Deworming: Less is More

Deworming: Less is More

Check to see if your dewormers are working by running two egg counts: one at the time of deworming and another one about 14 days later.

Photo: Michelle N. Anderson, Digital Managing Editor

Q. Last spring my veterinarian requested her clients stop deworming their horses. We now do no deworming and have her perform two fecal egg counts per year—one in the spring and one in the fall. I'm concerned this isn’t comprehensive. I own two horses and one is a 31-year-old senior with recurring uveitis and Cushing’s. Is this program of not deworming okay for my horses?

Heather, Pittsburg, Pa.

A. Your veterinarian has a point: There’s no doubt that the majority of horses in North America are over-dewormed, and the large majority of adult horses require very little deworming if at all.

Although all horses get internal parasites, the risks of actual disease caused by these are very low in adult horses. Foals and yearlings are more susceptible to infection and hence more at risk. However, in our guidelines for equine parasite control (published by American Association of Equine Practitioners) we do recommend a baseline of one to two annual dewormings that all adult horses should receive. The reason is that the fecal egg count mainly reflects the level of egg shedding with small strongyles, which are by far the most common and abundant parasites infecting horses. But other potentially important parasites such as tapeworms and large strongyles (bloodworms) are not readily reflected by egg counts.

Our research has shown that the bloodworms can become more common if horses are left completely untreated. Therefore we recommend these treatments as a safety precaution (discuss the specific class of anthelmintic drug you should use with your veterinarian). The most relevant time to consider a treatment for your horse is in the early to mid-fall, after he has grazed all summer. During the grazing season both tapeworm and strongyle burdens will accumulate to reach peak levels in the fall, so this is a good time to break the life cycle and lower the parasite burdens.

It’s important to routinely check the efficacy of your deworming drugs to make sure that they work as intended. Many people are living with a false sense of security by routinely use dewormers that have lost their effect. You can check your dewormers to see if they’re working by running two egg counts: one at the time of deworming and another one about 14 days later. We recommend testing at least six horses on every farm.

About the Author

Martin Krarup Nielsen, DVM, PhD, Dipl. ACVM

Martin Krarup Nielsen, DVM, PhD, Dipl. ACVM, is an associate professor of parasitology and the Schlaikjer professor in equine infectious disease at the University of Kentucky's Maxwell H. Gluck Equine Research Center, in Lexington. His research focus includes parasite diagnostic measures and drug resistance. Known as a foremost expert in the field of equine parasites, Nielsen chaired the American Association of Equine Practitioners’ (AAEP) parasite control task force, which produced the “AAEP Parasite Control Guidelines.”

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