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.
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.
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."