16/02/2010 Information for Worming Feature

Due to the growing problem of drug-resistant worms, people are advised to take a more informed and strategic approach when worming their horse.

Small redworms (cyathostomins) are the most important parasite affecting horses. But overuse of worming drugs has led to the development of drug-resistant small red worms. Researchers have found evidence of small redworm resistance to each of the three drug types, and the first incidence of multi-drug resistance was recently discovered in Brazil¹.

If a horse is infected with multi-drug resistant redworms, it may suffer the effects of a severe small redworm infection, which can include weight loss, colic, diarrhoea and death.

"Many horse owners simply buy drugs from veterinary supply stores and give their horse the drug 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 on worming drugs by doing Faecal Egg Count tests," says Paul Jepson, Chief Executive and Veterinary Director of The Horse Trust.

At The Horse Trust, horses are usually given worming treatments in March and November only. In between these months, a Faecal Egg Count (FEC) is carried out for each horse and additional wormers are only given to horses if their egg count comes back high. Out of around 100 horses at the sanctuary, very few need these additional worming treatments.

Staff at the sanctuary also actively manage the land to reduce the chance of worms being passed on between horses.

"When horses are moved between fields, we ‘poo pick’ the field they have left and rest the field for a few weeks to make sure the field is clean when the horses go back in," says Shirley Abbott, the Yard Manager at The Horse Trust. "Because of this ongoing management, we have very few problems with worms - the faecal egg counts of our horses usually come back very low."

In a recent study, Horse Trust-funded research, led by Professor Jacqui Matthews² of the Moredun Research Institute/University of Edinburgh, found that UK stud farm managers and other horse owners were worming their horses too frequently and unnecessarily³.

"The results of this survey are worrying - people still aren't taking the issue of drug resistance seriously," said Matthews. "Resistance to worming drugs is a genuine and growing problem in the UK and I would advise people to take practical steps to avoid it happening in their yard."

For stable managers who are worried about drug resistance, the main way of testing for this is to carry out a Faecal Egg Count Reduction Test (FECRT). However, Matthews warns that this test is relatively insensitive and 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 fewer horses, that all animals be tested. 

"There was a case last summer, where a yard in the south of England had been reported as having ivermectin -resistant redworms. But when we repeated the test with more horses, we found no evidence of drug resistance," said Matthews. "If you want to do a meaningful Faecal Egg Count Reduction test you need to include as many horses as possible."

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

"We are currently able to distinguish seven different species of small redworm as egg or larval stages that are found in dung or on pasture. If we find a drug-resistance link with a particular species then we can use this information when designing future control strategies. ”says Matthews.”  Any basic knowledge we can glean on small redworms and how they develop drug resistance is invaluable - as they say, to win you need to 'know your enemy and where equine welfare is concerned, cyathostomins are one of the top enemies"

¹ Molento et al - Anthelmintic resistant nematodes in Brazilian horses (Veterinary Record Issue 162, pg 384 - 385)

² Jacqui Matthews is the Moredun Professor of Immunobiology at the Royal (Dick) School of Veterinary Studies, University of Edinburgh, and The Moredun Research Institute.

³ Worming treatment only needs to be reapplied at the end of the Egg Reappearance Period, but many owners were applying medication more frequently.