2.6. Oxyuris equi (Pinworms) Clinical disease from pinworms historically was seen mostly in young horses. However, cases in adult horses appear to be becoming more common (Reinemeyer and Nielsen, 2014). Pinworm infections tend to be sporadic, and usually a subset of horses are affected out of a group. Clinical signs vary in intensity, but in severe cases, intense tail rubbing and hindquarter and/or perineal skin excoriations are seen. Horses may have patent pinworm infections without showing any specific clinical signs. As a consequence of the rubbing, horses can spread pinworm eggs throughout their environment, and transmission can potentially occur in many locations including stalls and from contact with grooming materials, tail wraps, fence posts, etc.
2.7. Gasterophilus spp. (Bots) Botfly larval infections are common in horses, and two species are primarily encountered in North America: Gasterophilus nasalis and G. intestinalis. Of these, G. intestinalis is by far the most common. Botfly larvae are generally mild pathogens and are rarely associated with clinical disease. However, they are a common incidental finding in the stomach during gastroscopic examination, causing unwarranted horse owner concern. The oral stages of infection have been associated with periodontitis with salivation, head shaking, lingual irritation, and chewing problems (Lind et al., 2012).
2.8. Habronema and Draschia spp. (Stomach Worms) Habronema and Draschia spp. are vector-borne nematodes, whose adult stages reside in the stomach, but they are rarely associated with clinical disease. Cutaneous habronemosis (summer sores) is a well-described manifestation of larvae being aberrantly deposited into wounds or on mucosal membranes by their fly vectors. This causes persistent granulomatous lesions that are often challenging to treat. In some arid regions of the U.S., particularly the Southwest, cutaneous habronemosis may be more common than clinical conditions caused by tapeworms or strongyles.
2.9. Onchocerca cervicalis (Neck Threadworm) The Neck Threadworm has gained increasing attention in recent years, although very few studies have focused on this parasite. Studies performed in the 1980s and 1990s demonstrated O. cervicalis prevalence in the 20-80% range in U.S. horses (Klei et al., 1984; Cummings and James, 1985; Lyons et al., 2000), but more recent surveys are lacking. Adult worms are considered refractory to anthelmintic treatment (Lyons et al., 1988), and females appear to remain viable for up to 20 years, which can explain why prevalence has been demonstrated to increase with horse age (Lyons et al., 1986).
Microfilariae of O. cervicalis may cause dermatitis of the ventral midline and/or limbs, shoulders, thorax, and withers, which responds positively to ivermectin treatment (French et al., 1988). As opposed to the adult stages, the microfilariae have been demonstrated to be susceptible to both ivermectin and moxidectin treatment (French et al., 1988; Monahan et al., 1995).
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