxation, and gestational length facilitates proper mare selection and neonatal survivability with induced parturition. Oxytocin is the current agent of choice to induce parturition in the mare. The method of oxytocin administration does not impact neonatal adaptability after induced birth. Because a low-dose oxytocin protocol is effective for inducing parturition in the mare, higher doses of oxytocin are unnecessary and may be inappropriate.


Abdominal Wall Hernias

The weight of the pregnant uterus on the caudal epigastric and caudal superficial veins may restrict drainage of the area, and a severe plaque of ventral edema can extend from the udder to the xiphoid. Mares with a ventral hernia present with a similar plaque of inflammatory edema. Damage to the musculature by external trauma can also cause edema. Progressive enlargement of a rent in the abdominal wall causes pain that makes affected mares walk slowly; often they are reluctant to move at all. Discomfort is displayed when the caudal abdomen is palpated. The defect and/or hernial contents are usually difficult to palpate transabdominally due to the edema. Transrectal palpation of the defect is seldom possible because of the presence of the fetus and the large, dependent uterus. Transabdominal ultrasound permits visualization of any abdominal contents that may be herniated through the rent in the body wall. Tears may be difficult to distinguish from separation of the musculature by severe edema, and careful ultrasonographic evaluation is indicated.

Treatment of Abdominal Wall Hernias

Initial treatment of a suspected hernia should be directed toward stabilization of the mare through confinement to a small area and restricting exercise. External abdominal support in the form of a belly-band should be used to transfer abdominal weight to the vertebral column. Administration of laxatives and reduction of the bulk of the ration will prevent constipation. If the foal is viable and mature, parturition should be induced to decrease the risk of uterine blood vessel rupture or enlargement of the defect to the point where abdominal discomfort and weakness leads to recumbency. Induction of parturition with the use of cloprostenol (two doses of 250-500 μg, 30 minutes apart), or oxytocin (20 IU, repeated as needed, or 50 IU in a saline drip) has been effective in this author’s experience. Delivery must be assisted because the mare will be unable to mount an adequate abdominal press. Excessive exertion should be minimized to prevent any enlargement of the hernia. A cesarean section should be considered if the prognosis for the mare is poor, and the viability of the foal is of primary concern. Surgery is also indicated if incarceration of a piece of bowel is suspected. If the pregnancy is not sufficiently advanced to permit delivery of a viable foal then abdominal support and supportive care should be maintained until induction or surgery is feasible.

Transverse, oblique, and ventral hernias may not be well delineated prepartum but when the edema resolves after delivery the rents will become more evident. An abdominal support should be worn for 2 to 3 months until the edema resolves and a fibrous ring forms at the sight of the hernia. Surgical repair with propylene or plastic mesh has been successfully performed. Smaller defects may heal by second intention. Reproductive capacity postrepair is not known; however, constant supervision and assistance during parturition is necessary so that further damage does not occur. In cases where defects are unable to be repaired, future pregnancies are unwise. Embryo transfer or other new assisted reproduction technologies may be the best option.


Placental Hydrops

Hydrops is a rare condition in the mare, with hydroallantois occurring more commonly than hydramnios. Hydroallantois causes rapid abdominal enlargement during the last trimester of pregnancy (), and a sudden increase in the volume of allantoic fluid during a period of 10 to 14 days. The pathophysiology of hydroallantois in the mare remains unknown. Some authors have suggested that the increase in fluid is a placental problem caused either by increased production of fluid or decreased transplacental absorption. Others have proposed that the etiology is related to placentitis and heritability. In these authors’ experience, examination of the fetus and fetal membranes have rarely demonstrated any consistent abnormality. One mare had diffuse mild placentitis (with leptospirosis) and another had histologic evidence of vasculitis when an endometrial biopsy was taken within 2 days of delivery of the fetus.

Mares may present with anorexia, tachycardia, severe ventral edema/plaques, abdominal discomfort, and labored breathing caused by pressure on the diaphragm. They typically have difficulty walking and often become recumbent. Uterine rupture may occur in advanced cases. Other complications associated with the excessive weight of the uterine contents include prepubic tendon rupture and development of abdominal wall and inguinal hernias.

Rectal palpation is diagnostic and reveals a huge, taut, fluid-filled uterus. The fetus cannot be balloted. Transrectal ultrasound imaging shows hyperechogenic allantoic fluid. The fetus is seldom observed as a result of the depth of the enlarged uterus. Transabdominal ultrasound will confirm the presence of excessive echogenic allantoic fluid and this approach does permit evaluation of fetal viability (movement and a heart rate of 80-100 bpm).

