Parasites: Anthelmintic Resistance

Written By Dr. Kris Hiney

Last month we discussed management strategies that will aid in lowering the amount of internal parasites to which your horse is exposed. However, even with the best management practices available, it still may be necessary to employ chemical means to eliminate parasites. However, the use of anthelmintics (drug class which eliminates parasites) should not be done indiscriminately. A growing concern within the equine industry is the international development of anthelmintic resistance among the common parasites which infest horses. With no new drugs on the near horizon, we should take a hard look at their responsible use and our current management practices.
First of all, we should discuss how much of an issue parasite resistance might be, and what owners might have inadvertently done to help create it. It has been known for some time that ascarids and small strongyles have developed resistance to some drug classes, including benzimadazoles and the tetrahydropyrimidines or pyrantel salts, which include pyrantel pamoates and pyrantel tartrate. The other major drug classes of dewormers are the macrocyclic lactones or avermectins/milbemycins. Many horse owners would know these more commonly as the ivermectins or moxidectrin. These dewormers have been extremely popular to use because of the broader spectrum of parasites which they eliminate. Compared to benzimadozoles and tetrahydropyrimidines which kill large and small strongyles, ascarids and pinworms, the macrocyclic lactones also eliminate bots and stomach worms and moxidectrin eliminates several of the larval stages of small strongyles. Because of their broader range of efficacy, this has led many individuals to rely almost exclusively on these drugs in their management plan. However, there is a growing body of evidence that suggests that resistance issues to these drugs have begun to develop.
Ascarid resistance to ivermectin has been demonstrated in the Netherlands, Italy, Canada and Denmark. Furthermore, even within strongyles, the egg reappearance period, or the time between when no eggs are detected in the feces following deworming, to when they are seen again, has shortened from 9-10 weeks down to 5-6 weeks.  In Brazil, an increasing number of colic deaths in horses have been attributed to a growing resistance in the strongyles population.   In a recent issue of Veterinary Parisitology looking at anthelmintic resistance of horses in Finland, treatment of infected horses with ivermectin only resulted in a 52% decrease in the number of eggs found in the feces and 63% of horses had resistant parasites. Ivermectins were more effective against strongyles, but 44% of treated horses had parasites with demonstrated resistance. Comparatively, pyrantel treatment of horses infested with strongyles resulted in only a 43% reduction in fecal egg counts with 79% of horses showing resistance issues. To clarify, it is the parasite within the horse which harbors resistance, not the horse itself.
So why is resistance a growing issue? As discussed already, many individuals have relied exclusively on one type of dewormer. Eventually, as is typical in nature, organisms adapt to their environment to be more successful and to pass on their genetic code to future generations.    Essentially, once the worms adapt, or a few individuals survive a purge deworming, they are able to pass on these enhanced genetics to a future generation of worms which will also have that advantage of being immune to that drug.   If they are never exposed to a different type of dewormer, essentially the horse owner is just developing a breeding program for resistant worms! One strategy to adopt when thinking about “breeding” worms is to increase the number of refugia, or the population of worms which have not been exposed to dewormers. These would be the worms that would be in horses that were untreated, the encysted larval stages or perhaps those in the pasture. These non-exposed worms actually help to dilute out the population of resistant worms, and allow them to breed and pass on their more inferior genetics. To this end, many veterinarians now recommend that a fecal exam be performed on a horse prior to choosing to deworm. In this manner, horses which may not have worms present, are not unnecessarily dewormed. Also, a strategic deworming plan can be implemented, targeting only those horses with a significant worm burden. For example, a target concentration of 200 eggs per gram can be used.    Only horses which test to have a higher fecal egg count would be dewormed, while those horses with a relatively low burden would be left untreated, and thus increase the refugia.  This lowers our usage of anthelmintics and targets their use only at individuals which need it.
As horse owners, we may also be inadvertently contributing to resistance issues by using poor practices when we do choose to deworm our horses. Every time we under dose our horses with purge dewormers, we are creating greater tolerance in the worms. Many times owners actually under-estimate the true weight of their horse when deworming, or have a horse which spits out part of the oral dose. One easy tip for owners is to make sure there is no feed present in the oral cavity of the horse when deworming, the presence of which makes it easier to spit out the dewormer.
So what should a horse owner do? Certainly selecting only those horses which are known to be high shedders (or have a significant number of eggs present in their feces) would be ideal. However, there is very little incentive for the average horse owner to purse this type of program.   It is typically cheaper to just deworm the horse, rather than testing it first, and then follow up with deworming. More or less it becomes a personal choice of which practice to follow. If owners are unwilling to perform fecal exams, then it is imperative that they do rotate within classes of dewormers. Slow rotation strategies, which entail using a particular class of dewormer for one year, followed by a different class the following year, is an effective strategy.  Using a fast rotation program, or continually rotating between classes for every treatment is an alternative rotational strategy.   No definitive studies have been performed as of yet to suggest which strategy may be best for avoiding resistance issues. Additionally, the use of a macrocyclic lactone at least twice yearly is recommended, as these are the types of dewormers which can eliminate stomach bots. These are typically given after the first hard frost of the year, and prior to the spring thaw in the spring.
Ultimately, it is in our best interest as horse owners to employ strategic decisions about deworming our horses. A combination of management strategies and informed intelligent decisions about which horses to deworm and what products should be used. Ideally, a sound plan can be developed with your veterinarian or equine professional that serves the needs of your own horse, and the greater equine community as well.

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