Abdominal discomfort in the peripartum mare poses a diagnostic dilemma for the equine clinician because of the difficulty in differentiating between normal uterine contractions and other sources of abdominal pain. When a periparturient mare displays abdominal discomfort she may be experiencing a reproductive problem including uterine torsion or rupture, vaginal tear involving the peritoneal cavity, hematoma of the uterine wall, or a uterine artery rupture. Possible lesions in other abdominal organs include rupture of the urinary bladder or cecum, large colon impaction or torsion, or vascular compromise of a segment of bowel as a result of mesenteric rents or trauma. Several of these conditions can cause the affected mare to rapidly become depressed and febrile, with accompanying signs of shock and toxemia. A prompt and accurate diagnosis followed by aggressive medical and/or surgical intervention can often prevent an otherwise fatal outcome.
Recently, transabdominal ultrasonography has become an integral part of the diagnostic evaluation of the equine abdomen. The quantity and cellularity of fluid accumulated in the ventral abdomen is readily seen with a 3.5-MHz probe. However, detection of abnormalities in peritoneal fluid is still extremely useful when managing equine colic. The significance of these abnormalities in peripartum mares that are experiencing abdominal discomfort has only recently been widely appreciated. Intuitively, because peritoneal fluid composition reflects the pathophysiologic state of the visceral and parietal mesothelial surfaces, one would anticipate that the mechanics of the foaling process (and certainly obstetrical manipulations) would be likely to incite some changes in peritoneal fluid composition.
Obtaining A Peritoneal Fluid Sample In A Periparturient Mare
The mare is sedated with xylazine hydrochloride (0.3 mg/kg of body weight, IV) and butorphanol tartrate (0.01 mg/kg, IV) if needed. The most dependent portion of the ventral abdomen is clipped, shaved, and aseptically prepared. The abdominocentesis is performed as far cranial as possible, and approximately half an inch to the right of midline, to avoid penetrating the spleen or gravid uterus. An 18-gauge, 1.5-inch needle is introduced through the skin and slowly advanced into the abdominal cavity. If no fluid is obtained, the needle should be repositioned. Sometimes rotation of the needle or injection of a small volume of air with a sterile syringe is necessary to facilitate drainage of fluid. A minimum of 1.0 ml of peritoneal fluid should be collected in a tube that contains sodium ethylenediaminetetraacetic acid (EDTA) as an anticoagulant. In clinically ill mares, it maybe difficult to obtain a sample, especially if the mare is dehydrated or has a large ventral plaque of edema that extends cranially from the mammary glands. If difficulty is experienced in obtaining a sample, a local anesthetic block can be made over the site and a stab incision made through the skin, subcutaneous tissue, and muscular fascia with use of a number 15-scalpel blade. A blunt teat cannula or sterile female catheter can then be carefully advanced into the peritoneal cavity. The incision is allowed to heal by second intention.
Potential risks of an 18-gauge needle used to perform abdominocentesis include inadvertent laceration of the spleen or intestinal puncture (enterocentesis). Contamination of the sample with blood is more likely to be caused by penetration of superficial blood vessels or vessels in the abdominal musculature. This does not affect the usefulness of the sample because as much as 17% blood contamination does not alter the interpretation of the nucleated cell count or the total protein concentration of peritoneal fluid samples. Inadvertent enterocentesis may cause a transient increase in the nucleated cell count. Studies have shown that intestinal puncture with a needle rarely causes clinical signs of disease. Likewise, repeated abdominocentesis at 24- to 48-hour intervals has been shown to not alter the peritoneal fluid composition. Thus monitoring progressive changes in the peritoneal fluid may alert the clinician to the presence of a deteriorating condition in the abdominal cavity.
Peritoneal Fluid Analysis
The peritoneal fluid is visually inspected for color and clarity (turbidity). Specific gravity and total protein concentration (TPr) estimations can be made with a hand-held refractometer. Total white blood cell (WBC) counts can be determined manually with a hemocytometer, or measured by using an automated analysis system. Differential white blood cell counts can be made by viewing 100 cells on a smear that is stained with Wright’s stain. A direct smear can be made if the WBC count exceeds 10,000 cells/|xl. When the count is low the sample should be centrifuged to concentrate the cells. The white blood cell population of the peritoneal fluid consists of a mixture of nondegenerate neutrophils and large mononuclear cells. The latter are a combination of mesothelial cells that desquamate from the peritoneal surface, as well as blood-borne monocytes and macrophages that have migrated into the peritoneal cavity.
Normal values for an adult horse may vary between laboratories. A sample is classified as a transudate if the total protein concentration is less than 2.5 g/dl and the nucleated cell count is less than 5000 cells/μl. Modified transudates are characterized by an increase in TPr concentration or WBC count. If the total protein concentration exceeds 3.0 g/dl and the nucleated cell count exceeds 10,000 cells/μl then the sample is classified as an exudate. The normal differential WBC count is approximately 60% neutrophils and 40% mononuclear cells. As much as 70% neutrophils (%N) is considered normal in equine peritoneal fluid. In acute inflammatory processes (i.e., peritonitis) the %N in peritoneal fluid may increase to 85% to 100%.
Assessment of cell morphology through cytologic examination is an extremely important part of any peritoneal fluid analysis. The morphologic characteristics of the cell types can be used to differentiate between septic and nonseptic inflammation. Nondegenerate neutrophils predominate in transudates and mild exudates. Degenerate neutrophils are characterized by nuclear pyknosis, karyorrhexis, karyolysis, and cytoplasmic vacuolization. A large number of degenerate neutrophils indicates bacterial toxin-induced cell disruption, and they predominate in septic effusions, and a guarded-to-grave prognosis is warranted. Detection of phagocytosed bacteria confirms the presence of a septic process.