The new equine parasitic threat
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.
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.