University of Maryland

ANSC 232- Horse Management

Anthelmintics Can’t      Solve This Problem

Student:                                                                        Instructor:

Jaelen Cherry                                                                Dr. Burk

December 1, 2020

Introduction

Horse owners face many challenges in successfully caring for their horses and protecting them from harm. However, without proper management practices, adequately caring for them becomes difficult. One particular issue that requires special management practices is parasites in horses. Parasites are organisms that require a host in order to survive. The host in this case would be horses, where the parasites drain them of their nutrients, and use this as a food source to repopulate. This can lead to poor feed efficiency, poor performance, or death in serious cases. On the bright side, there are known management practices and anthelmintics to swiftly take care of these parasites. Yet, parasite management is a growing issue in the horse industry, where traditional parasite management is becoming less effective as time passes. This is because of the overreliance on anthelmintics to treat infected horses. An anthelmintic is a treatment that is effective in killing or paralyzing parasites, inhibiting their ability to lay eggs. The overuse of anthelmintics as a result of poor parasite management practices leads to anthelmintic resistance in  parasites, endangering the horse industry as a whole. This paper will discuss the importance of a proper parasite management plan, as well as how anthelmintic dependence contributes to the anthelmintic resistance issue, and what may be possible solutions.

What is Anthelmintic Resistance

Anthelmintic resistance in parasites is caused by Darwin’s theory of survival of the fittest in the natural selection of parasites, where those with a beneficial mutation of resistance survive to produce more resistant offspring. These parasites are also able to develop ways to avoid drug effectiveness, for instance developing in difficult areas for anthelmintics to reach (Capela and Moreir and Lopes 2019). In normality, the parasites that possess this resistance are dominated by their non-mutated counterparts. However, once an anthelmintic is introduced, the resistant strains will now become dominant, because of the reduced population of non-resistant parasites. Another possible cause of anthelmintic resistance is the improper use of them. This includes not finishing the prescription, and not following the proper administering directions, non-lethal doses, and not deworming a significant portion of a horse population (Naylor 2020). These errors can lead to parasites surviving past the treatment, which can lead to additional use of anthelmintics to combat this. One can see how this would pose a significant issue if large numbers of horse owners participated in these detrimental actions.

Anthelmintic Resistance in Nematodes

        In a study conducted on anthelmintic resistance in equine nematodes, researchers concluded that the issue of anthelmintic resistance often leads back to improper use of anthelmintics from owners (Matthews 2014). The study also concluded that macrocyclic lactones such as moxidectin and ivermectin showed emerging anthelmintic resistance. Moxidectin and ivermectin were released in the early 1980s- late 1990s, which offered a newer anthelmintic to combat parasites (Griffiths 2020). Moxidectin is used in dewormers such as Quest, while ivermectin is used in Zemerictin. A reason for this emerging resistance is that both moxidectin and ivermectin are effective in treating many parasites such as ascarids, large and small strongyles, bots, and tapeworms (Figure 1). Ivermectin is shown to have a higher emergence of resistance in acraids, which are large roundworms known for causing blocked arteries, as well as damage to vital organs in young horses (Lane 2014). Moxidectin specifically is established as an “all-in-one” type anthelmintic,  to be used for almost all parasites. This is a cause of concern since the overuse of moxidectin can contribute to a rise in resistant strains.

The Strongyle Problem

        Cyathostomins, also known as small strongyles are parasites that encyst in the hindgut of horses, that may cause ulcerations to the mucous membrane of the intestinal tract. Another complication is larval cyathostominosis, a fatal colitis in horses. Small strongyles are becoming more abundant in horses today (Naylor 2020). However, treating them is not as easy as using a simple dewormer, since many dewormers are ineffective. In stage three of the larvae process of cyathostomins, anthelmintics become very ineffective. This stage is particularly concerning since the small strongyles begin to encyst into the intestinal walls. Only macrocyclic lactones such as moxidectin  and benzimidazoles such as fenbendazole are effective in treating horses with this stage of small strongyle development (Peachey 2020). However, as stated earlier, moxidectin is showing to have cases of resistance rising. Fenbendazole is not saved from this issue either, where its effectiveness is dependent on higher  to kill cyathostomins when in the stage three encysted fase, which poses the risk of toxicity if too much is used (Succeed Veterinary Center). This is causing many researchers to raise their eyebrows at possible solutions to fix this growing problem. On one hand, cyathostomins need to be managed in the horses experiencing the encysted stage of infestation requiring moxidectin and fenbendazole, but on the other hand, overreliance on these anthelmintics will cause for soon render them useless to resistant strains.

Solutions to This Issue

In order to slow the emergence of anthelmintic resistant parasites, owners must first understand the completely eradicating parasites is unlikely to occur. However, this does not mean that proper management practices can not help reduce the impact these parasites have on the horse population.