Treatment of Placental Hydrops

During the last few years the medical facility at these authors’ clinic has seen an increase in the number of hydroallantois cases presented to the clinic. Once the diagnosis of hydroallantois is confirmed, fetal viability is determined, and udder development and the milk electrolytes are assessed to estimate the level of fetal maturity. Those mares that present early in gestation may undergo elective termination of pregnancy by IM injection of cloprostenol (500 μg, 2 ml IM ql2h until delivery). Cases that occur later in gestation, or those with profound abdominal enlargement, may have large volumes of fluid within the uterus and require controlled drainage of the fluid before expulsion of the fetus. The reason for the controlled drainage is that excessive uterine distention alters total body fluid balance and venous return to the right heart. Sudden loss of this large volume of fluid may result in hypovolemic shock. The effect of the fluid loss is exacerbated by the sudden expansion of the abdominal venous circulation once the uterine weight is reduced. In the short term, abdominal support (i.e., belly band), IV fluids, steroids, broad-spectrum antibiotics, and antiinflammatory medication will provide systemic support for the mare. Slow siphoning of the allantoic fluid is then attempted. Once the size of the distended uterus has been reduced, the authors have used oxytocin (20 IU IV given repeatedly or 50 IU in a saline drip) or cloprostenol (two doses of 500 μg, 30 minutes apart) to promote fetal expulsion. In these authors’ experience, cloprostenol has provided a smooth progression of labor, with stage 2 occurring 30 to 60 minutes after the second dose. Mares that present within the last 2 to 4 weeks of pregnancy may be managed by partial drainage. The aim in these cases is to maintain the pregnancy for as long as possible in order for additional fetal maturation to occur.

The technique for drainage involves several considerations. Location (stocks or stall) is determined by individual preference and the condition of the mare. The process takes 2 to 3 hours, so comfort is a factor. The clinician should initiate supportive care by placing an IV catheter and administering a slow infusion of a crystalloid fluid. A tail wrap and sterile surgical preparation of the mare’s perineum is essential. The equipment includes a 24- to 32-French sharp thoracic trocar catheter, a two-way plastic adapter, sterile plastic tubing, a sterile sleeve, and buckets to collect the allantoic fluid. Smaller-sized catheters will take longer for fluid removal. The technique involves sterile passage of the catheter through the vagina and cervix, and sharp puncture of the chorioallantois. The sharp trocar is removed and the two-way adapter is used to connect the catheter to the tubing. The catheter is held in place by the clinician’s arm within the vagina. Controlled gradual drainage can then be performed into the buckets. Some pericervical separation of the placenta is common.

Several mares have been successfully treated in these authors’ hospital with this technique. In a few cases (6) that were within 2 to 4 weeks of term, maintenance of the pregnancy has been attempted after partial drainage of the allantoic compartment. These mares were treated with additional antimicrobial therapy, antiinflammatory medications (flunixin meglumine, pentoxifylline), agents with possible tocolytic activity (isoxsuprine, clenbuterol, albuterol), and altrenogest. In cases where partial drainage is attempted, fetal death may occur as a result of fetal asphyxia that results from varying degrees of placental separation. Iatrogenic fetal infection, secondary to contamination of the placental fluids during drainage, is also a problem. In most cases attempted to date, the fetus has become infected with Escherkhia coli and subsequently died. One mare died after 72 hours as a result of rupture of a uterine artery. The fetus in that mare had remained alive and exhibited normal parameters when monitored by transabdominal ultrasound.

Owners should be advised that the fetus is usually lost in mares with hydroallantois. However, early recognition of the problem, and prompt intervention, provides a good prognosis for the mare both physically and reproductively. Complications that should be anticipated when managing a mare with hydroallantois include hypovolemic shock, dystocia, and retention of the fetal membranes. The hypovolemia requires rapid volume expansion with use of large volume crystalloid infusion (as high as 40 ml/kg) alone, or in combination with hypertonic saline (4 ml/kg). The use of colloid fluids such as hetastarch (10 ml/kg) might also be beneficial. Dystocia may be associated with incomplete cervical dilation and uterine inertia. Malpositioning and malpostures are common. Therefore manual assistance to deliver the foal is necessary. Management of retained fetal membranes is discussed elsewhere in this text (see “Retained Fetal Membranes”). Because it is possible that a heritable component to this condition exists, breeding to a different stallion may be prudent.


