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


DVM360 MAGAZINE


Treating by necessity

Although they only amount to about 20 percent to 30 percent of the animals, high egg shedders distribute more than 80 percent of the eggs onto the pasture. By treating these animals more intensively, you not only help them directly, but you also help other horses indirectly, by not allowing the heavy egg shedders to contaminate the pasture. "By using a selective treatment approach, the more heavily parasitized horses that need the treatments will get them," says Kaplan. "Less treatments are given to the horses that don't need to be treated."

By not treating the low egg shedders, you're allowing them to contaminate the pastures with a relatively small number of eggs. Therefore, you're not applying any selection pressure for resistance on the parasites from the low egg shedders. On the other hand, if you treat the high egg shedders with drugs shown to be effective, you can decrease total egg shedding by as much as 95 percent.

If you're using a drug such as ivermectin or moxidectin, which should decrease the egg counts by 99.9 percent, then the treated animals are shedding only a small fraction of all eggs on the pasture. Therefore, if you treat half the animals with these effective drugs — and do nothing with the other half (low egg shedders) — you will decrease the total egg shedding by about 95 percent. After treatment, about 98 percent of all the eggs will be shed by the untreated animals, supplying what is called "refugia," the non-drug-selected portion of the population that helps dilute any resistant worms.

By following this regimen, the evolution of resistance is greatly slowed. Although there is still selection toward resistance taking place, it's occurring at a much slower rate, because you're allowing all of the unselected refugia to remain in the overall parasite population. This concept is the key idea behind the selective treatment of high versus low egg shedders. Not only does it prevent a lot of unnecessary treatments, but it also allows you to focus your treatments on the horses that really need it — and slow down the progression of resistance in the process.

Checking egg counts

The overriding theme of horse parasite control is an irrational fear of horse parasites dating back to the 1960s. Horse owners and veterinarians have fallen into the same trap — a paranoia of horse worms. The belief is that we have to treat frequently; otherwise, our horses are going to get sick and die of worms. We think that if we don't treat, we are either being negligent owners, or we, as veterinarians, might be liable for horses that get sick and die. The traditional deworming strategy does not match the reality of the biology. That's why a change is necessary, Kaplan says.

Once owners and veterinarians start checking egg counts, which is the key to success, they'll see that many of their horses most often have zero egg counts, leading them to question, "Why am I having to treat him all the time?" Or if they use a drug and do an egg count before and after treatment — and realize that the egg count did not decrease — then they'll ask themselves, "Why am I treating this horse with this drug?"

Winds of change

Once members of the equine community realize that these horses are healthy after six months without treatment, they will start to realize the futility of their former approach. However, this change in mindset doesn't come easy — it requires a total shift to initiate change.

This trend has already begun, Kaplan says. "Recently, increasing numbers of veterinarians are picking up on this message and modifying their thinking and approaches about deworming," he says. "We've got a long way to go, but in the last year or two we're actually starting to see a shift, which is encouraging."

Because moderate control efforts will keep adult horses healthy, it's unnecessary to treat every two months. For about 50 percent of horses (low egg shedders), a treatment once or twice per year is adequate. For the 25 percent that are moderate egg shedders, three or four treatments per year is recommended. The remaining 25 percent of high egg shedding horses need four to five treatments per year. In addition, it's important to remember that climate and type of dewormer used also affect the outcome.

If veterinarians continue to apply deworming treatments to adult horses six times per year — which has become customary practice — Kaplan insists the drug resistance problem will only worsen. "Strategies to decelerate further selection of drug resistance, thereby extending the lifetime of currently effective anthelmintics, need to be implemented whenever possible," says Kaplan. "This goal can best be achieved by treating the right horse with the right drug at the right time."


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Source: DVM360 MAGAZINE,
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