What to do About Worms

Resistance to dewormers is increasing. How does a horse owner cope?

by R. Paul Schwab, DVM

If you haven’t heard already, I’ll break the bad news: we are already starting to see resistance to our current dewormers for horses. The worse news is that there are no new dewormers due to come out anytime soon. What does this mean? We need to utilize other methods of parasite control besides just giving dewormers every eight weeks. 

Dewormer resistance is not a new problem, especially on the East Coast. There has been resistance to most dewormers in goats and sheep for several years, but more recently it has become an issue in the horse population. We are just starting to see some of this resistance here in the Northwest now.

Without going into too much detail, resistance has probably developed over the years from our various rotational deworming strategies. Slow rotation, or using one product at regular intervals, without monitoring by checking fecal egg counts may allow resistant worm populations to dominate the pasture without being killed. Fast rotation schedules, or rotation of different products at regular intervals, may be selecting for resistance in all the classes used in the rotation if not monitored with fecal egg counts. 

What Can We Do? 

First of all, remember how horses actually pick up worms: the worm eggs and larvae live in the horses’ manure or the grass and ground just around the manure pile. If the horse grazes on pasture contaminated with these parasites (we are talking thousands of eggs in one manure pile), they will eat the eggs or larvae. These eggs or larvae then go through several stages of development within the horse and turn into adult worms that lay more eggs in the manure to contaminate your pasture. Removal of this manure is one strategy to reduce pasture contamination. 

What about harrowing? Remember, the worms stay within the manure pat. If you spread it around, you are actually spreading the larvae and eggs around the whole pasture instead of containing them in the manure pile. If you are not removing manure, you are better off leaving the “islands” of manure pats because usually—though not always—horses will avoid eating around the manure. Harrowing will help only if the ambient temperature is above 85˚, sunny, and the pasture can have no animals on it for at least four weeks (two months or more would be better).

Other practices that may help with reducing pasture burden include grazing on fields used recently for hay and rotating pastures with other livestock species (cattle, sheep, etc.).

Multiple-Horse Areas

We have always treated herds as a whole. If you deworm one, you assume they all have the same exposure and deworm everyone at the same time. We may need to rethink our strategies and identify individual animals that have more susceptibility to parasites. Geriatric and young horses, in general, are more prone to parasite infestation, but there also adult horses that may shed more worms and have less immunity to fight off parasite burdens. Using fecal sample monitoring, we can try to identify these animals and utilize that information to determine how often to deworm individual animals that shed more eggs into the environment.

Monitoring Egg Counts

Another way to utilize fecal egg counts is to monitor the effectiveness of individual deworming medications on the parasites on your farm. This is done by having a fecal test done the day that you deworm your horse. A follow-up fecal sample should then be taken ten to fourteen days after deworming. The formula for reduction is as follows:

[(Pre-Treatment Eggs/gram feces – Post-treatment Eggs/gram) / Pre-Tx EPG X 100]

Numbers above 90% indicate that the product is efficacious, and numbers below 80% indicate resistance. Numbers between 80-90% are questionable. This formula is a good tool but was developed with other species, not horses, so it should not be a gold standard. Further research may adjust this formula in the future, but for now it is very helpful to give you a general idea.

This is a very hot topic right now and much is still to be learned. As equine veterinarians and horse owners, we are in for a challenge over the next few years to get our arms around this issue. We have all trained ourselves to follow a schedule that until now has been the most consistent way to keep horses healthy and free of parasites. My advice is to sit down with your veterinarian to discuss a plan for parasite control for your individual farm situation. Utilize your veterinarian’s lab to assess your horses’ parasite load. You may actually find that your horses need to be dewormed far less often than you thought.