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
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."