Equine parasites are becoming more resistant to dewormers, several veterinarians and equine parasitologists say. They differ on how to handle the problem.
“Multiple studies across the country are showing that entire classes of dewormers are no longer working against small strongyles,” says Frank Hurtig, DVM, MBA, director of Merial Veterinary Services of Duluth, Ga.
“The threat of parasites is nothing like it was through the 1980s,” Dr. Hurtig says. “The difference between then and now (is that) small strongyles are not as pathogenic as large strongyles, especially bloodworms.
“We have to make sure that we preserve the effect of dewormers in the future,” he says. “Anthelmintic resistance is a problem. We are just now learning about drug resistance from other species—specifically, sheep and goats.”
Of the three major chemical classes of dewormers, Hurtig says, well-documented resistance to small strongyles has been demonstrated against benzimidazoles, one of the older classes of dewormers.
He also cites 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, that showed 40 percent of the farms surveyed had small strongyles that were resistant to pyrantel, another chemical class of dewormer.
Hurtig says if parasites develop resistance to ingredients like the macrocyclic lactones ivermectin and moxidectin, virtually all dewormers would be ineffective against small strongyles.
“Ironically, resistance is the direct result of the ready availability and ease of administration of dewormers,” says Harold C. McKenzie III, DVM, MS, Dipl. ACVIM.
Dr. McKenzie is an associate professor of equine medicine at Marion duPont Scott Equine Medical Center at Virginia Tech in Leesburg.
“It is more difficult to give advice on deworming now than several years ago, when there were so many different classes of compounds and there was little drug resistance of nematodes,” says Gene Lyons, Ph.D., a parasitologist at the University of Kentucky’s M.H. Gluck Equine Research Center in Lexington.
“Opinions vary about parasite resistance,” Dr. Lyons says, “but the bottom line is that in areas like central Kentucky, horses on most, if not all, Thoroughbred farms already have drug-resistant nematodes.”Ivermectin and moxidectin used in spring and fall, Lyons says, “will help ration the efficacy against small strongyles. Even though activity seems to have diminished, these compounds are still the most effective compounds on these parasites.”
“The benzimidazoles (fenbendazole and oxibendazole) and pyrantel pamoate are virtually ineffective against small strongyles,” Lyons says, “but seemingly are effective still on other species of nematodes.”
“Macrocyclic lactones (ivermectin and moxidectin) treatments are becoming less effective on small strongyles, based on counts of eggs per gram of feces returning post-treatment quicker than initially,” he says.
“Ascarids (roundworms) have been found to be resistant to ivermectin now,” he says. “There are reports that the benzimidazoles still seem effective on ascarids but pyrantel pamoate is less so in some situations.
“One success in parasite control has been the great reduction of the very important large strongyles in the genus Strongylus,” Lyons says. “Definite drug resistance of these parasites has not been shown at this time.”
“The prevalence of bloodworms is low across the United States,” Hurtig says. “Bloodworms were—and are—readily affected by ivermectin and moxidectin.”
“We need to use dewormers more carefully and base their use on the actual parasite burden in the horse and in the herd,” McKenzie says. “Parasite resistance is very real, especially in ascarids and in small strongyles.”
“It is vital that horse owners know which dewormers work on their farms,” says Hurtig. “Though not as accurate as we would like, fecal egg counts are the only tool we have to try and measure dewormer effectiveness.”
He suggests that only veterinarians or veterinary technicians run the McMasters or modified McMasters tests to ensure accuracy and that they are conducted properly.
Leading parasitologists suggest that 200 to 300 eggs per gram of manure is a reasonable cut-off point to justify deworming a horse.
“Resistance is a farm issue, not an animal issue,” Hurtig says. “Experts suggest there is a tipping point: If about 25 percent of worms are resistant to dewormers, then the farm will most likely be resistant and this appears to last forever.
“Eighty percent of the high egg count comes from 20 percent of the animals,” he says. “We need to focus the extra treatments on that 20 percent.”
To keep the percentage of resistant parasites diluted, Hurtig suggests not treating all of the horses every time. “If egg counts are low, don’t deworm,” he says.
John Byrd, DVM, of Horsemen’s Laboratory in Mahomet, Ill., agrees. His lab conducts fecal egg counts for horse owners by mail and e-mails results. He started the service in 1992, after almost 20 years as an equine practitioner.
“As veterinarians, we need to educate the horse owners about proper evaluation of the horse’s worm control program through stool samples,” Dr. Byrd says. “Many veterinarians feel that the influence on their horse owners’ worm control program has been lessened due to the availability of paste dewormers at the feed store.”
Byrd also cautions practitioners to teach horse owners the proper technique for paste deworming to make sure the horse ingests all of the dewormer. Occasionally, the post-deworming egg count comes back close to or the same as the pre-deworming count. The only explanation seems to be that the horse did not ingest the proper dose of dewormer.
