Page 111 - September 2022

EQUINE HEALTH
Threadworms can survive for many generations without the presence of a horse.” - Dr. Craig Reinemeyer
“Most practitioners recommended
starting early in order to kill Strongyloides (threadworm) infections, but this has been found unnecessary. Threadworms that foals can get directly from the mare through the milk during the first days of life were thought to be a problem and it was common to either treat the mare during her last month of pregnancy to try to prevent transmission of these, or to treat the foal very early in life to kill them after they were transmitted,” he explains.
“But it turns out that these treatments aren’t necessary. This parasite is only occasionally pathogenic; it’s not usually very damaging. You won’t lose a foal from this parasite. It’s also very easy to treat if we do diagnose a foal with a problem, and foals develop immunity to threadworms by the time they are 5 to 6 months old.”
The other issue is that no matter how you treat the mare or foal, the foal will eventually pick these worms up from pasture. “Threadworms can survive for many generations without the presence of a horse,” says Reinemeyer.
“So, these early dewormings are the first thing we were doing wrong and as a consequence, we’ve been exposing other worms, especially ascarids,
to sub-optimal doses (those that are not at high enough levels to kill all the ascarids) of whatever drug we were using for threadworms. As a result, the non-targeted parasites have been developing resistance to these drugs. Any time you treat a foal with a drug like ivermectin or moxidectin
you are treating every worm inside
that horse—wherever it might be
in the body. That’s not the
case when using Panacur at
the single one-day dose,
or Strongid (a pyrantel
product which does
not leave the gut).
A worm that is
migrating through
the lungs or
liver will not
have much
exposure
to that
drug,” he
explains.
“What we’ve done over time with our deworming methods helped to select for resistant worms that are no longer susceptible to ivermectin and moxidectin in these young horses,” says Reinemeyer.
Another disadvantage of frequent and repeated treatment is that it reduces the effectiveness of treatments. “Worms are much less susceptible to anthelmintic treatment when they are immature (and traveling through the liver and lungs and are not in the gut). As an individual worm gets older, it becomes more susceptible to these drugs. When we deworm a foal at a month of age, any ascarids present at that time are just tiny larvae and not in the gut, and we are not going to kill very many of them,” he says.
Research over the years at the Gluck
Center, University of Kentucky, showed that oxibendazole at standard label dosage was only about 40% effective against ascarid populations that were only 28 days old. “When the researchers waited until these worms were adults in the gut and laying eggs, the same drug used at the same dosage was over 95% effective. By deworming frequently and starting too young, we compromise efficacy of the drug and select for worm resistance,” he explains.
The recommendation today is to never deworm foals that are less than 60 days of age. After that,
the status of individual foals
can be checked with fecal exams. “We’ll have to accept some positive samples in the interest
of prolonging the efficacy of the few drugs we still have,” says Reinemeyer.
“We’ve always overdone treatment for ascarids in an attempt to halt
passage of eggs in feces. Once
the ascarid eggs are passed into
the environment, they can survive
5 to 10 years or more. We were probably a little paranoid about letting the worms mature to pass eggs, but we need to give up the unrealistic idea that we can eradicate worms. We must learn to tolerate them, live with them, and the goal should be to keep them down to levels that don’t compromise the health of horses,” he says.
“We are lucky with ascarids in that most horses eventually develop immunity to them just like they do threadworms. You rarely find ascarids in adult horses, and in those few instances we don’t know why they have these worms,” he says.
We need to back off in early deworming of young horses, and we also need to know which drugs are effective when we do treat. “Resistance to ivermectin and moxidectin has been demonstrated with ascarids in many regions of the country, but the biggest challenge is for breeding farms with many foals,” says Reinemeyer.
“Pyrantel is still a pretty good drug. There are some populations of ascarids that have shown resistance to it, but not very many. We still have Panacur (fenbendazole) and oxibendazole (Anthelcide). In most herds, ivermectin also still works, but evolving resistance in some worm populations is something we need to be aware of,” he says.
The discovery of worm resistance to ivermectin is discouraging, not just because there are now two types of worms that are
resistant, but because many people always thought ivermectin
was a bullet-proof panacea against worms.
“Parasitologists now envision a grim future for
“What we’ve done over time with our deworming methods helped to select for resistant worms that are no longer susceptible to ivermectin and moxidectin in these young horses.”
- Dr. Craig Reinemeyer
Strongyloides are only occasionally pathogenic and
not usually very damaging. Foals develop immunity
to threadworms by the time they are 5 to 6
months old.
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