Cindy weighed 60kg more than expected.

Owners urged to check their horse's weight before dosing

There is a growing resistance to wormers because many owners are not giving their horses the appropriate dosage for their weight, according to animal health experts.

A survey carried out last year by the British Riding Club revealed that 44 per cent of horse owners surveyed do not weigh their horse before worming. Not weighing puts horses in danger of under-dosing, which causes worms to develop resistance to the wormers used and can also have a serious impact on how worm burdens can be managed in the future.

The issue of weight was recently highlighted in case studies from a private yard in Suffolk. Cindy, for example, is a 13.hh 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, when means that she has been significantly under-dosing.

Experts say that resistance to wormers is 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 within 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 this process continues, only resistant worms remain the the population. Frequent dosing or under-dosing can cause resistance to occur, reducing the effectiveness of the wormer.

Vet Wendy Talbot said that ideally owners should be weighing their horses at least once a year using the accuracy of a weighbridge.

"Several feed manufacturers offer a portable weighbridge service," said Wendy.  "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."

Image (C) Zoetis