Horse owners should be taking a more informed and strategic approach to parasite control in horses, according to The Horse Trust in Britain.
The trust believes many horses are being dosed unnecessarily and, with the growing problem of worm resistance, targeted use of deworming drugs is becoming essential.
The trust says most of the horses in its care receive only two chemical treatments a year for parasites. However, this strategy is dependent upon regular monitoring of eggs counts and active pasture management.
"Many horse owners simply buy drugs from veterinary supply stores and give their horse the drug regularly," says trust chief executive Paul Jepson, who is also its veterinary director.
"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," he says.
Small redworms, or cyathostomins, are the most important parasite affecting horses.
However, there is clear evidence of development of drug resistance, almost certainly due to overuse of worming drugs.
Researchers have found evidence of small redworm resistance to each of the three drug families, 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.
Horses in the trust's care 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 given to horses only if their egg count comes back high.
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," explains yard manager Shirley Abbott.
"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 and University of Edinburgh, found that British 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," Matthews says. "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 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.
 Cyathostomin eggs. |
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," Matthews says.
"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.
She and her collaborator, Dr Jane Hodgkinson, at the University of Liverpool, are investigating whether particular species of small redworm are associated with 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," Matthews says.
"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."
The Horse Trust, founded in 1886, is the oldest horse charity in Britain. It operates a home for around 100 retired working equines at Speen, in Buckinghamshire, and runs a programme centred around welfare, science and education.
It is the largest provider of grant funding for equine welfare in Britain.
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