Worming your horse

Professor Jo-Anne Murray
PhD, MSc, PgDip, BSc (Hons), BHSII, RNutr, PFHEA
University of Glasgow Veterinary School

Having a worming programme in place is an essential component of managing any horse. Parasitic worms can have a detrimental effect on your horse’s health and well-being, including poor body condition, colic and general ill health. These worms are parasitic as they live off or in the horse without any benefit to the horse in return. These internal parasites can cause serious damage to the horse’s intestines and other internal organs, with potentially fatal consequences. Therefore, it is important to monitor worm burden in horses, implement an appropriate worming programme and ensure good pasture management.

Which worms infect horses?

There are many types of worms that use the horse as a host, including:

  • Small redworm (Cyathostomes)
  • Large redworm (Strongyles)
  • Large roundworms (Ascarids)
  • Pinworm
  • Tapeworm
  • Bots

Small redworms (Cyathostomes)

Small redworms are the most common intestinal worm in horses and account for approximately 90 percent of the worm burden in horses. Adult worms are small (1-2 cm long) and live in the large intestine of the horse. The adult worms lay eggs that pass to the pasture when faeces are voided. The larvae (immature worms) then hatch from the eggs on to the grass for the horse to eat when grazing. Once ingested these larvae travel to the large intestine and burrow into the gut wall. Once in the gut wall they can mature or hibernate, and this hibernation can last for weeks or years.

Larvae that are burrowed in the gut wall, also known as encysted larvae, are difficult to treat with worming drugs, not all types of wormers will treat these. There is also no way of detecting how many encysted larvae are present in the gut wall as only adult worms produce eggs that can then be detected in the faeces. Therefore, it is recommended to give your horse a wormer that treats encysted larvae one per year, usually in the winter as eggs and larvae on the pasture cannot continue to develop to the infective stages in cold temperatures thus limiting further contamination.

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