The Disease
Parasite resistance is an emerging and significant threat to the health and welfare of horses. This chapter provides guidelines for managing parasite resistance in equine populations, particularly focusing on controlling gastrointestinal parasites such as strongyles, ascarids, and cyathostomins.
Parasite resistance refers to the ability of gastrointestinal parasites to survive and reproduce despite the administration of anthelmintic drugs (wormers). It is part of the overarching syndrome of antimicrobial resistance, that therefore should be considered a disease in its own right.
Parasite resistance is an emerging problem and may affect all age groups of horses. Morbidity (the occurrence of disease) is most frequent in young animals at pasture, however all horses play an important role in the parasite lifecycle and the impact of significant worm burdens can impact on the welfare of all horses.
The primary parasites of emerging resistance are
1. Parascaris spp (Ascarids) causing respiratory disease in young foals and intestinal disease (colic and diarrhoea)
2. Cyathastomins (Small ‘redworms’) causing severe life-threatening diarrhoea and potentially leading to a failure to thrive in older horses
3. Anoplocephala spp (Tapeworm) causing colic, both mild and lifethreatening conditions
In addition, there is significant concern about the re-emergence of large strongyle species, especially Strongylus vulgaris, that can disrupt the blood supply to the intestine.
Other parasites of lower pathogenicity play an important role in responsible parasite control programs, and may lead to overuse of anti-parasitics to impact on parasites of low pathogenicity (Oxyuris equi, Stonglyoides westerii).
Importance
A failure to act to limit parasite resistance will lead to irreversible impacts on animal health and welfare
At an individual premises basis, it will render pastures, or potentially an entire site unsuitable for equestrian use.
Parasite resistance is of highest risk where large populations of youngstock are maintained together. Therefore it is of particular importance to breeding and rearing units.