Effect Of Obstetric Conditions On Peritoneal Fluid

Uterine Torsion

In this author’s experience presurgical peritoneal fluid samples from uncomplicated uterine torsion cases have not revealed any values outside of the normal range. If neglected or misdiagnosed, mares may develop significant uterine compromise that results in changes in the composition of the peritoneal fluid. WBC counts in excess of 10,000 cells/μl in conjunction with total protein levels above 3.0 g/dl are cause for concern. When abnormalities are detected, the expense of a ventral midline celiotomy to evaluate the condition of the uterus may be justified. Alternatively, results of peritoneal fluid analysis may support a decision for euthanasia on economic grounds.

Normal Foaling Process

In prepartum animals and in mares with uncomplicated deliveries (oxytocin-induced or natural foaling), peritoneal fluid is clear to yellow unless it is red-tinged as a result of blood contamination. The WBC count in the postpartum samples may be increased compared with the prefoaling values, but should remain within the normal range for the laboratory. This slightly increased peritoneal fluid mononuclear cell count (WBC still <5000 cells/μd) seen in foaling mares may be caused by normal hemodynamic changes in the immediate postpartum period. When the fetus is delivered, pressure on the great vessels is removed and the volume of peritoneal fluid may decrease. Thus, the cellularity of the remaining fluid would be expected to increase. Although the WBC count in clinically normal postdystocia cases also remains at less than 5000 cells/μl, the increased cellularity is the result of an influx of neutrophils, probably in response to hyperemia and increased endothelial permeability. However, any bruising and inflammation within the uterine wall is generally not severe enough to cause leakage of protein-rich fluid. The TPr should remain at less than 2.5 g/dl.

Obstetric Cases

It may be beneficial to obtain a peritoneal fluid sample from referred obstetric cases. In most instances the foal will be dead, and the fluid analysis will provide a baseline that can document preexisting conditions (e.g., uterine tear) before further vaginal intervention. Evidence of a laceration would warrant an immediate cesarean section. The duration of the dystocia before successful fetal extraction does not appear to affect the composition of the peritoneal fluid. In difficult obstetric cases that have been subjected to prolonged manipulations before referral to a veterinary hospital, the peritoneal fluid profile is generally not altered from the normal range. The author has managed cases with an emphysematous or macerated fetus in which the composition of the peritoneal fluid samples was not abnormal. Dystocia itself does not necessarily cause significant changes in the peritoneal fluid. If an experienced obstetrician performs the vaginal manipulations and/or fetotomy the fluid should remain grossly normal.

The author has studied the peritoneal fluid from more than 50 cases that remained clinically normal after resolution of a dystocia. None of the median values changed significantly in the peritoneal fluid of these postdystocia mares. However, although the median WBC counts remained within the laboratory reference limit (<5000 cells/μl), some mares did develop slightly increased cell counts. Although TPr exceeded normal limits (as high as 3.4 g/dl) in 3 cases, the cell count never exceeded 10,000 cells/μl in mares that remained clinically normal. The preponderance of neutrophils may reach 90% in some cases. Only one mare had more than one peritoneal value elevated on either day 1 or day 2. In this author’s experience, those mares that are destined to become clinically ill will have at least two of the TPr, WBC count, and percent neutrophil values significantly elevated above the normal reference range.

As might be expected, mares that experience postdystocia complications have significantly higher median peritoneal fluid values for TPr, WBC count, and percent neutrophils than do mares that make an uneventful recovery. The markedly increased WBC counts in the mares with uterine tears or with vaginal lacerations involving the peritoneal cavity will cause the peritoneal fluid to appear cloudy, with a dark orange color from the increased erythrocyte numbers. These cases are likely to have TPr, WBC count, and percent neutrophil values that exceed 3.0 g/dl, 15,000 cells/p.1, and 80%, respectively. WBC counts often exceed 50,000 to 100,000 cells/μl. A mare with a partial thickness uterine tear is unlikely to have elevated TPr or WBC counts immediately postpartum. However, the peritoneal fluid values may be increased within 2 to 3 days depending on the severity of the damage to the uterine wall.

