A Changing Paradigm on Deworming Part 1

Work smarter, not harder, at preventing anthelmintic resistance.

When I got my “smart phone,” I happily programmed my horses’ deworming dates into its calendar, so that I’d get a reminder every few months when it was time to treat my small herd.

Turns out, that wasn’t “smart deworming” at all.

Dr. Cyprianna Swiderski, a veterinarian at Mississippi State University, says there's a new paradigm in deworming, a smarter way to manage parasites while guarding against resistance to the drugs that expel worms from the body (also known as anthelmintics). She advocates a targeted deworming strategy, evaluating horses’ individual worm loads and managing them accordingly, rather than potentially over-treating some less-infected horses by simply going by the calendar.

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First, let’s talk about how resistance builds to these drugs. Tom Kennedy is senior vice president of research and development at AQHA Corporate Partner Farnam. He says resistance to anthelmintics is “an unavoidable thing. If you remove a population of parasites from an animal (by deworming), you’ve removed the susceptible population.” But no dewormer will kill every single parasite every time, and “the ones that don’t respond to the anthelmintic are the ones that remain, and they’ll be the ones that populate the pasture or paddock.”

It is, in effect, “selecting for a worm that has the potential to survive in the face of treatments with that particular class of anthelmintics,” Tom says. And rotating dewormers will actually breed resistance to multiple classes of dewormer.

Enter the concept of “refugia” – a population of parasites that has not been exposed to an anthelmintic. They survive by either being outside the horse (in the pasture) when he is treated, or by being encysted inside him (only a few types of dewormer will kill encysted larvae; the others will leave them unscathed). As these parasites reproduce, they create future generations of worms that the dewormer will be effective against.

It may seem odd to think about managing a worm population instead of trying to obliterate it, but by understanding the development of drug resistance, horse owners can ensure the continued efficacy of the current selection of dewormers – important since there are no new ones coming in the foreseeable future.

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“We really have to husband the anthelmintics that we have,” Tom says. “To use them more correctly will prolong their lives as effective anthelmintics.”

Smart Deworming

Dr. Swinderski and Tom say that veterinarians must become more involved in deworming programs, helping horse owners make sense of all the variables. The protocol that Dr. Swiderski advocates focuses on the management of small strongyles because those are of primary concern in adult horses. (Ascarids are the major parasites in horses under age 2.)

One place where veterinarians can help is with evaluating the local climate to determine precisely when the small strongyles are most active. As general rules of thumb, in hot summers – 85 degrees Fahrenheit and above – the infective stage of small strongyle larvae die off. In cold winters – 45 F and below – there are smaller numbers of those infective larvae. Cold temperatures don’t kill the larvae, but they do prevent earlier stages of larvae from molting into the stage where they are infective.

So if you live in the sultry South, your small strongyle season will start in the fall, once temperatures drop below 85. If you’re in the northern latitudes, the parasite season will begin in the spring, once temperatures rise above 45.

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To start with a “smart deworming” plan, your veterinarian will perform a fecal egg count test at the beginning of your horse’s parasite season. The results will give you a good idea of the kind of horse you’re dealing with.

Because the horse has just gone through a season where there were few small strongyles in the environment, if he is excreting a large number of worm eggs, that means he is highly permissive to a small strongyle infection. His body allowed ingested larvae to complete their life cycle so that parasite eggs were excreted. It’s important to identify these horses because they are the primary sources of contamination of the pasture.

The highly permissive type of horse is what Dr. Swiderski calls a “Typhoid Mary,” and he will have to be dewormed more frequently than other types. Other horses will have a minimal egg count, reflective of a stronger immune system and a better ability to naturally fight off a parasite infection. Still others will fall somewhere in the middle.

Check back to learn more about disease resistance in Part 2!