Worm resistance a weighty issue in horses, survey finds

Using a tape to calculate a horse's weight.
Using a tape to calculate a horse’s weight. Here’s another method to try. © Zoetis

A British animal health company has warned of the dangers of under-dosing of deworming products in horses, with nearly half of all horse owners surveyed not weighing their horses before treatment.

Under-dosing can cause worms to develop resistance to the wormers used and this can have a serious impact on the management of worm burdens in the future.

The British Riding Clubs Horse Health survey commissioned by Zoetis last year, revealed that 44% of the horse owners asked do not weigh their horse before worming.

» How do you work out weight without scales?

The survey was completed online by 559 horse owners in the UK, during February 2014. It contained 21 questions on general horse health, care and management.

Knowing the current weight of the horses that you wish to treat before purchasing your wormers can help ensure that you buy the correct dose in each case.

Zoetis vet Wendy Talbot said that, ideally, horses should be weighed at least once a year using the accuracy of a weighbridge. “Several feed manufacturers offer a portable weighbridge service,” she said.

“Weigh tapes, although less accurate, are a great way to keep tabs on weight once you have the accuracy of the weighbridge weight to refer to and have factored in any discrepancies. Remember that many horses will change in weight throughout the year.”

Cindy weighed in 60kg more than expected.
Cindy weighed in 60kg more than expected.

The issue of weight was recently highlighted in some case studies from a private yard in Suffolk. Cindy is a 13.1hh Connemara mare in ‘summer’ condition. Her owner was very surprised when the weighbridge clocked her up to 380kg – a full 60kg more than was expected. Usually Cindy’s owner splits one wormer, designed for dosing a horse up to 600kg in weight, between two similarly sized ponies, which means she has been significantly under-dosing.

Pop weighed more than the maximum on the dewormer tube.
Pop weighed more than the maximum on the dewormer tube.

Pop is a 16.1hh Irish Sport Horse gelding. His owner admitted that he is carrying a few surplus pounds but was shocked when he tipped the scales at 638kg. She had recently wormed him for a weight of 600kg – the maximum weight of the worming syringe she had bought.

Warmblood gelding Dec weighed in at 674kg.
Warmblood gelding Dec weighed in at 674kg.

Dec is a 16.3hh warmblood. He is lean, lithe and competition fit but still weighs in at 674kg.

“Our case studies give a very real perspective on how easy it is to under-estimate the weight of our horses,” Talbot said.

Resistance to wormers is also a growing problem. When a parasite population previously controlled by a drug is no longer susceptible to that drug it is known as resistance. The active ingredient of the horse wormer kills the sensitive parasites in the population, but those parasites not affected go on to create new generations of resistant parasites.  Eventually, if the process continues, only resistant worms remain in the population.

Frequent dosing or under-dosing can cause resistance to occur, reducing the effectiveness of the wormer.

»  More on equine parasites




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