The normal parturient process in the mare does not entail epithelial loss from the endometrium. An intact endometrium appears to be able to prevent absorption of endotoxin and bacteria, whereas devitalization permits diapedesis and peritoneal contamination. Peritonitis is likely to develop subsequent to severe necrotizing endometritis as areas of transmural necrosis extend through the myometrium to the uterine serosa. Complete perforation of the uterine wall is not necessary for peritonitis to develop if traumatic obstetric manipulations have damaged the uterine wall. However, recent research has proven that even a fetotomy procedure does not alter the composition of the postpartum peritoneal fluid if it is performed correctly.

Cases with rupture of the uterine artery and associated development of a broad ligament hematoma tend to have markedly elevated TPr values (as high as 5.0 g/dl), but the WBC count is likely to remain within the normal range (<5000 cells/μl). Broad ligament inflammation around a uterine artery hematoma may explain the high protein levels seen in these cases. Specific gravity values are inevitably increased if the TPr is elevated. A tear in the broad ligament subsequent to a uterine artery rupture invariably results in a bloody tap, with an elevated red blood cell count in the peritoneal fluid. Even if a clot has contained most of the hemorrhage within the broad ligament, there is often considerable blood loss into the peritoneal cavity. In this author’s opinion these mares should not be transported, because movement could destabilize the clot and prove fatal. Postpartum hemorrhage is discussed in detail in site.

Rupture of the mesocolon is unlikely to cause an immediate increase in the WBC count. However, the compromised segment of bowel soon loses its integrity and a massively increased WBC count (as high as 150,000 cells/μl) can occur within 48 hours as peritonitis ensues. An intussusception of the uterine horn can cause an elevated TPr (3.0 g/dl), but the WBC count tends to remain low unless necrosis has developed in more chronic cases. Retroperitoneal abscessation can be a life-threatening complication following a dystocia. Affected mares exhibit signs of toxemia, and the peritoneal fluid is likely to develop an increased TPr content (3.0-5.0 g/dl) and a massive increase in the WBC count (often exceeding 100,000 cells/μl). These retroperitoneal abscesses often develop from infected hematomas. Thus, retroperitoneal hematomas in a postpartum mare warrant prophylactic broad-spectrum antibiotic coverage.

Repeated abdominocentesis is indicated in cases where clinical signs suggest that a parturient related abdominal lesion may be present, because the peritoneal fluid constituents may change within hours. Several studies have shown that repeated abdominocentesis is not detrimental to the horse and does not cause significant changes in the peritoneal fluid composition. If indicated, a series of peritoneal fluid analyses may provide useful information about the progression and seriousness of the condition. A single, elevated peritoneal fluid value (either TPr, WBC, or percent neutrophils) is likely to be an incidental finding, whereas two or more elevated values may signal the onset of clinical abnormalities. This author’s clinical experience has been that if a postpartum peritoneal fluid sample has TPr greater than 3.0 g/dl in conjunction with WBC count greater than 15,000 cells/μl and a WBC differential count of greater than 80% neutrophils (especially if degenerative changes are present) then a potentially life-threatening lesion is likely. However, peritoneal fluid values should not be viewed in isolation. An abnormal peritoneal fluid analysis must be considered in conjunction with the history and clinical signs that are exhibited by the mare.


Oocyte Transfer

Oocyte transfer is the placement of a donor’s oocyte into the oviduct of a recipient. The recipient can be inseminated within the uterus or within the oviduct. Placement of the oocyte and sperm within the recipient’s oviduct is more accurately termed gamete intrafallopian transfer (GIFT).

The first successful oocyte transfer was done in 1989; however, the technique was not used for commercial transfers until the late 1990s. Oocyte transfer is currently used to produce offspring in subfertile mares in which embryo transfer is not successful because of various reproductive problems. These problems include ovulatory failure, oviductal blockage, recurrent or severe uterine infections, and cervical tears or scarring. In some cases, the cause of reproductive failure cannot be diagnosed; however, oocyte transfer can be successful.

Sychronization Of Donors And Recipients

Oocytes are collected from preovulatory follicles between 24 and 36 hours after the administration of human chorionic gonadotropic (hCG; 1500-2500 IU, IV) to a donor mare or between 0 and 14 hours before anticipated ovulation. Criteria for hCG administration are as follows:

• Follicles greater than 35 mm in diameter

• Relaxed cervical and uterine tone

• Uterine edema or estrous behavior present for 2 or more days

Some mares, especially older mares, do not consistently respond to hCG. In these cases, this author uses a combination of the gonadotropin-releasing hormone (GnRH) analog, deslorelin acetate (2.1 mg implant; Ovuplant), followed by an injection of hCG (2000 IU, IV) between 4 and 5 hours later.

