What is resistance and why does it matter?

By Wendy Talbot on 23 May 2023

What is resistance and why does it matter?


In the same way that antibiotics are becoming less effective in humans and in animals worms are becoming resistant to the drugs in some wormers and can survive, despite the horse being wormed.


Resistance to wormers is a serious and 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.  The development of these resistant worms increases if wormers are used indiscriminately.1


The incorrect use of wormers, including under-dosing can cause resistance to occur, as can the effectiveness of the wormer. Under-dosing means that that parasites which are only partially susceptible to the drug may survive and reproduce instead of being killed as expected with the correct dose. Dosing correctly and when required is the key to managing the issue of resistance in our horses. 2


Why blanket worming doesn’t work

‘Blanket worming’ means using the same wormer or rotating products every 6-8 weeks for all horses, without necessarily knowing which parasite you are targeting or why. Resistance development has been shown to be affected by treatment intensity, with a modeling exercise showing that increasing the treatment frequency increased the speed at which resistance developed. However, the results also indicated that reducing the number of annual treatments from six to four, while continuing to treat all horses, is likely to have only a minimal benefit in slowing resistance development, therefore it is crucial that we use diagnostics to actively select which horses to treat.3


Can we eliminate worms completely?

It is not possible to eliminate worms completely and attempting to try to will only increase the pressure for resistance. Refugia are the worms that, for whatever reason, remain unexposed to the wormer used at each treatment. These worms importantly contribute to the next generation and may then be susceptible to the next wormer used. They helpfully counter the population of worms generated from resistant stock and it is thought that they are important in slowing the development of resistance. The largest and most important source of refugia is parasites on the pasture and those parasites in horses that are not treated.  A smaller source of refugia is any stages of worms not treated by a wormer (i.e. larval stages).3


Continued wormer resistance will lead to untreatable parasite burdens that could be a potentially fatal health risk to our horses, so it is vital to take expert advice before using a wormer. Vets and RAMAs/SQPs have the knowledge to guide you on the most up-to-date methods of worm control and it’s important to ask their advice before making any decisions about worming your horse.



  1. AAEP (2019) Parasite control guidelines.
  2. Matthews JB (2008) Equine Vet Educ, p 552-560
  3. Nielsen MK, Sauermann, C.W., Leathwick, D.M. (2019) The effect of climate, season, and treatment intensity on anthelmintic resistance in cyathostomins: A modelling exercise. Veterinary Parasitology, 269, 7–12,



Wendy graduated from Bristol University in 1999. She then went on to complete a residency at Liverpool University and holds a European Diploma in Equine Internal Medicine. After working in practice for 13 years, she joined Zoetis in 2012 as the National Equine Veterinary Manager.

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