But in some cases, even if the horse does get its proper dose of dewormer, some veterinarians say, parasite infection still be present in egg counts.
“It’s not that dewormers are no longer effective,” says Robert Holland, DVM, Ph.D., “but rather that a couple of the chemical classes of dewormers are not quite as effective as they used to be.” Dr. Holland is associate director of outcomes research at Pfizer Animal Health in Lexington, Ky.
“Resistance is not widespread among all areas of the country,” he says. “Continuing to strategically rotate among the three chemical classes is still the safest and most effective way to ensure the horse is protected from parasites.”
According to McKenzie, rotational deworming has been of some help in trying to slow the development of resistance, but is not the most effective method.
“It is better to use a targeted approach with different treatments based upon a horse’s level of infestation,” he says.
Wendy Vaala, VMD, Dipl. ACVIM, cautions that misconceptions exist about drug resistance. “Drugs don’t create resistance,” she says. “Random mutations occur in certain parasites that render them resistant to certain drugs. Frequent use of those drugs selects for the resistant parasites and allows them to become the dominant (resistant) population. Don’t rely on one drug repeatedly. That will select for resistance.”
Dr. Vaala is senior equine technical service veterinarian for Intervet Schering-Plough Animal Health.
“We can slow the resistance down,” Vaala says, “by remembering good husbandry practices and pasture management strategies together with purposeful selection of dewormers and the proper identification of the horses that need to be dewormed. Fecals are an essential part of that program.”
Vaala says part of the resistance problem is due to the frequent use of dewormers without monitoring. This practice seems to be most common wherever there are well cared-for horses.
She says the problem is more pronounced in regions where there are large numbers of intensively managed young horses, mares and foals, such as in Kentucky.
Vaala says some parts of the country don’t have as many parasite issues. Lots of pasture, low stocking density and/or more arid temperatures can lower the parasite infection rate.
“We need to move away from panic, and get back into science,” she says. “No blanket recommendation will work. We need to strengthen communication between the veterinarian and the horse owner. Veterinarians need to become re-involved in designing and monitoring deworming programs.”
Hurtig says the rotation of deworming products makes no difference in the infection levels of the horse.
Rotation was originally thought to be the best method of using different chemical classes of dewormers to kill all parasites in an animal. The belief was that chemical classes had to be changed to keep resistance low. Hurtig says scientists have shown that rotation is not necessary, or desirable.
“It is common practice among horse owners to rotate dewormers, depending on the season,” he says. “It would make more sense for them to figure out what works for their farm, and use that.”
“Horse owners should not blindly rotate their dewormers without understanding what drug they are using and why,” Vaala says. “Drug use should be based in science.”
“A variety of dewormers are easily accessible on the market,” Holland says. “The problem is if a dewormer is not given in proper dosage and timing, it might not be as effective as it could be. Inadequate treatment dosages allow residual worm populations to survive and encourages resistance.
“Assuming every horse in the barn is the standard 1,100 pounds,” he says, “often leads to underdosing.”
“By deworming the most heavily infested horses, we can reduce pasture contamination dramatically,” says McKenzie.
“Several methods (to reduce evironmental contamination) have proven benefit, some have unproven benefit, and some are impractical,” Lyons says. “Removal of fresh feces, chain harrowing, leaving pasture vacant of horses for long periods, and feeding horse specific benign fungi to kill strongyle larvae have all been tried.”
Recently back from the World Assn. for the Advancement of Veterinary Parasitology conference in Calgary, Alberta, Vaala says new information was presented for a fecal test that would help identify active tapeworm infections.
Another challenge for veterinarians: the teaching of these concepts of diluting resistant parasites with susceptible parasites on a farm is recent, says Hurtig. Some veterinarians may not have been taught these concepts in school.
“Large-animal parasitology concepts taught at veterinary schools may be quite variable,” Hurtig says. “There are very few veterinary parasitologists with strong backgrounds in equine parasitology currently teaching in schools.”
This article first appeared in the October 2009 issue of Veterinary Practice News.
Parasitologic Wish List
• Robert Holland, DVM, Ph.D., thinks it would be great to have a better diagnostic system to measure parasite load in a horse. “For now, the fecal egg count exam is really the gold standard when it comes to testing for parasitic infection in the horse,” Dr. Holland says. .
• Frank Hurtig, DVM, MBA, says work is under way to try to validate other equine diagnostic technology..
• Wendy Vaala, VMD, says tapeworm eggs are difficult to detect in routine fecals, making an infection diagnosis difficult. “Their heavier eggs don’t float in a fecal egg count, so they are often overlooked,” Dr. Vaala says. “A blood test can identify exposure, but we need a more sensitive test for tapeworms.”.
• Parasitologist Gene Lyons, Ph.D., points to this success: A molecular method has been developed to detect the presence of the most pathogenic nematode, the bloodworm (Strongylus vulgaris).