It may be time to seriously rethink how we are deworming our horses. When we heard of ivermectin resistance being documented in cows (see August 2002), we warned that the same problem could occur in horses. It appears it has.
The Journal of the American Veterinary Medical Association reports the failure of ivermectin to eliminate roundworm infestation in 20 out of 37 foals treated with ivermectin. In some foals, the counts kept increasing between the time of treatment and a recheck 12 to 13 days later.
The foals were residing on a large Thoroughbred breeding farm, where roundworm density was likely to be high. The ivermectin failures occurred mostly in foals that had been born on the farm and were regularly dewormed with ivermectin since birth, both suggesting the emergence of an ivermectin-resistant roundworm on this farm.
The foals that did respond to the ivermectin were those that had not been lifetime residents of the farm and/or had been treated previously with dewormers other than ivermectin.
Whether this meant those foals had not yet picked up heavy burdens of the ivermectin-resistant worm possibly unique to this farm, or the roundworms in those foals remained sensitive to ivermectin because they had not been intensively treated with ivermectin, was not clear. Either way, the possibility that ivermectin, which combines the broadest dewormer efficacy at the highest safety profile, might become widely ineffective is frightening. The thought of returning to the days of major strongyle damage is a sobering thought to anyone who remembers it.
Theoretical Options
For a glimpse into what this could mean in terms of deworming options, we researched the current state of knowledge regarding ivermectin resistance in ruminant animals.
Although we found some conflicting information, most studies report that once a parasite has become resistant to ivermectin it will also be resistant at least partially, if not completely, to moxidectin and avermectin (the same class as ivermectin) as well. Therefore, it’s not safe to assume that if ivermectin isn’t working for you, moxidectin will. The good news is that one study reported at least one strain of these highly ivermectin/moxidectin-resistant parasites was sensitive to the older dewormer classes of the benzimidazoles and to levamisole.
Much more work needs to be done to establish both how widespread ivermectin/moxidectin resistance in parasites might be, how this came about, how to avoid it and how to treat these resistant strains. While there is no evidence yet that this problem is extensive, we can no longer ignore the possibility of ivermectin/moxidectin resistance.
The problem is most likely to surface on farms with a high density of horses and on breeding farms with many young horses where parasite density is high. Intensive use of ivermectin under those circumstances puts considerable “pressure” on the parasites to evolve to a resistant form.
It’s not known yet if rotational deworming using other agents would delay the resistance developing, but it’s one approach under consideration. Other experts contend that rotational deworming might contribute to the emergence of multiple resistant forms. More definitive research under farm conditions is needed.
At this point, we can’t consider paste dewormers a magic bullet and need to pay close attention to all the details of the big picture in parasite control. This means a return to the basics of a comprehensive program, including:
• Good sanitation with regular manure removal.
• Not feeding animals off the ground.
• Use of clean (no horses on it for 12 months) or at least rested (no horses on it for at least 70 days) pastures for young horses whenever possible.
• Avoid overcrowding horses.
• Maintain horses in good flesh and good general nutrition to maximize the health of their immune response, which helps them eliminate parasites naturally.
As for the possibility of drug resistance, we took pointers from other livestock-management techniques, as these groups keep their animals under more intensive, crowded conditions and must deal with this problem more often. Guidelines include:
• Do not overworm but, when you do deworm, be sure not to underdose the animal. Base dosage on an accurate body weight. Underdosage encourages the development of resistance. However, this isn’t a call to overdose your horses either.
• Consider using one dewormer class for a year at a time, or until it becomes ineffective, as judged by a fecal worm-egg count. As a rule, parasites tend to develop resistance to all drugs within a class, but not between classes. Therefore, just changing to another drug within a major class won’t help avoid resistance. You must change the actual class of drugs (see our chart at end of story).
• Isolate new animals, do a fecal test, deworm and retest. If parasites aren’t eliminated, deworm again with another class of drug until they are.
• Deworm only high-risk animals (pregnant, nursing, young, old or ill/debilitated animals) or animals with high fecal-egg counts. Not routinely treating animals with only low counts ensures that a number of nonresistant parasites will be present in the environment to crossbreed with any that may be developing resistance. This will prolong the interval to the development of drug resistance.
• Ask your vet to do regular fecal examinations, both to determine if your dewormer is still effective and also to identify those animals that need to be dewormed.
Bottom Line
Drug resistance isn’t a new problem. We all know routinely reaching for antibiotics for every minor injury, or antibiotic use in newborn foals “just in case,” encourages drug resistance.
Although we’ve been lucky until now in that resistance to ivermectin was not a problem, this appears to be changing. Reliance on good management, general health and frequent fecal parasite-egg counts to identify when the horse needs to be dewormed, rather than routine, timed dewormings, may be in order.
We have outlined four options for your deworming schedules. At this point, there is no solid research available to recommend one schedule over another. We strongly advise you to discuss your concerns with your own veterinarian and make a decision based on what’s best for each individual horse.
Also With This Article
Click here to view ”What To Do Now.”
Click here to view ”Tapes And Bots.”
Click here to view ”Know Your Dewormer Drug Class.”
Click here to view ”Deworming Schedules.”
Click here to view ”Is Your Horse At High Risk’”
