Published November, 2012

In regard to the state of equine parasite control, Martin Nielsen, DVM, PhD, EVPC, believes the horse industry is currently in the midst of a "revolution." With new research on parasite control and anthelmintic resistance currently taking place at a rapid rate, equine deworming recommendations are also changing.

Nielsen outlined the do's and don'ts of "sustainable equine parasite control" on Practitioners' Day at the 9th International Conference on Equine Infectious Diseases, held Oct. 21-26 in Lexington, Ky.

The main goal of sustainable equine parasite control is to minimize disease risk associated with parasite infestations while trying to prevent said parasites from becoming resistant to all forms of dewormers.

"As parasitologists one of our commitments is to come up with the most current, evidence-based advice (for horse owners)," even if it means readdressing and revising existing recommendations, said Nielsen, an assistant professor of equine parasitology at the University of Kentucky Gluck Equine Research Center.

To that end, Nielsen and colleague Craig Reinemeyer, DVM, PhD, of East Tennessee Clinical Research, developed a list of key points to remember about successful sustainable equine parasite control, based on current research.

Don't use an anthelmintic without knowing its efficacy against the intended parasite population. Nielsen explained that small strongyles (cyathostomes) exhibit at leastemerging resistance for all anthelmintic classes--benzimidazoles, tetrahydropyrimidines (pyrantel), and the macrocyclic lactones (ivermectin and moxidectin); ascarids (Parascaris equorum) have widespread resistance against the macrocyclic lactones; and all drugs are effective against large strongyles (Strongylus vulgaris). Nielsen recommended using fecal egg count reduction tests, especially on high shedders, to evaluate efficacy. Aim for a threshold of 90% efficacy for the benzimidazoles and tetrahydropyrimidines, and a 95% threshold for macrocyclic lactones.

Don't treat at fixed interval frequencies year-round. Different types and life stages of parasites flourish at different types of the year, and there are parasite seasons and off-seasons with huge differences in parasite transmission," he relayed.

Don't rotate blindly between drugs. "This is a myth that's difficult to kill," Nielsen said. "(All the research) that comes out shows rotation doesn't do squat against slowing resistance. Besides, there isn't much to rotate between since it doesn't make sense to rotate to a drug to which there is already resistance"

Don't treat adult horses during the off-season. "Adult horses should be treated during the active parasite season (i.e., grazing season) only," Nielsen said. "Treating them during winters is an unneeded used of dewormers."

Don't treat the entire herd before moving to new pasture. Anthelmintic resistant eggs develop and are expelled via feces during and immediately after deworming of the entire herd, Nielsen said. If these resistant eggs are deposited in a clean pasture with no nonresistant parasite larvae to dilute out the resistant ones, the vast majority of the parasites horses ingest will be drug resistant, he explained. The good old "dose and move" advice is no longer recommended.

Don't treat pregnant mares just prior to foaling. While pregnant mares can harbor potentially dangerous parasites for foals, such as ascarids, the main source of parasite infection for young horses is the environment. Additionally, there is no evidence of periparturient rise in egg counts in the foaling mares, he said. "Pregnant mares should be treated just like other adult horses," Nielsen relayed, and if this is the case, there is no real incentive for additional dewormings around foaling.

Don't treat at the first frost. "Frost doesn't kill infective strongyle larvae or eggs of the roundworm Parascaris equorum," Nielsen said. "When the first frost occurs, the grazing season is typically over, and so is the active parasite transmission season. Treatment at this time has been associated with an increased risk of parasitic disease and could potentially accelerate the development of drug resistant worms. Rather than targeting treatments at the time of the first frost, parasite treatments should be performed well ahead of this, and the first frost should mark a time period with less intensive deworming."

Don't intentionally under-dose any anthelmintic treatment. Essentially, under-dosing only adds to the anthelmintic resistance problem by exposing parasites to a drug, but not at a high enough dose it kill them.

Don't use formulations not labeled for horses. "We don't know how drugs not labeled for specific uses will work in the animal; it hasn't been studied," Nielsen said. "It doesn't make sense to extrapolate a ruminant dose and use it for a horse. How do you know if it works?" Additionally, he noted, dewormers labeled for other animals still contain the three classes of drugs in equine dewomers, so they likely won't do anything to combat anthelmintic resistance. Finally, Nielsen cautioned not to administer products in a different manner than they're intended to be, such as administering an injectable dewormer into an apple and giving it to the horse. "We have no idea what the plasma concentrations of the active ingredient will be, and we may very well underdose the horse," he said.

Don't use a standard treatment program for all horses in a stable. All the horses on a specific farm have different egg counts: some high, some low, and some in the middle. Therefore, each horse should be treated individually. For example, a high egg shedder will need more aggressive treatment than a low shedder. Consider implementing fecal egg counts into horses' annual veterinary care to determine just how much deworming they require.

Now that all the "don'ts" are out of the way, Nielsen said, what should owners do when it comes to deworming?

For adult horses, Nielsen recommended fecal egg counts to determine the each horse's specific deworming requirements. He also recommends an annual fecal egg count reduction test to see which dewormers work on specific farms and which aren't as effective.

He suggests providing a basic treatment foundation for all horses of two yearly treatments with effective dewormers to suppress a possible resurgence of theStrongylus species and tapeworm. Finally, he recommended targeting high shedders more aggressively than low shedders, as determined by fecal egg count reduction tests.

For younger horses, Nielsen noted that owners should deworm more frequently than adult horses. He the following deworming schedule:

• Two to 3 months of age: Treat horses for ascarids with a benzimidazole;
• Six months of age: Take a fecal sample to determine whether to treat for ascarids or strongyles, and then treat accordingly (he cautioned that the drugs that still work against ascarids, most likely will not work against strongyles and vice versa);
• Nine months of age: Treat horses for strongyles using an efficient drug (most likely pyrantel, ivermectin, or moxidectin, he said); and
• One year of age: Treat horses for tapeworms and strongyles.

Geographic location might impact the exact deworming schedule, so consider discussing these deworming plans with a veterinarian prior to implementing it.

Erica Larson, News Editor at The Horse