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,
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)
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.
This cyathostome is at the L3 (infective) stage.
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.
However, the control strategy is to understand egg counts and create a program to minimize shedding. (PHOTO: COURTESY OF
DR. ED KANE)
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
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.