A NEW PARADIGM
“We are facing a significant paradigm shift in equine parasitology,” says Martin K. Nielsen, DVM, PhD, Dipl. EVPC, assistant professor in the Department of Veterinary Science at the University of Kentucky Gluck Equine Research Center. “New deworming strategies should be focused on reducing transmission of parasites, keeping parasite burdens below harmful levels, and treating clinically affected horses. In this manner, a population of susceptible parasites is maintained to dilute the resistant genes in the population.”
Nielsen was chair of the subcommittee of the American Association of Equine Practitioners (AAEP) that launched new equine parasite control guidelines in 2013.
This strategic approach to deworming entails administering anthelmintics (anti-parasite drugs) only as often as necessary to reduce fecal egg counts. Ultimately this will improve the effectiveness of current deworming medications, eliminate unnecessary treatments, reduce costs and the negative impact on the environment. Over time, an appropriate program will reduce contamination of pastures and re-exposure in horses.
Dr. Nielsen is working with Penn State University Extension’s equine team, which received a grant from the Northeast Sustainable Agriculture and Education program to travel across the commonwealth educating horse owners about strategic deworming and nondrug parasite control methods.
“It is critically important to understand the basics of parasite resistance and develop a deworming program that will work for your farm,” says Donna Foulk, Penn State Equine Natural Resources Educator. Good pasture management in the form of manure composting, pasture rotation, mowing, and dragging are also an integral part of farm parasite population management.
Most horse owners believe if they don’t deworm all of their horses frequently, then they are not doing everything they can to protect them, but this belief is being rightly challenged. Although controlling the parasite load is important, an integral objective of parasite control is preventing environmental contamination, thereby decreasing transmission and re-infection.
RESISTANCE IS REAL Today, the major parasites that cause most health problems in adult horses are small strongyles and tapeworms in adult horses, and roundworms (ascarids) in young horses.
Despite the plethora of products, there are only three major chemical classes of dewormers: benzimidazoles, pyrantel salts, and macrocyclic lactones (avermectin/ivermectins).
Resistance to benzimidazoles is the most widespread. A study led by Ray Kaplan, DVM, Ph.D., Dipl. EVPC, of the Department of Infectious Diseases at the University of Georgia’s College of Veterinary Medicine, showed 40 percent of the farms surveyed had small strongyles that were also resistant to pyrantel.
THE NATURAL IMMUNE RESPONSE
Horses tend to fall into one of two categories regarding their natural immune resistance to intestinal parasites: first, how easily infected they are, and second, by how many eggs are then produced (shed) by the female worm.
Approximately 70 percent of our horse population in the United States have a good natural immune response to intestinal parasites and are considered low shedders, meaning their own bodies keeps the parasites in check and at very low numbers within the body.
High shedders are the most susceptible to parasitic infestation. Approximately 20–30 percent of our horse population can’t naturally keep their intestinal small strongyles in check. While high shedders only account for a small part of the horse population, they produce 80 percent of the parasite eggs on the pasture.
CHECKING FOR RESISTANCE A Fecal Egg Count Reduction Test (FECRT) is a simple way to determine if your horse needs to be dewormed and which products are still effective. Fresh manure is analyzed for parasite eggs both before and after deworming a group of horses.
At least five horses should be included in a FECRT test. Once egg counts are taken, the horses are dewormed, and a second egg count is taken after 14 days to determine the percentage of parasite reduction. Pyrantel and benzimidazoles should show at least a 90-percent reduction. Ivermectin and moxidectin should show a reduction of at least 95 percent. If resistance is found, then the entire farm will most likely be resistant, a condition that is usually permanent.
