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Gasterophilus spp in Horses

By Thomas R. Klei, PhD, School of Veterinary Medicine and Louisiana Agriculture Experiment Station, Louisiana State University

Horse bots, which are found in the stomach, are the larvae of bot flies, Gasterophilus spp. Three major species are distributed worldwide, and a number of minor species are found in parts of Europe, Africa, and Asia. The adult flies are not parasitic and cannot feed; they survive long enough to mate and lay eggs and die as soon as the nutrients remaining from the larval stage are used, usually in ~2 wk. The three important species can be differentiated in any stage of their development. The eggs of G intestinalis (the common bot) are glued to the hairs of almost any part of the body but especially the forelimbs and shoulders. The larvae hatch in ~1 wk when stimulated, usually by the animals’ licking. The eggs of G haemorrhoidalis (the nose or lip bot) are attached to the hairs of the lips. The larvae emerge in 2–3 days without stimulation and crawl into the mouth. G nasalis (the throat bot) deposits eggs on the hairs of the submaxillary region. They hatch in ~1 wk without stimulation.

The larvae of all three species apparently stay embedded in the tongue or the mucosa of the mouth for ~1 mo, after which they pass to the stomach, where they attach themselves to the cardiac or pyloric portions and, in the case of G nasalis, to the mucosa of the first part of the small intestine. After development for ~8–10 mo, they pass out in the feces and pupate in the soil for 3–5 wk, after which the adult emerges. The main pathogenic effect is caused by larvae, which attach by oral hooks to the lining of the stomach. This induces erosions and ulcerations at the site of attachment and a hyperplastic reaction around it. However, oral stages may cause sinus tracts in which mucopurulent discharges form, especially along the lingual border of the upper, more posterior cheek teeth.

Clinical Findings and Diagnosis:

Bots cause a mild gastritis, but large numbers may be present with no clinical signs. The first instars migrating in the mouth can cause stomatitis and may produce pain on eating. The adult flies may annoy horses when they lay their eggs. Specific diagnosis of Gasterophilus infection is difficult and can be made by demonstrating larvae as they pass in the feces. In the USA, the presence of gastric infections during the winter months is often assumed. History of the individual horses, knowledge of the local seasonal cycle of the fly, and observation of the yellow to cream-white bot eggs (1–2 mm) on the horse’s hairs all help identify the presence of the parasite in a given herd.


In temperate areas, it is assumed that most animals are infected by the end of summer. Ivermectin is effective against oral and gastric stages of bots and, when used as part of a routine parasite control program, provides effective bot control throughout the season. In subtropical or tropical areas, some transmission may occur throughout the year. Moxidectin is effective against gastric stages. Current recommendations for control include at least one treatment annually, at the end of the bot fly season. In some locations where the bot fly season is long, additional treatments may be necessary. Although there is no satisfactory method to protect exposed horses from attack by the adult flies, bot control programs, when applied on a regional basis to all horses, markedly reduce fly numbers and larval infections.