2.3. Anoplocephala perfoliata (Tapeworms) Necropsy surveys performed in Kentucky reported Anoplocephala perfoliata prevalence of approximately 50% both prior to and after the introduction of widespread use of cestocidal drugs in horses (Lyons et al., 1984; 2000). The prevalence in other parts of the U.S. is unknown; however, tapeworms must be considered common in grazing horses.
Several studies have documented an association between the presence of tapeworms and colic originating from the ileocecal region (Nielsen, 2016a). Tapeworms produce small mucosal erosions at the site of attachment, and infection has been associated with ileal impactions, ileocecal intussusceptions, and spasmodic colic (Nielsen, 2016a). However, most horses infected with tapeworms tend to have relatively few worms, and these likely produce little in the way of pathogenic consequences.
In arid regions or in horses that are not kept on green pastures, tapeworms are unlikely to require primary attention in parasite control programs.
2.4. Parascaris spp. (Roundworms; Ascarids) This parasite is common in breeding operations worldwide, and it is the most important helminth pathogen in foals. Migrating larvae can cause signs of airway inflammation, including cough and nasal discharge (Clayton, 1986), although this appears to be rare. The most clinically important manifestation of Parascaris spp. infection in horses is a verminous small intestinal impaction, which occurs in a small subset of infected foals. This condition is associated with a guarded prognosis for survival, and it can be further complicated by intestinal volvulus, intussusception, and rupture, as well as post-operative adhesions in foals that required surgical intervention (Nielsen, 2016b).
Current evidence suggests that deworming a heavily parasitized foal with an efficacious anthelmintic can trigger an impaction of dead or dying worms in the intestinal lumen (Nielsen, 2016b). Claims have been made that this may be primarily associated with anthelmintics with a paralytic mode of action (such as ivermectin and pyrantel), and it remains possible that benzimidazole-type drugs may represent a safer treatment choice for Parascaris spp. infections due to their non-paralytic mode of action and slower onset of effect. However, these claims are not supported by strong evidence.
Parascaris spp. infections may occasionally be diagnosed in apparently immunocompetent adult horses, but clinical disease is an extremely rare event in this age group.
2.5. Strongyloides westeri (Threadworms) Strongyloides westeri infection is common in young foals (Lyons and Tolliver, 2014), but this parasite is rarely associated with disease. Further, it is generally not considered part of the panel of pathogens known to commonly cause diarrhea in this age group. Larvae of this parasite are known to transmit lactogenically from mares to foals, but it should be kept in mind that it can also infect through fecal/oral and percutaneous routes. In addition, S. westeri is a facultative parasite, which can maintain a full life-cycle in the environment without entering a host. Thus, focusing solely on interrupting one route of transmission may not be successful.
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