Oocytes collected 36 hours after hCG administration to the donor are transferred immediately into a recipient’s oviduct. Oocytes collected 24 hours after drug administration to the donor are cultured in vitro for 12 to 16 hours before transfer. The advantage of collection of oocytes between 32 and 36 hours after hCG administration to the donor is that the oocytes do not require culture in vitro. However, donors could ovulate follicles before oocytes are collected. The collection and culture of oocytes at 24 hours after hCG administration to the donor are often easier to schedule; the oocyte can be collected in the afternoon and transferred the next morning. This method requires expensive equipment and training for tissue culture, however. In a modification of these procedures, oocytes are collected 24 hours after hCG and immediately transferred into the recipient’s oviduct to allow maturation to complete within the oviduct. With this latter method, recipients are inseminated 16 hours after transfer.

Oocyte Collection

Oocytes are usually collected by one of two methods. In one method, the ovary is held per rectum against the ipsilateral flank of the mare. A puncture is made through the skin and a trocar is advanced into the abdominal cavity. The ovary is held against the end of the trocar while a needle is advanced through the trocar and into the follicular antrum.

In this author’s laboratory oocytes are collected by using transvaginal, ultrasound-guided follicular aspirations. For this procedure, a linear or curvilinear ultrasound transducer is used with the transducer housed in a casing with a needle guide. Before the procedure, the rectum is evacuated and the vulvar area is cleaned. The mare is sedated (xylazine HC1, 0.4 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV) and a substance to relax the rectum (propantheline bromide, 0.04 mg/kg, IV) is administered. A twitch is applied. The probe is covered with a nontoxic lubricant and placed within the anterior vagina lateral to the posterior cervix and ipsilateral to the follicle to be aspirated. The follicle is positioned per rectum and stabilized with the apex of the follicle juxtaposed to the needle guide. A needle is advanced through the needle guide to puncture the vaginal and follicular walls. In this author’s laboratory, a 12-gauge, double-lumen needle is used (Cook Veterinary Products, Spencer, Ind.). The follicular fluid is aspirated from the follicle by using a pump set at a pressure of -150 mm Hg. After removal of follicular fluid, the lumen of the follicle is lavaged with 50 to 100 ml of flush (typically modified Dulbecco’s phosphate-buffered solution or Emcare [ICP, Auckland, New Zealand]) that contains fetal calf serum (1%) or bovine serum albumin (0.4%) and heparin (10 IU/ml).

Equipment used to handle the oocyte is warmed to 38.5° C before use because the oocyte is sensitive to temperature changes. On collection, the follicular aspirate and flush are poured into large search dishes and examined under a dissecting microscope to locate the oocyte. Aspirations of preovulatory follicles are often bloody because the follicle has increased vascularity as ovulation approaches. The oocyte is approximately 100 μm in diameter and is surrounded by a large mass of nurse ceils — the cumulus complex. Cumulus cells, or the corona radiata, appear as a ring surrounding the oocyte. When the oocyte matures, the cumulus complex becomes less distinct. The corona radiata appears clear in the bloody flush solution and can be observed by the naked eye.

Oocyte Evaluation And Culture

On collection, cumulus oocyte complexes (COC) are evaluated for cumulus expansion (graded from compact to fully expanded) and for signs of atresia. Oocytes are determined to be in a stage of atresia when the COC is clumped and/or sparse, the corona radiata is fully expanded, or when the ooplasm is shrunken and dark or severely mottled. Oocytes with a fully expanded cumulus (marked separation of cumulus cells with expansion of the corona radiata) are considered mature and are transferred as soon as possible into a recipient’s oviduct. Oocytes with a moderately expanded cumulus complex (translucent COC with moderate separation of cumulus cells and incomplete expansion of corona radiata) are cultured before transfer. On occasion, the donor does not respond to hCG and the follicle does not begin to mature. Consequently, the granulosa cells that line the follicle are collected in small, compact sheets, and the oocyte is frequently not retrieved. If an immature (compact COC with little or no separation of cumulus cells) oocyte is collected, special culture conditions are required, including a maturation medium with additions of hormones and growth factors.

