Inter-relationship between Gasterophilus larvae and the horse's gastric and duodenal wall with special reference to penetration.

The degree of penetration into the stomach and duodenum of the horse by bot fly larvae, Gasterophilus intestinalis (De Geer) and G. nasalis (Linnaeus) (Diptera : Gasterophilidae) was evaluated. Evidence of larval perforation of the stomach or duodenum was not found on gross inspection. Palpation of the intact stomach and duodenum was not effective in establishing the existence of Gasterophilus larvae within the organs. Findings suggest that larvae of both species produce an ulcer of similar depth within the gastrointestinal wall. The ulcer depth produced by larvae did not correlate with the normal, unaffected thickness of the particular stomach or duodenum. Tissue proliferation beneath ulcers of the stomach and duodenum was not correlated with the depth of the ulcer. Proliferation of the tissue beneath the ulcers of the stomach generally exceeded that found under duodenal ulcers. Gastric wall beneath the G. intestinalis ulcer frequently attained a thickness equal to or greater than the normal stomach wall. Histopathological examinations below the ulcer revealed intense fibrosis. Duodenal thickness below the G. nasalis ulcer was typically less than normal and resulted in an attenuated wall. Histopathological analysis of the affected duodenum revealed severe loss of submucosal glands in a sharply demarcated area below and surrounding the lesion. Fibrosis of the underlying lamina propria mucosae and tunica submucosa was appreciable but failed to restore the original thickness of the duodenal wall. Host tissue response and moderation of the parasite' s behavior reduce the chances of direct perforation of the gastrointestinal tissue.