Pfizer Animal Health is reminding horse owners of the importance of treating encysted small redworm larvae this autumn.

According to the company, these hidden, dormant parasites can account for up to 90% of the redworm burden in a horse (1). Furthermore, encysted small redworm larvae won't show up in a faecal egg count (FEC). Even if a horse has shown a negative or low count it could still be harbouring several million harmful larvae, hidden within the gut wall (2)

Encysted small redworm are a potential time bomb, with the ability to survive inside a horse for up to two years. They usually 'wake-up' in the late winter or early spring, developing and emerging from the gut wall all at the same time. Such a sudden mass emergence can cause a disease syndrome known as 'larval cyathostominosis', causing diarrhoea and colic with up to a 50% mortality rate (2). Treating encysted small redworm successfully in the late autumn or early winter is vitally important in order to prevent this serious risk.

Pfizer says that Moxidectin, the key ingredient of EQUEST, is recognised as the only single dose treatment for encysted small redworm. It has been shown to kill the larvae in-situ, without resulting in severe inflammation of the gut wall that other multi-dose treatments may cause (3). In addition, Moxidectin is licensed for persistent activity against small redworm over two weeks, killing larvae ingested as the horse grazes for up to two weeks after treatment.

Ben Gaskell, Pfizer's veterinary advisor said: "A responsible and sustainable worming programme involves good pasture management, regular diagnostic tests and targeted worming, carefully planned using specialist advice. However, it is also vital to remember the danger of encysted small redworm. Even if a horse's FEC is clear, there could still be a significant encysted small redworm burden that must be treated properly every year in late autumn / early winter."

1) Bairden K. et al (2001) Veterinary Record 148, 138-141
2) Dowdall S.M.J. et al (2002) Veterinary Parasitology 106, 225‑242
3) Steinbach T. et al (2006) Veterinary Parasitology 139, 115‑131

PS: Whilst you're here, take a moment to see our latest job opportunities for vets.