Parasite management enables horse owners, specifically horse barn owners, the ability to prevent infection, and to contain infections when they are spotted. These preventive and treatment measures are intergirl in maintaining control of the parasites. One measure that can be taken is manure management. Parasites are able to be spread through fecal matter of horses because many of these nematodes infect the intestinal walls (Rogers 2013). This means preventing horses from coming into contact with parasite infested feces is important. One way to do this is to remove feces from stalls and paddocks day, to prevent parasites from migrating from host to host. Another important measure is preventing horses from feeding directly on the ground by installing feeders above ground level. However, this is not always avoidable since pasture horses can come into contact with infected feces. Though, in order to reduce the risk of parasites inhabiting a pasture, long grasses can be mowed and maintained.

Another practice that can be done is utilizing fecal ID counts. Fecal ID counts allow horse owners to detect the largest shedders of parasites in their horse population, while also learning whether their anthelmintic is effective in treating that parasite. If FEC’s rise abruptly, and consistently, this is a warning sign for anthelmintic resistance in that horse, indicating that anthelmintic is no longer as effective in killing the parasites (Griffiths 2020). FEC’s are also important to the scientific community when analyzing these parasites. FEC’s allows scientists to measure what parasites are more prominent, and study how anthelmintic resistance emerges. If more horse owners utilized FEC’s, a clearer approach of preventative measures would be understood.  

Another step that owners can take is to avoid the traditional methods, as these have been proving to be less effective in treating the persistent parasites of today. The traditional method of routine deworming was thought to prevent eggs from maturing, inhibiting their ability to populate a host (Lesté-Lasserre 2020). However, this method contributes to the issue of anthelmintic resistance by using them even when they are not required. Each time an anthelmintic is used, it allows for resistant strains to be selected. Instead, anthelmintics should be used sparingly, only when a horse requires treatment.

Lastly rotational use of anthelmintics can also prove to be effective in reducing the risk of resistance (Wood 2020). This can be done annually when deworming horses for parasites. An example of this would be with moxidectin and fenbendazole, where one can be used each year to prolong the effectiveness of both anthelmintics.

Opposition Response

Although anthelmintic resistance is a growing problem, perception of said problem is not cut and dry. Convincing horse owners to subscribe to the notion of thinking towards the future to prevent anthelmintic resistance becomes far more difficult when their horses are experiencing parasite issues already. Not only this, but who is to say that these preventative measures will guarantee these resistant strains to persist in the future? To this, the answer would be there isn’t. However, this does not mean that scientists can not develop a new anthelmintic treatment to combat the small strongyles. For instance, with large strongyles, ivermectin became very effective in reducing the number of large strongyles in the horse population, to the point where it is no longer the main parasite concern. This too could be done for small strongyles if enough time is provided to scientists to do so. In order for this to occur, preventative measures must take place sooner, rather than later. Although these measures may not be as effective in treating already infected horses, they are much effective in preventing mass spread of parasites within a horse population.

Conclusion

In conclusion, anthelmintic resistance must be addressed in order to prevent resistant strains from endangering the horse population. More research must be done on developing and implementing new/current anthelmintics to treat the growing concerns of parasites. With this, more education must be provided to those within the industry on the importance of a proper parasite management plan in order to slow the spread of parasites. With these measures, this issue can be greatly improved upon.

Figure 1. Shows anthelmintics such as moxidectin and ivermectin, and the parasites they are effective against.

References

Capela, R., Moreira, R., & Lopes, F. (2019, November 15). An Overview of Drug Resistance in Protozoal Diseases. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6888673/.

Griffiths , J. (2020, October 5). Risk and Reality: Horse Parasite Control and Anthelmintic Resistance. The Horse. https://thehorse.com/169288/risk-and-reality-horse-parasite-control-and-anthelmintic-resistance/.

Lane, T. J. (2014, December). Parasite Control in Horses - Management and Nutrition. Merck Veterinary Manual. https://www.merckvetmanual.com/management-and-nutrition/health-management-interaction-horses/parasite-control-in-horses.

Lesté-Lasserre, C. (2020, June 24). Equine Parasite Control: Deworming and Beyond. The Horse. https://thehorse.com/features/equine-internal-parasites/.

Matthews, J. B. (2014, October 25). Anthelmintic resistance in equine nematodes. International journal for parasitology. Drugs and drug resistance. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4266799/.

Naylor, R. (2020, November 2). Emerging threats in equine parasite control. Veterinary Practice. https://veterinary-practice.com/article/emerging-threats-in-equine-parasite-control.

Parasitic Colitis in Horses (Small Strongyles): Equine GI Disease Library. (2017, July 19). https://www.succeed-vet.com/equine-gi-disease-library/colitis/parasitic-colitis/.

Peachey, L. (2020, November 2). Larval cyathostominosis. Veterinary Practice. https://veterinary-practice.com/article/larval-cyathostominosis.

Rogers, K. (Ed.). (2013, April). Anthelmintic. Encyclopædia Britannica. https://www.britannica.com/science/anthelmintic.

Wood, C. (2020, January 22). Management and control of internal parasites in horses. Horses. https://horses.extension.org/management-and-control-of-internal-parasites-in-horses/.