Parasites: Anthelmintic Resistance

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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.

Equine Parasite Management

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Written By Dr. Kris Hiney

Last month we introduced you to the major internal parasites which can plague your horse. This month we will discuss management strategies that you can use to decrease the parasite load on your horse, in part through an understanding of their life cycle. We can actually use the horse’s environment to help decrease our reliance on de-wormers and do our part to aid in the battle of anthelmintic resistance.
If you remember the life cycle of our most insidious parasite, the small strongyles, you know that the tiny infective larvae hatch from eggs outside of the horse. They then use the dew or moisture present on the grass to be able to wriggle around in the blades of grass and await your horse to come along and ingest them. Since they need this moisture as part of their life cycle and to be mobile, horses housed in stalls and dry lots are far less likely to be able to pick up infective larvae. It is pasture grazing, therefore, which is the key to the strongyles’ survival. Worm larvae will tend to be located in the thicker grass areas of the pasture and down in the thatch layer, where moisture remains longer.  The highest potential for infection will occur if your horse crops the grass close to the ground.
If you observe horses natural feeding patterns, horses tend to graze pastures into areas of roughs and lawns. The lawns, characterized by short grasses,  are the areas which are cropped closely to the ground and the roughs, which have longer grass, are the areas where horses choose to defecate and avoid grazing. Obviously the larger the area in which horses are kept, the less likely they are to graze near infective piles of horse manure. This will decrease their chances of picking up larvae. As stocking density of the pasture increases, or vegetative growth decreases, such as in times of draught, the horses will be forced to eat nearer these thick areas of grass just teeming with swarms of larvae. If the grass becomes too short, supplemental hay should be provided to avoid forcing the horse to graze in the roughs. Additionally, the pasture can be mowed to keep the roughs from spreading further into the pasture.
Many people employ dragging the pasture to break up manure piles and spread them through the pasture to prevent the formation of roughs. However, if you use this strategy, you must understand that you are effectively dispersing the eggs and larvae far more thoroughly than they could ever do themselves. Even on their own, larvae can spread 4 to 12 inches from their original pile, and even further if aided by heavy rainfall. Therefore, if you drag the pasture, keep the horses off the pasture for at least two weeks. Preferably the dragging should be done in the hottest part of the year in order to expose the larvae to heat and dehydration. Cool temperatures allow the larvae to survive longer, so it is not advised to drag during the spring and fall.  If you must drag in cooler weather prevent the horses from grazing for an even longer period of time. As strongyles larvae are especially hardy and can survive winter quite easily, this is really not a good strategy for trying to kill the larvae.  Finally, if you are going to spread manure on pastures as a means of disposal, never spread fresh manure. Make sure it has been thoroughly composted before applying it to your pasture.
In an ideal world, pasture rotation allows the best management strategy to reduce strongyles infestation  in your horses. Horses grazing in fresh new pasture will avoid grazing near manure piles, and have a lower chance of re-infesting themselves. Letting pastures lie dormant will also allow any eggs or larvae present to die before horses are introduced. If space and equipment allows, putting pastures into hay production will allow parasites to die as well. Finally, if you own multiple species of animals, grazing pastures alternatively between cattle, goats and sheep will reduce your parasite burden, as the worms are host specific. Obviously all of these strategies do require a significant amount of acreage and fencing to be effective and may not work for everyone.
Remember, for strongyles elimination, heat is your friend. Only drag pastures during the hottest part of the year, and do not allow horses back onto the pasture for at least two weeks. Use separate pastures for winter pasture and summer pasture. Remember, winter does not kill the parasites. In cooler climates, parasites will not die after emerging from their dormant state until about June, May in hotter climates. If you do have a clean pasture, before you turn horses onto it, chemical deworming can prevent parasite infestation. Horses that are dewormed should be held on dry lots for several days before turning them out. This will allow all the eggs that the mature female has deposited to pass through your horse’s digestive tract. When your horses enter their new pasture, they won’t be bringing any “friends” with them!
What about the other parasites in your horses life other than strongyles? There are certainly management strategies which will help control their populations as well. For ascarid control, remember that these worms are primarily a problem for young horses. If possible and space allows, rotate which pastures house young horses with adult horses. However, even this may not be completely effective as ascarids can remain alive in the environment for several years. Essentially, if foals and young horses have been housed in a pasture, it is fairly likely that ascarids are present. Unfortunately, as ascarids don’t involve the same strategy for survival as strongyles, they can also infest the young horse in stalls and dry lots. This is typically why young horses are dewormed more frequently than older horses.
Stomach bot larvae and adult fly control are unfortunately only going to be controlled through the use of anthelmentics. The adult form can fly for miles so even if you have a great deworming program, if your neighbors do not, their flies will simply fly over to your property to lay eggs on your horse.
Tapeworms are relative newcomers when discussing parasites in horses. While not new to the horse, they are new to us, so not as much is known about them. They are believed to have a similar susceptibility to climate as the small strongyles, but may be hardier. More horses in northern climates have been exposed to tapeworms, which would indicate that these parasites are relatively cold resistant, but may have a susceptibility to heat. Therefore, follow similar management protocols as you do for small strongyles control.
From looking at the parasites life cycle and their means of infesting horses, it is clear that horses are often dewormed more frequently than is really necessary. As anthelmintic resistance becomes a growing issue in horses, we need to understand the ways in which we can manage horses to reduce their parasite burden. Next month we will tackle the issue of anthelmintic resistance and discuss which deworming strategies might be the most correct option for your horse.