Field observations of the host-parasite relationship associated with the common horse bot fly, Gasterophilus intestinalis.

Behavior of Gasterophilus intestinalis (Diptera: Gasterophilidae) was observed in the laboratory and field over a period of 10 years. Mating occurred in a frenzied manner as flies emerged from pupae. Males attempted to copulate with females that had not fully emerged from the pupal case. Mating was never observed in the field or near the horse. Observations suggest that fecal piles of the horse are used as the mating site for the newly hatched flies. Activation of host-seeking behavior was found to occur early and remain throughout the life span of the adult as it sought multiple hosts for oviposition. Evidence is presented supporting the use of olfaction and vision in finding the host. Interest by flies in shadows and linear high-contrast objects was observed. Methods used by the flies to maintain close proximity with the host are discussed including flight with the host and avoidance of host defenses. Recapture methods were successful in determining the time taken to reestablish at the host, and extent of travel to the host. The presence of kairomone(s) within horse secretions is suspected.

Use of detomidine hydrochloride as an adjunct for studying first-stage Gasterophilus intestinalis (Diptera: Gasterophilidae) in the tongue of the horse.

A synthetic alpha-2 adrenergic agonist, detomidine hydrochloride, was used in the study of in vivo activity of Gasterophilus intestinalis (Diptera: Gasterophilidae) during migration in the tongue of the horse. Use of the drug allowed the investigator to manipulate the tongue and closely observe the movement patterns and tissue disturbance caused by burrowing first-stage larvae. Detomidine hydrochloride should be utilized in studies of drug efficacy and larval biology, whenever possible, to avoid the need to sacrifice the horse.

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.