Worms: a heavy burden

This horse has encysted small redworms in its gut

With the help of The Horse Trust, Horse Deals takes a look at drug-resistant redworms

Whether your best friend’s a top performer or companion, worming is something every owner has to consider. But due to the growing problem of drug-resistance, people are now advised to take a more informed and strategic approach to worming. Small redworms (cyathostomins) are the most important parasite affecting horses. However, overuse of wormers has led to the development of drug-resistant small redworms. Researchers have found evidence of small redworm resistance to each of the three major drug types and the first incidence of multi-drug resistance was recently discovered in Brazil.

If a horse is infected with multi-drug resistant redworms, it may suffer a severe infection, including weight loss, colic, diarrhoea and death. “Many owners simply buy wormers and give them to their horse regularly. However, many horses have a low worm burden, so don’t need to be treated as often. Owners could reduce the risk of drug resistance and save money by having faecal egg count tests carried out,” says Paul Jepson, chief executive and veterinary director of The Horse Trust.

By faecal egg counts, he doesn’t mean owners should look at their horse’s droppings and try to count how many worms they contain! Instead, a small sample — approximately enough to fill a small yoghurt pot — is sent to a company such as Westgate, who will test it in their laboratory.

Due to its management regime, The Horse Trust now has very few problems with worms

It’s recommended that you start a worm count programme when worming is due for most horses on the yard. When the results are returned, only those with a high count need wormers. The test is then repeated at three-monthly intervals. If records are kept of each horse’s counts, you will see a pattern emerge.

At The Horse Trust, horses are usually wormed in March and November only. In between these months, a faecal egg count is carried out for each horse. Out of around 100 horses at the sanctuary, very few need additional worming treatments. Staff at the sanctuary also actively manage the land to help reduce the chance of worms being passed on between horses.

“When horses are moved between fields, we poo pick the field they’ve left and rest it for a few weeks to make sure it’s clean when the horses go back in,” says Shirley Abbott, yard manager at The Horse Trust. “Because of this ongoing management, the faecal egg counts are usually low.”

Recent Horse Trust-funded research led by Professor Jacqui Matthews of the Moredun Research Institute/University of Edinburgh found that UK owners were worming horses too frequently and unnecessarily.

“The results are worrying because people still aren’t taking the issue of drug resistance seriously,” says Prof Matthews. “Resistance to worming drugs is a genuine and growing problem and I’d advise people to take practical steps to avoid it happening.”

Owners could reduce the risk of drug resistance and save money by having faecal egg count tests carried out – Paul Jepson

The life cycle of worms

The main way of testing for resistance is through a faecal egg count reduction test. However, Prof Matthews warns that this can give erroneous results if too few horses are tested. If possible, she recommends that a minimum of 10 horses be tested for a specific drug type or in yards where there are less horses, all animals are tested. “There was a case last summer where a yard had been reported as having ivermectin-resistant redworms. When we repeated the test with more horses, we found no evidence of resistance,” she says. “For a meaningful test, you need to include as many horses as possible.”

Prof Matthews’s group is in the early stages of developing a test to quickly screen for horses infected with drug-resistant small redworms. Currently, she and Dr Jane Hodgkinson of the University of Liverpool are investigating whether particular species of small redworm are associated with resistance to drugs such as ivermectin and moxidectin.

“We’re now able to distinguish seven different species of small redworm as egg or larval stages in dung or on pasture. If we find a drug-resistance link with a particular species we can use this information when designing future control strategies,” she explains. “Any basic knowledge we can glean on small redworms and how they develop drug resistance is invaluable. As they say, you need to ‘know your enemy’ and where equine welfare’s concerned, cyathostomins are among the leading enemies.”