In recent years researchers have updated their recommendations for controlling internal parasites, or what many horse owners refer to as worms, in horses. Historically, horse owners were taught internal parasites can potentially wreak havoc on horse health, causing a variety of health issues from dull coats to fatal colic. “I am not a fan of all this doomsday talk,” says Martin Nielsen, DVM, PhD, DVSc, Dipl. EVPC, ACVM, professor of equine clinical sciences at Aarhus University, in Denmark. “Parasitic disease is so extremely rare, and the overwhelming majority of horses tolerate parasites well.”
Instead, Nielsen, who chairs the American Association of Equine Practitioners’ (AAEP) parasite control task force, which produced the AAEP Parasite Control Guidelines, recommends horse owners implement a parasite control program that minimizes the risk of developing parasitic disease and delays further parasite resistance to dewormers.
Nielsen reviewed common equine parasite misconceptions among horse owners.
Misconception #1: Deworm your horse at the first frost.
Nielsen: Parasites generally do very well at freezing temperatures. The colder it is, the longer eggs and larvae on pasture will survive. Rather than deworming when it is very cold, it makes better sense to deworm during active strongyle parasite transmission, which would be at substantially warmer temperatures (45℉-85℉).
Misconception #2: High fecal egg counts mean many parasites in horses.
Nielsen: No correlation exists between fecal egg counts and parasite counts inside the horse, so a higher egg count cannot be interpreted as a larger worm burden. We use egg counts for three purposes:
- to determine deworming efficacy (very important)
- to screen for high shedders (i.e., horses that produce lots of eggs and add substantially to the infection pressure)
- to test for ascarid parasites because these often require different dewormers
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Misconception #3: Some horses do not have any internal parasites.
Nielsen: This does not exist. All horses have parasites all the time. Some have more and some have less, but they all have them. No deworming agent will completely eliminate all parasites from the horse.
Misconception #4: Treating horses at less than the labeled dose is effective.
Nielsen: Horses should be dewormed at the labelled dose. Treating with half the dose does not provide any benefits or protect the horse against adverse effects, and it is a common misconception that halving the dose only kills half the parasites. No such linear relationship exists.
Misconception #5: Label information tells you everything you need to know.
Nielsen: Label information represents historical information from when the products were first introduced decades ago, and it does not account for the current drug resistance levels we see. The only information horse owners can trust is that obtained through appropriate resistance testing done with their own horses.
Misconception #6: One dewormer can target all parasites.
Nielsen: This does not exist. No current products can be expected to effectively treat all target parasites in a horse. Testing is the only way forward.
Misconception #7: Drug rotation prevents parasite resistance to anthelmintics.
Nielsen: Drug rotation was practiced for decades and did not prevent resistance. On the contrary, we now have multidrug resistance and many cases of complete treatment failure (where none of the products work effectively).
Take-Home Message
As is often the case with equine health care issues, the best parasite control program a horse owner can implement is developed with the input of a veterinarian familiar with your management system and your horses.