Most horseowners today recognize that “one-size-fits-all” parasite control programs are not effective. For this reason, horse owners/caretakers should create a parasite control program that:
- reduces transmission
- keeps worm burdens low
- treats clinically affected horses
Fecal egg count tests can determine which horses are shedding the most worm eggs. Once identified, deworming treatments can be directed at those animals.
“Of the three classes of dewormers, many experts believe that macrocyclic lactones are the only class of medications to which key equine parasites of adult horses have not developed resistance in the United States,” says Hoyt Cheramie, DVM, MS, manager, Merial Veterinary Services. Within the macrocyclic lactone class, the two most common products are ivermectin and moxidectin.
Used for parasite control in cattle, sheep and horses, ivermectin and moxidectin have a similar mode of action but are different in their chemical properties. These differences may affect the likelihood of resistance occurring.
“Equine parasite resistance is becoming more of a concern as we continue to investigate how well each anthelmintic class and product is currently working,” Dr. Cheramie says. “While we have seen resistance in other anthelmintic classes, we had not seen it pop up in macrocyclic lactones until recently. I and other equine veterinarians I’ve spoken with are particularly concerned with moxidectin resistance, which we’ve seen occur in other species and in horses in other countries.”
A study conducted between 2001 and 2004 — published in 2006 — of Australian sheep farms using only a moxidectin drench for parasite control found nearly three times more resistance to brown stomach worms than those using ivermectin. Additionally, prevalence of resistance was significantly higher — up to 77 percent — on the sheep farms where moxidectin had been used for more than two of the preceding five years.
With reports of resistance to moxidectin in sheep and Brazilian horses, some experts in equine parasitology caution that moxidectin use in horses should also be limited. Based on theories of selection for resistance, some experts believe that regular use of moxidectin may increase the risk of producing macrocyclic lactone resistance.
Even though ivermectin and moxidectin are both members of the same anthelmintic class, they do have differences. In horses, ivermectin is not labeled for control of encysted larval stages of small strongyles. Therefore, encysted small strongyles should be slow to develop resistance to ivermectin as a population that hasn’t had an opportunity to be exposed to the drug. However, while moxidectin is extremely effective in eliminating larval stages of small strongyles, that also may mean small strongyles may be more likely to develop resistance to moxidectin.
“If macrocyclic lactone resistance were to develop in one product, it would most likely occur with all macrocyclic lactone products,” Dr. Cheramie says. “There are some differences in relative effectiveness of each macrocyclic lactone to different species of parasites, but all products would be adversely affected if resistance were to develop.”
With no new parasite control products or classes on the horizon, adds Dr. Cheramie, it’s increasingly important to use all parasite control products carefully.
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