Sustainable control of cyathostomin infections in practice

Summary Cyathostomins are the most prevalent helminths in horses and are found in nearly all grazing groups. These parasites have been shown to exhibit widespread anthelmintic resistance and can cause clinical disease, so they are a growing concern. The emergence of large numbers of larval stages from the large intestinal wall can cause larval cyathostominosis, a severe colitis that has a case fatality rate of around 50% in referral hospitals. Effective control of cyathostomins requires a combination of excellent paddock management, testing to identify horses that require treatment, and strategic treatments of high‐risk individuals where testing does not provide useful information. Faecal egg count (FEC) tests are valuable tools for reducing anthelmintic treatments. These tests assess strongyle egg levels in dung, and because many horses have low egg shedding, testing can substantially reduce the amount of treatments administered by identifying such low egg‐shedders. However, egg shedding does not correlate with cyathostomin counts in individuals, meaning that some horses may exhibit negative/low FEC results despite having substantial worm burdens, particularly of encysted larval stages. To mitigate the risk of harbouring pathogenic encysted larval burdens, it was previously recommended that horses in northern temperate regions receive an annual larvicidal treatment in late autumn or winter. This blanket approach to treatment likely increased selection for anthelmintic resistance. To avoid further selection for resistance in these regions, cyathostomin treatments in autumn/winter should be guided by risk assessment. High‐risk horses should receive a cyathostomin larvicidal treatment, while low‐risk horses may not require treatment. For those concerned about withholding anthelmintic treatment, a cyathostomin‐specific ELISA (Small Redworm Blood Test, Austin Davis Biologics) can confirm if a horse has a low burden that does not necessitate treatment. This review provides a background to cyathostomins and their control, together with considerations for integrating this ELISA into sustainable control approaches to manage these important infections of horses.