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Ectoparasites - Gastrophilus

Gastrophilus intestinalis:Common horse bot.
G. nasalis: Throat botfly

General Description: Adults are brown, hairy, and bee-like, about 18 mm. long, with one pair of wings. The reddish mature larva (bot) is 2 cm. long. The narrow hooked anterior end tapers from a broad, rounded body. There is a single row of spines per segment on Gastrophilus nasalis larvae, and two rows per segment on G. intestinalis.

Life Cycle: Larvae over winter attached to the mucosa of the stomach of the host and can be so numerous that the stomach tissue cannot be seen. In late winter or early spring they release their hold and are passed in the faeces. The larvae burrow into the ground and pupate. The adults emerge in 3 to 10 weeks, depending on temperature. Egg laying begins in early summer. The female flies dart onto the horse and cement eggs singly onto individual hairs. Eggs of the three major species differ in color and placement. G. intestinalis lays pale yellow eggs on the forelegs and shoulders, up to 1000 per female fly. Moisture and friction from the horse licking itself cause the eggs to hatch in about 7 days. G. nasalis lays about 500 yellow eggs around the chin and throat, which hatch in a week without stimulation. The female G. haemorrhoidalis (not reported in NZ) lays 150 black eggs around the lips of the horse, which hatch in 2 to 3 days. After hatching, G. intestinalis larvae are licked into the host's mouth, but G. nasalis and G. haemorrhoidalis larvae burrow under the skin to the mouth. After a month wandering in the mucosa of the tongue and cheeks, the larvae of all 3 species migrate to the stomach, where they overwinter.

Location: Stomach.

Geographical Distribution: G. intestinalis and G. nasalis are common in most parts of the world where horses are raised. G. haemorrhoidalis is uncommon.

Significance: Moderate. The annoyance from adult flies may be of more significance than the presence of bot larvae in the stomach. However, if the larvae are very numerous they may block the opening to the intestine.

Effect on Host: Gastric myiasis; bot larvae attach to stomach mucosa. Migration by larvae under the skin and mucous membranes causes linear lesions open to infection. Attachment to the gut can cause haemorrhage, ulceration, and intestinal blockage if uncommonly large numbers are present. There are virtually no clinically apparent effects with low levels of infection. Deposition of eggs by adult flies causes nervousness. Horses may stand with their chins and lips against the flanks of other animals for protection from the "dive-bombing" flies.

Diagnostic Information: Eggs may be identified by color and site, and larvae by the arrangement of the spines.

Control: Removal of eggs by frequent grooming and providing shelter from Gastrophilus flies will help control these parasites. Regular treatment with ivermectin is necessary to provide good control of the bot fly.

Gastrophilus– adult fly   Gastrophilus– egg attached to hair
ophilus– larvae attached to stomach lining   Horse leg with bot eggs

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