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The new equine parasitic threat
Understanding biology is key to battling the emergence of resistant parasites in horses


DVM360 MAGAZINE



Worm wars: Selective treatment is the new deworming strategy, says expert Dr. Ray Kaplan, especially as the threat of resistance intensifies. This photo, at 1300x magnification, shows the anterior end of Cylicostephanus longibursatus.(PHOTO: COURTESY OF DR. SUE HOWELL)
For the past several decades, anthelmintics have fought off equine intestinal parasites. In recent years, however, things have changed, as some of these parasites have risen to what veterinary expert Dr. Ray Kaplan calls "the chemical challenge." "Today most horse owners continue to follow recommendations that are based on knowledge that is 30 to 40 years old," says Kaplan, DVM, PhD, Dipl. EVPC, and associate professor at the University of Georgia, College of Veterinary Medicine in Athens, Ga.


This cyathostome is at the L3 (infective) stage.
To complicate the issue further, the parasite of dominance has also shifted. According to Kaplan, prior to the early 1980s, large strongyles, including Strongylus vulgaris, were the dominant intestinal parasites seen in horses, frequently causing colic due to their migrating arterial stages. Since that time, however, he explains that cyathostomes (small strongyles) have emerged as the leading culprit, frequently accounting for nearly 100 percent of the strongyle worm egg output of grazing horses — and recognized today as the principal parasitic pathogen of the horse. "Contributing to the pathogenic potential of these parasites is the problem of drug resistance, which is now reaching alarming levels," Kaplan cautions.


­However, the control strategy is to understand egg counts and create a program to ­minimize shedding. (PHOTO: COURTESY OF DR. ED KANE)
Anthelmintic-resistant cyathostomes are now highly prevalent in horses. Even where drugs are still effective, the egg reappearance period (ERP) following treatment has become significantly shorter. Despite this reality, one positive for the horse world is the fact that small strongyles have a relatively low level of pathogenicity. "If they were more pathogenic worms, we would have been in a huge world of hurt in the equine industry a long time ago with all the resistance we have," says Kaplan.

Now that we've had a quick history lesson on where we've been on this subject, let's take a closer look at the current state of the deworming debate. (Note: The following discussion pertains to adult horses only. Treatment regimens and concerns for foals and yearlings are substantially different than those for adult horses.)

Resistance is inevitable

The development of resistance can't be stopped, but veterinarians can control it, Kaplan says. The nature of the biology of genetic selection for resistance in parasitic worms can even be described mathematically, explains Kaplan. "It starts out being a very rare trait — and is invisible — as it initially increases over time," he says. "Every time you treat, you're increasing the relative number of resistant worms by killing off the susceptible worms. By the time you're seeing decreased efficacy of the drugs, you've passed the threshold — and there is nothing you can do to change the course of the development of resistance."

At this point, the game is basically over. To combat resistance before reaching the threshold, you must change the selection pressure you're putting on the parasite population. "In regard to treating cyathostomes with ivermectin and moxidectin, we're not at that end point yet, though it appears we're getting close," says Kaplan. "For now, we don't know how close we are. However, based on all appearances?— with eggs coming back faster and faster after treatment — it looks like we're finally to a point where ivermectin- and moxidectin-resistant strongyles are a realistic inevitability in the near term."

Consequently, Kaplan has submitted a grant for a research project to commence in 2010 that could help us understand how close we actually are.

Modernizing control

It's important for equine practitioners to be educated on parasite biology in order to deal with the emerging issues surrounding parasite control practices effectively. When it comes to developing a treatment approach, veterinarians must remember that every horse is infected with cyathostomes in different stages of development.

For example, disease (when seen) is not caused by adult worms, but rather by larval worms emerging from the mucosa, causing lots of gastrointestinal damage. We see this all the time in horses at a subclinical level. If you see eggs in a horse fecal sample, that means there are adult worms in the animal's gut — and somewhere in the recent past the larvae came out of the mucosa. Even so, as long as the population is maintained at a biological equilibrium, it causes very little health damage to the horse.

"Nearly all anthelmintics fail to kill the encysted larval forms that are causing the pathology," says Kaplan. "Therefore, when you deworm a horse, you are only killing the stage of the parasite that is not really harming the horse — the adult parasite."

The benefits of this approach, says Kaplan, are that you're decreasing the amount of egg shedding that is occurring onto the pasture. Decreasing egg shedding (and therefore preventing future infections) is beneficial to promote the health of horses as well as to provide equine practitioners with a strategy to slow down the resistance.

About 50 percent to 60 percent of all the adult horses have very low egg counts (less than 200 eggs per gram). In fact, many of them will have egg counts of either zero or approaching zero, whether you treat them or not. Therefore, those horses don't need to be treated very often. Because they are not shedding many eggs onto the pasture, they just need one or two properly timed treatments during the year.

The bottom line is that the threat of worms to the health of an adult horse is highly overrated. If most adult horses were never treated for small strongyles in their lifetime, they would probably appear perfectly healthy. However, a certain percentage of horses are more affected than others — these animals, which amount to approximately 20 percent of horses, may develop clinical signs if you don't control the parasites. "As we get more resistance and treatment failures, those are the animals that are going to start showing the problems," cautions Kaplan.


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