Page 110 - September 2022

EQUINE HEALTH
DEWORMING HORSES:
PROBLEMS WITH DRUG RESISTANT WORMS
Tby Heather Smith Thomas
here are several kinds of internal parasites
that are damaging to horses. Most horse
owners routinely deworm their horses to keep these parasites under control. The problem now is that some of our best deworming drugs are no longer as effective for killing worms as they were in the past. Some of the parasites are becoming resistant to their effects. Rotating dewormers can help - and knowing how and when to change drugs, but isn’t a complete answer to this challenge.
Knowing the parasites’ life cycles helps us create a more effective plan for dealing with them, utilizing a combination of chemical dewormers and careful pasture management. Dennis French, DVM, University of Illinois, says the important thing to understand is that routine deworming based on a calendar schedule is not the way to go. This is an inefficient way to
get rid of parasites, but can also be detrimental and lead to more resistant worms. “We’ve had resistance problems with the benzimidazoles for many years. Now there are increasing reports of ascarid resistance to the avermectins (ivermectin and moxidectin),” he says.
The pharmaceutical companies have not been coming up with any new drugs, so we need to take a different direction. We need a different mindset to try to break the parasite cycle by determining which horses are shedding parasite eggs onto the pasture, and only treating them.
A relatively small percentage of horses in any group are shedding the highest number of eggs. “If we treat those individuals and minimize recontamination of the pasture, the rest of the herd will be fine,” French explains. If we reduce the use of dewormers, targeting just the shedders, we prolong the effectiveness of deworming drugs.
Since the advent of modern deworming drugs - the various benzimidazoles in
the 1960’s and ivermectin in the 1980’s, horsemen have been diligently dosing their animals to try to keep them free of worms. Now we have an increasing number of worm populations that have developed resistance to many of our products. They are not as effective anymore and we need to develop different strategies.
Dr. Craig Reinemeyer, East Tennessee Clinical Research, Inc., says that one
major change should be relaxation in how intensively we deworm foals. “Traditionally, we started deworming them very early, often at less than a month of age, and repeated the treatments as often as monthly or bimonthly during the first year of life. In hindsight, this was a very bad idea,” he says.
“Knowing the parasites’ life cycles helps us create a more effective plan for dealing with them, utilizing a combination of chemical dewormers and careful pasture management.”
108 SPEEDHORSE September 2022
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