On identification and evaluation, the oocyte is washed and placed in a transfer or collection medium. A commonly used medium for the culture of maturing oocytes is tissue culture medium (TCM) 199 with additions of 10% fetal calf serum, 0.2 mM pyruvate, and 25 mg/ml gentamicin sulfate. A carbon dioxide (C02) incubator must be used to establish the proper culture conditions of 38.5° C in an atmosphere of 5% or 6% C02 and air.

Oocyte Transfer

Mares that will receive oocytes should be young (preferably 4-10 years of age) with a normal reproductive tract. Oocytes are transferred surgically; therefore, adequate exposure of the ovary is essential to facilitate transfers. Mares with short, thick flanks and short broad ligaments are not good candidates for recipients. Both cycling and noncycling mares have been used as oocyte recipients. When cyclic mares are used, they must be synchronized with the donor; thus, hCG is administered to the estrous donor and recipient at the same time of day. Before the mare can be used as a suitable recipient, her own oocyte must be aspirated. Anestrus and early transitional mares are suitable noncyclic recipients. During the breeding season, a high dose of a GnRH analog or injections of progesterone and estrogen (150 mg progesterone and 10 mg estradiol) can be administered to reduce follicular development in potential recipients. Noncyclic recipients are given injections of estradiol (2-5 mg daily for 3-7 days) before transfer and progesterone (150-200 mg daily) after transfer. In mares that are not having estrus cycles, pregnancies must be maintained through the use of exogenous progesterone.

Oocytes are transferred through a flank laparotomy into standing sedated mares. Recipients are placed in a stock and a presurgical sedative (xylazine HC1, 0.3 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV) is administered. The surgical area is clipped, scrubbed, and blocked with a 2% lidocaine solution. Immediately before surgery, additional sedation is administered (detomidine HC1, 9 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV). An incision is made through the skin approximately midway between the last rib and tuber coxae, and the muscle layers are separated through a grid approach. The ovary and oviduct are exteriorized through the incision. The oocyte is pulled into a fire-polished, glass pipette, and the pipette is carefully threaded into the infundibular os of the oviduct and advanced approximately 3 cm. The oocyte is transferred with less than 0.05 ml of medium.

Insemination Of Recipients

In a commercial oocyte transfer program, use of stallions with good fertility is essential. Cooled and transported semen is often provided. When fresh semen from fertile stallions and oocytes from young mares was used in different experiments, insemination of the recipient only before (12 hours) or only after (2 hours) oocyte transfers resulted in embryo development rates of 82% (9/11) and 57% (8/14), respectively. In a commercial oocyte program, mares were older with histories of reproductive failure and cooled semen from numerous stallions of variable fertility was used. Pregnancy rates when recipients were inseminated before or before and after oocyte transfer were significantly higher than when recipients were only inseminated after transfer (18/45, 40%; 27/53, 51% and 0/10, respectively). These results suggest that the insemination of a recipient before transfer with 5 X 108 to 1 x 109 progressively motile sperm from a fertile stallion is sufficient. However, if fertility of the stallion is not optimal, insemination of the recipient before and after transfer may be beneficial.

After insemination and transfer, the recipient’s uterus is examined by ultrasonography to detect intrauterine fluid collections. The uterine response to insemination often appears to be more severe when recipients are inseminated after transfer than when they are inseminated only before transfer. The uterus is evaluated and treated once or twice daily until no fluid is imaged. Recipients with accumulations of intrauterine fluid are treated similar to ovulating mares, with administration of oxytocin or prostaglandins to stimulate uterine contractions or with uterine lavage and infusion.

Future Of Oocyte Transfer

Oocyte transfer has proved to be a valuable method of obtaining pregnancies from mares that cannot carry their own foal or produce embryos for transfer. Because the mare does not have to ovulate or provide a suitable environment for fertilization or embryo development, the oocyte donor is only required to develop a preovulatory follicle with a viable oocyte.

The transfer of oocytes and a low number of sperm (200,000 motile sperm) into the oviduct of recipients has been successful. Pregnancies could be produced with GIFT when sperm numbers are limited, such as from subfertile stallions and from sex-selected or frozen sperm.

Through the use of this technique at this author’s laboratory, pregnancies have been recently produced from oocytes that were frozen and thawed and from oocytes that were collected from the excised and shipped ovaries of dead mares. These advances provide excellent methods to preserve the genetics of valuable mares.