Anthelmintic Resistance
Benzimidazoles: (Panacur, Safequard, Anthlecide) – widespread resistance
Pyrantel: (Strongid, Exodus, etc.) – developing resistance
Ivermectin – (Eqvalan, Equimecterin, etc.) – early signs of resistance
Moxidectin – (Quest) – early signs of resistance
PASTURE MANAGEMENT AND COMPOSTING The key to parasite control is pasture management. Eggs are deposited onto the ground from the manure of an infected horse and develop into larvae. Rain and wind spread the larvae, and the larvae are swallowed during grazing. Removing manure from the pastures is one of the best preventative measures barn owners can take. Harrowing a pasture exposes parasites hidden in deep grass or in manure piles. However, to be effective at killing parasites and not just spreading them, harrowing should only be done during hot, dry periods when sunlight exposure will help kill strongyle larvae. Proper composting of manure and bedding can virtually eradicate parasites. Noncomposted horse manure should never be spread on pastures as this will increase the level of parasite contamination.
CUSTOMIZING YOUR DEWORMING PROGRAM
Knowing which horses are shedding the most parasites, what parasites are common in your horse population, which drugs are most effective against those parasites on your farm, and finally, how your climate affects parasite lifecycles will give you and your veterinarian the tools to protect your horses. Deworming for adult horses should be designed with the following principles in mind:
A basic foundation of anthelmintic treatments should be considered for all horses.
Identify horses that are high shedders using FECRT.
Evaluate the efficacy of the dewormers at least every three years.
Treat one or two times per year to target large strongyles, tapeworms, and bots in early fall after the grazing season.
Focus deworming treatments during peak transmission (usually spring thru fall)
Consider using a product labeled for encysted larvae in horses that have a history of parasite infection and disease.
Remember, foals do not have natural resistance to parasites and benefit from a regular monthly deworming program.
ON THE HORIZON
Dr. Nielsen and colleagues from the University of Massachusetts have identified a naturally occurring bacterium that secretes a protein that kills both insect and parasite worms safely in mammals. One form of delivery may involve giving a culture of this bacterium in a probiotic that will kill off all of the parasites within that animal.
KEEPING IT NATURAL
There is nothing more natural than ensuring that your horse’s immune system is running smoothly so that you can eliminate or lesson the frequency of chemical dewormers. Routine chemical worming can be an unnecessary toxic burden on horses and can overstress the liver and kidneys
While a natural program may work well for healthy horses that are not under stress and are living in a properly managed environment, an integrative approach that includes strategic use of chemical dewormers may be more prudent once you’ve done your research.
Consider the age and health of the horse, feed quality, activity level, travel and susceptibility to worms, environment, size and number of horses on pasture and manure management.
Low shedders are able to contain parasite infections and may only need to be supported with additional probiotics, enzymes, and immune-enhancing herbs, homeopathic products and diatomaceous earth. These options may not work for high shedders or boarding stables with high populations where high shedders are not managed. Whichever options you employ, have your veterinarian perform fecal egg counts before and 14 days after any deworming efforts and calculate the percent egg count reduction to ensure your program is working.
Carolyn Crew is an award winning freelance journalist and Equine Assisted Learning Practitioner. Her company, ECHO (Equine Connections Helping Others), holds experiential learning workshops for both personal and corporate leadership development. www.echoconnections.com
References:
Kaplan RM, Hodgkinson JE, Thamsborg SM, Nielsen MK. Background and goals. In: Kaplan RM, Nielsen MK, eds.
Proceedings of the Equine Parasite Drug Resistance Workshop 2008:3.
Von Samson-Himmelstjerna G. Anthelmintic resistance in equine parasites—potential clinical relevance and implications for control. In: Kaplan RM, Nielsen MK, eds. Proceedings of the Equine Parasite Drug Resistance Workshop 2008:10.
Kaplan RM. Biological considerations in evaluating drug efficacy and resistance in equine strongyle parasites using fecal egg count data. In: Kaplan RM, Nielsen MK, eds. Proceedings of the Equine Parasite Drug Resistance Workshop 2008:14.
Kaplan RM, et al. Prevalence of anthelmintic resistant cyathostomes on horse farms. JAVMA
2004;225(6);903-910.
(Gould et al., 2012)