Question: I recently bought a 12-year-old Thoroughbred and need to start him on a deworming program. What’s the best strategy to prevent parasites?
MARTIN NIELSEN, DVM, PhD, Diplomate ACVM & EVPC
Answer: First of all, we can’t completely prevent parasites in horses, no matter how clean their environments are or how diligently we treat them with dewormer medications. Every single horse in the world with access to pasture has small strongyles in his body; most horses also have tapeworms. Other parasites, like large strongyles, have become quite rare, although we continue to monitor for them as well.
Parasites enter a horse’s body through the digestive system by latching onto the grass he eats. They are most likely to survive and develop in pastures when conditions are also favorable for grass growth—during the spring, summer and fall (depending on your climate). Parasite eggs won’t hatch when temperatures drop below about 42 degrees Fahrenheit.
For several decades, the general goal of deworming programs was to eradicate parasites from horses altogether. This turned out to be not only impossible but also counterproductive: Aggressive deworming efforts resulted in widespread levels of drug resistance. Worms exposed to these drugs adapted themselves so that they would no longer be affected by some of them. For example, the large majority of small-strongyle populations are now resistant to both benzimidazole and pyrantel-type dewormers.
Our new goals are to: 1) prevent parasitic disease—conditions in which parasite levels are high enough to produce symptoms such as colic, diarrhea and weight loss—and 2) maintain effective dewormers for the future. To find this happy balance we recommend deworming only as often as necessary for your particular horse. The best way to determine the appropriate program for him is by measuring the levels of parasite eggs exiting his digestive system, or “shedding,” via his manure.
Ask your veterinarian for instructions on how to collect and submit manure samples for a fecal egg count. Ideally, these tests should be performed at least twice a year. The timing doesn’t have to be exact, but it makes sense to do it around the start and end of the grazing season (the months when grass is growing in the pasture). Egg counts will give you an idea of whether your horse is a low, moderate or high strongyle shedder.
The overwhelming majority of mature horses belong to the low-shedding category. If this is the case with your horse, he won’t need more than two annual dewormings—in the spring and fall, if possible. (Again, this timing will vary depending on where you live and how long your grazing season is.) Some horses require even fewer treatments. In rarer cases, horses who fall into the high-level strongyle-egg-shedding category and live in climates where the grazing season lasts longer than five or six months may require more frequent doses. Consult your veterinarian about the appropriate program for your individual horse.
Currently, the most effective dewormer products for strongyle parasites are macrocyclic lactones: ivermectin and moxidectin. If the lab detects tapeworms in your horse’s manure sample as well, these are best treated in the fall, toward the end of the grazing season. One effective tapeworm treatment is praziquantel, which is available in combination with either ivermectin or moxidectin in some products. A double dose of pyrantel pamoate is another effective option. I do not recommend daily dewormer supplements.
It is a good idea to monitor how well your chosen treatment is working by regularly performing a fecal egg count reduction test: Test a manure sample before administering the dewormer, then test again two weeks later to see if the egg count was significantly reduced.
Keep in mind that excessive deworming will contribute to the drug-resistance problem, so treat your horse with no more than the number of doses prescribed for his egg-count status.
Born and raised in Denmark, Dr. Martin Nielsen earned his veterinary degree at the Royal Veterinary and Agricultural University in 2001 and spent three years in equine veterinary practice before completing his PhD at the University of Copenhagen. For the next four years, he served as an assistant professor and a member of the large-animal hospital’s night duty emergency team at the University of Copenhagen. He was awarded the Danish Research Council’s Young Elite Scientist Award in 2009 for his research in parasitology. A diplomate of both the European Veterinary Parasitology College and the American College of Veterinary Microbiologists, he is now an associate professor at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, where he continues to study equine parasites and, among other things, their resistance to drugs. Dr. Nielsen is also the co-editor-in-chief of Veterinary Parasitology and has published many journal articles and book chapters.
This article was originally published in the March 2018 issue of Practical Horseman.