The larvae of certain dipterans are capable of developing in the tissues of many domestic animals. This results in a condition called myiasis. There are two types of myiases: (1) facultative myiasis — larvae are free-living, but can become parasitic under certain conditions; and (2) obligatory myiasis — larvae are always parasitic, i.e., without a proper host, the flies cannot complete their life cycle.
• Complex metamorphosis with egg, larval stages (instars), pupa, and adults.
• Separate sexes with adult females laying eggs or larvae on host or in environment.
• Larvae hatch from eggs; three larval stages (maggots).
• Third larval stage pupates with adults emerging from pupae.
Obligatory Myiasis-Producing Flies
Adults of the obligatory myiasis-producing flies tend to resemble honeybees. They have only vestigial mouthparts and, therefore, do not feed. They are quite annoying to their hosts as the females buzz around the animal, laying her eggs. These flies are extremely host- and site-specific. Because they are so specific, the third-stage larvae can be provisionally identified to genus based on host and site alone. However, first- and second-stage larvae must be differentiated from larvae of the facultative myiasis-producing flies, particularly if in an abnormal host.
OESTRUS OVIS (SHEEP NASAL BOT)
• Females fly around nostrils of sheep and goats during the hottest part of the day; deposit tiny, white to yellow, first-stage larvae; crawl into nasal sinuses, and develop into large (3 cm), dark brown third-stage larvae.
• Third-stage larvae crawl out of the nostrils or are sneezed out; pupate in the ground; adults emerge 3-6 weeks later; if begin pupating in fall, pupate overwinter and adults emerge in spring; larvae can also overwinter in nares of host.
• Larvae produce a purulent rhinitis or sinusitis leading to head shaking, restlessness, snorting; may lead to damage of cribriform plate and subsequent brain injury.
• Diagnosis based on seeing large, dark brown larvae dropping out of nostrils; postmortem diagnosis achieved by sawing skull in half longitudinally, rinsing key areas with water, and examining the rinsings for larvae with a magnifying lens.
• Ocular myiasis of humans has been reported.
GASTEROPHILUS (OR GASTROPHILUS) SPP. (HORSE STOMACH BOT)
• Three species in horses — Gasterophilus nasalis, Gasterophilus hemorrhoidalis, Gasterophilus intestinalis.
• Adult females attach elongate, operculated eggs to hairs of the intermandibular space (Gasterophilus nasalis), the lips (Gasterophilus hemorrhoidalis), or forelegs and shoulders (Gasterophilus intestinalis) during late summer and early fall.
• Eggs around the mouth hatch spontaneously; those elsewhere hatch in response to sudden warmth provided by the breath of the horse.
• Larvae penetrate and migrate in oral mucosa and tongue; eventually reach the stomach or duodenum and attach to the wall.
• Third-stage larvae pass out in feces in spring; pupate in the soil for 3-9 weeks.
• Generally considered benign parasites except for the annoyance associated with the adults; mucosal and submucosal inflammation and mucosal ulceration of the duodenum has been associated with infections of Gasterophilus nasalis in ponies.
• Diagnosis is based on seeing eggs attached to hairs or distinctive third-stage larvae in feces; one can also find larvae at necropsy; usual attachment sites are the first ampulla of the duodenum for Gasterophilus nasalis, the nonglandular part of the stomach at the margo plicatus or in the saccus cecus for Gasterophilus intestinalis, the duodenum and rectum for Gasterophilus hemorrhoidalis.
HYPODERMA SPP. (CATTLE GRUBS; HEEL FLIES)
• Two species infesting cattle and bison — Hypoderma bovis (northern cattle grub), Hypoderma lineatum (common or southern cattle grub).
• Hypoderma lineatum is present in the southern United States; both species are present in the northern United States and into Canada.
• Entire life cycle of both species takes about 1 year to complete.
• Adult Hypoderma lineatum becomes active with the start of warm weather, remaining active for about 2 months; Hypoderma bovis becomes active about the time Hypoderma lineatum stops, remaining active into summer.
• Larvae hatch spontaneously, crawl down hair shaft, penetrate skin, and migrate through the subcutaneous tissues.
• Larvae, in 4 — 5 months, come to rest in either the submucosal connective tissue of the esophagus (Hypoderma lineatum) or the epidural fat (Hypoderma bovis); remain there for about 3 months.
• Resume migration to subcutaneous tissues of the back, cut breathing holes and increase in size as they develop to the third-stage larvae.
• When fully developed, larvae exit through breathing holes, fall to ground, and pupate with adult flies emerging in 4-5 weeks.
