1. Goals of Parasite Control
The true goal of parasite control in horses is to keep animals healthy and reduce the risk of clinical illness. The goal is NOT to eradicate all parasites from a particular individual. Not only is eradication impossible, the inevitable result of pursuing this goal is accelerated development of parasite drug resistance.
The goals of any parasite control program can be summarized as follows: • To minimize the risk of parasitic disease.
• To delay further development of anthelmintic resistance and maintain efficacious drugs for as long as possible.
2. Introduction
2.1. Cyathostomins (Small Strongyles) Equine strongyles represent the most commonly encountered internal parasite group in horses worldwide. Cyathostomins (small strongyles) are truly ubiquitous in grazing horses, but they are rarely associated with disease. Acute Larval Cyathostominosis is a disease syndrome caused by mass synchronous excystment of cyathostomin larvae from the large intestinal walls leading to a generalized typhlocolitis. This disease is described with a 50% case-fatality rate in the United Kingdom (Reid et al., 1995, Lawson et al., 2023), but it is rarely encountered in the U.S. and does not appear to constitute a major concern.
In arid regions or in horses that are not kept on green pastures, strongyle parasites are unlikely to require primary attention in parasite control programs.
2.2. Large strongyles The group of large strongyles includes Strongylus vulgaris, which is considered the most pathogenic equine gastrointestinal helminth. Also known as the bloodworm, this parasite can cause thromboembolism and life-threatening peritonitis (Pihl et al., 2019). Decades of intensive anthelmintic treatment appear to have reduced the occurrence of this parasite to often undetectable levels within the U.S. (Nielsen et al., 2012a). However, in Scandinavian countries, where anthelmintic products are available on a prescription-only basis and treatment intensities are substantially lower than in the U.S., a re-emergence of this parasite has been reported (Nielsen et al., 2012b; Tydén et al., 2019) and infection has been associated with clinically significant disease (Nielsen et al., 2016; Hedberg-Alm et al., 2022).
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