What is wormer resistance?
Resistance is a word that’s bandied around a lot these days — usually in relation to antibiotic medication — but what does it actually mean? “Resistance is something that’s developing in lots of drugs,” says Claire Shand, a suitably qualified person (SQP) from Westgate Labs. “In relation to wormers, it means that the drugs are becoming less effective at killing the worms. Some worms possess genetics that makes them naturally stronger against wormers.” These genetics protect the worms from being killed by wormers. Worms will reproduce, passing their wormer-resistant genes on to their offspring. As time goes on, more resistant worms are born, decreasing how effective each wormer is. There’s currently a degree of resistance to wormers used in horses, but it’s the situation in sheep that’s causing alarm, with experts keen to avoid similar circumstances arising in our equine population. “Endemic disease caused by parasites costs UK sheep farmers £84 million per year,” says Claire. “If horse owners aren’t careful, we’ll be heading in a similar direction.” “There are areas of land containing worms that are completely resistant to all the worming drugs available for sheep, meaning sheep can no longer graze there,” explains vet Alistair Todd MRCVS, from Kernow Veterinary Group. “At the moment, resistance to horse wormers tends to depend upon the individual livery yard, stud or farm and its worming history. For example, there are some stud farms where wormer resistance is a problem, largely due to them blanket worming in the past.”