Gastrophilus Equi

GASTROPHILUS EQUI, the horse botfly, an insect of the family Tachinidae (this family now includes the former family Oestridae), of the order Diptera. G. equi lays eggs and glues them to the hair of a horse (see figure 1). The larva developing in the egg emerges from the egg shell and bores into the epidermis, where it creates tunnels. Under the influence of itching, the horse scratches the skin with its teeth and swallows the botfly larvae from the tunnels opened by scratching, as well as larvae emerging from the eggs. In the stomach, the larva molts twice and reaches 1.5–2 cm. Its body is broad, covered with small spines (see figure 2); at the head end, there are two powerful hooks with which the larva attaches itself to the stomach wall, where it leaves a mark in the form of a funnel-shaped depression. In the vicinity of the resulting ulcer, papillomatous growths may appear, sometimes taking on a carcinomatous character. Up to 1,000 larvae may be found in a horse's stomach (see figure 3). Subsequently, the larvae detach from the stomach wall, pass with its contents into the intestine, and are expelled. In the ground, the larva pupates. A winged insect emerges from the resulting barrel-shaped pupa. The larvae of G. equi contain a poison that acts strongly on horses. Upon intravenous injection of extracts, horses die. Larvae taken from horses suffering from pernicious anemia possess even greater toxicity. K. and R. Seyderhelm attribute a causal role to G. equi larvae in the etiology of the aforementioned disease, which, however, is disputed by other authors. Van Es and Schalk observed severe intoxication upon injection of extracts from Gastrophilus equi larvae only in those horses that were infected with these parasites. In such

Figure 1.

a case, there is more evidence to speak of a manifestation of anaphylaxis. There are cases of abnormal localization of G. equi larvae; for example, when found in the esophagus of a horse, paralysis of the larynx and esophagus was observed, which depends on the toxic action of the parasite. Adult larvae can be guest-parasites (see), for example, in the stomach of a dog and even a bird upon accidental ingestion by these hosts of parasites that have fallen out of the horse's intestine. There is only one old report from Siberia regarding the finding of a G. equi larva in the human stomach. However, there are many known cases where phase I larvae of G. equi are guest-parasites in human skin. According to the research of E. Pavlovsky and A. Stein, the Gastrophilus equi larva creates a tunnel for itself in the thickness of the epidermis, specifically in its Malpighian layer. On the surface of the skin, the tunnel made by the larva is visible in the form of a reddish or bluish-crimson raised stripe, which appears on fresh areas of skin and gradually

Figure 3. Part of the mucous membrane of a horse's stomach with G. intestinalis larvae attached to it (after Kholodkovsky). GASTROCIRROSIS disappears in old areas after peeling. Subjectively, itching and partly burning are felt. The larva's tunnel can be quite tortuous; it advances 7–8 centimeters per day. The larva itself can be extracted if the epidermis is opened with a needle right in front of the fresh end of the stripe on the skin. The larva is white, covered with numerous small spines, and has good armament at the head end, also in the form of spines and hooks. The G. equi larva can penetrate human skin in two ways: 1) upon contact of sweaty skin with horse hair on which there are G. equi eggs with developed larvae; the larva emerges from the egg and bores into the human epidermis; 2) upon the gluing of G. equi eggs directly to the hairs of human skin; the larva, upon emerging, immediately bores into the epidermis. When investigating cases of guest-parasitism of G. equi larvae, it is necessary to look for G. equi eggs on the hairs at the old beginning of the tunnel. Besides G. equi, other species are known—G. haemorrhoidalis, G. veterinus, and others (see also Larva migrans and Horsehair worm).