• Egg-laying activity disturbs animals; they run about aimlessly (gadding) in an attempt to escape the flies; results in loss of production.
• Larval infestation leads to carcass damage and damage to hide from the breathing holes; if animals are treated when larvae are in resting sites (esophagus, spinal canal), signs of bloat/choke or central nervous system disease can result.
• Diagnosis is made by finding either the eggs on hairs of the legs or the larvae in the back.
• Infestations of horses and humans have been reported, although rare.
COCHLIOMYIA HOMINOVORAX (AMERICAN PRIMARY SCREWWORM)
• Only fly in North America attracted to uncontaminated skin wounds of domestic animals; will infest any living warm-blooded animal (including humans) with a wound.
• Adult females lay many eggs in batches of 15-400 at the edge of wounds.
• Larvae hatch spontaneously and enter wound feeding on secretions and living flesh; become third-stage larvae in 5-7 days.
• Larvae drop to ground, burrow in soil and pupate; adults emerge in one to several weeks.
• Life cycle can be completed in as little as 24 days; larvae cannot overwinter where soil freezes.
• Fatal if not treated; can kill a full-grown steer in 5-7 days.
• Massive eradication efforts used insecticidal treatment of all infested animals and release of sterile flies to eliminate this parasite from the United States and Mexico. Because females mate once and the wild population of the fly is relatively small, release of billions of sterile males swamps the population and significantly reduces the chance of a successful mating.
• Occasionally reenters the United States in imported animals; larvae encountered in wounds (particularly of imported animals or animals in border areas with Mexico) must be differentiated from facultative myiasis-producing flies. If encountered, it must be reported to state and federal authorities.
• Primarily parasites of rabbits and rodents; will infest dogs and cats; although rare, most frequent cause of endemic human myiasis in North America.
• Adult females lay eggs near entrances to burrows or along rabbit runs.
• Larvae hatch in response to presence of animal, crawl into fur, and enter subcutaneous tissues of host through natural body openings.
• Cut breathing holes and develop to large (up to 3 cm), black third-stage larvae in subcutaneous cysts.
• In dogs and cats, generally found in neck and head region in late summer and early fall; also found in aberrant sites including anterior chamber of the eye and the brain.
• Generally benign unless secondary bacterial infection of cyst occurs or larvae migrate to aberrant sites.
• Diagnosis is based on finding characteristic cysts with breathing holes in which second- or third-stage larvae are usually present.
Facultative Myiasis-Producing Flies
GENERA OR SPECIES INVOLVED
• Lucilia (green or copper bottle flies)
• Phoenicia (green bottle flies)
• Phormia (black blow flies)
• Calliphora (blue bottle flies)
• Sarcophaga (flesh flies)
• Cochliomyia macellaria (secondary screwworm)
• Normally, adults lay eggs in carrion or feces; also attracted by suppurative wounds, necrotic areas, skin soiled with urine, feces, or vomitus (bacterial growth generates odors attractive to flies); condition called fly strike or strike.
• Females feed and lay eggs; larvae hatch; feed on necrotic debris and exudates.
• Larvae can cause further damage; some may invade healthy subcutaneous tissue producing large cavities or tunnels; host becomes anorexic and weak.
• Infestation can lead to death as a result of septicemia, toxemia, or shock.
Diagnosis of maggot infestation is not difficult because the larvae are easily observed in the wound or within the hair coat. Species diagnosis, however, is dependent on morphologic characteristics of the larvae, particularly of the spiracular plates on the posterior end of the larvae. Depending on the situation, larvae may need to be differentiated from those of Cochliomyia hominivorax. In that case, collect larvae, preserve in 70% ethanol, and submit to proper authorities for identification.
Treatment and Control
For bots of sheep, horses, and grubs of cattle, avermectins are effective. Treatment of cattle should be done immediately after fly season ends but before the larvae reach the esophagus or spinal cord; destruction of larvae in these tissues causes severe inflammatory reactions and clinical signs corresponding to the locality of the larvae. The danger period for treatment is estimated to be 6-8 weeks before the larvae appear in the back, which occurs around mid-September in the southern United States, late December in areas such as Ohio, and late January in the more northern areas of the United States. Although adverse reactions are considered to be rare occurrences these days, they must still be kept in mind in designing parasite control programs for cattle.
Infestations of Cuterebra hominivorax are reportable; should it occur, treatment and control requirements will be outlined at that time.
Treatment of Cuterebra consists of manual extraction of the larva.
Treatment of facultative myiasis infestations includes debriding area, applying appropriate insecticides, and, if present, treating secondary bacterial infections.