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Parasite Control:
New Research Shows Parasites are Becoming Resistant to Ivermectin & Pyrantal Pamoate

Daily wormer? Two way rotation? Three way rotation? Annual rotation? Or dosing only after routine fecal testing detects a problem? Parasite control is hardly an exact science. Clearly, we don't want our horses walking around with such a grievous parasite load that their health suffers. And so we choose one of the above ideas, and pat ourselves on the back for being good owners.

Unfortunately, some of the anthelmintics we have relied on for years are becoming predictably ineffective against some of the most common equine intestinal parasites. Routine use of ivermectin and both pyrantal pamoate and pyrantal tartrate in horses, cattle and sheep have largely killed off the gene pool of parasites susceptible to those drugs...and created a huge population of the gene pool that are resistant.

In the Tijuana River Valley, and indeed, in San Diego County in general, cases of habronemiasis (summer sores) are on the rise. Habronemiasis is caused by a small stomach worm run amok...a small stomach worm that used to be easily controlled with ivermectin. In fact, the most recent research, and still the current prescription for summer sores is ivermectin, both topically, and administered orally on an accelerated schedule during the summer months. A couple of years ago, it worked. Now, however, ivermectin appears to be wholly ineffective. If you doubt the research suggesting ivermectin is no longer a viable agent, watching a horse be eaten alive by summer sores will quickly make you wonder.

While ivermectin appears to be ineffective for eradicating the harbonema larvae that cause summer sores, as well as roundworms, ivermectin is still more effective than other drugs for controlling small strongyles. But recent research indicates that small strongyle eggs reappear more quickly after worming with ivermectin than previous studies had shown.

Parasite resistance to the drugs routinely used to eradicate them is nothing new; it is quite simply natural selection at work.

Are our standard worming protocols literally too much of a good thing? Growing contingents of horse people are starting to think so. Many experts agree that routine fecal testing is a valid -- and probably smarter -- option. Instead of dosing on a schedule, test on a schedule. Use the results to determine which parasites, if any, are in abundance, and then select the wormer best suited to take care of that particular parasite. Testing again, two weeks after worming, will tell you if the treatment was effective.

Sound like too much work or expense? Not really. Test-at-home kits, which allow you to send a sample to a lab for processing within 24 hours, run around $20. They even help you choose the appropriate class of dewormer. It's not as simple as just following a rotation program and dosing on a schedule. But it's a far more accurate way to address the problem.