For decades, the accepted deworming practice has been to rotationally deworm horses every two months. But in the face of parasite resistance, rotational deworming has quickly become a problem. Here are the facts on deworming, including the importance of fecal egg counts and how to adjust your deworming program.

The Problem of Parasite Resistance

We have just a few different wormer active ingredients in our arsenal: fenbendazole, pyrantel, ivermectin, and moxidectin. Though there are countless different wormer brands, they all use these same ingredients to kill off intestinal parasites. The problem occurs when parasites develop a resistance to these ingredients. As parasites develop a resistance, they pass that resistance on to latter generations. And, according to The Horse, once a parasite species has developed a resistance to a particular drug, they are unlikely to ever become susceptible to that drug again.

This can pose a serious problem: If parasites become resistant to the effective wormers that we do have, we will eventually reach a point where we can’t kill off parasites with a wormer. We will need to develop new worming products, and that can take time. It’s not a situation we ever want to be in.

Parasite resistance can differ from farm to farm, especially depending on how frequently a farm deworms their horses.

The New Deworming Protocol

Selective deworming is now the accepted standard protocol to avoid parasite resistance. Rather than deworming every two months, when you use selective deworming, you deworm your horse only when necessary to reduce fecal egg counts.

A fecal egg count determines the number of parasite eggs present in each horse’s manure. You will need a fecal egg count for each horse in your stable, and these must be performed repeatedly to monitor parasite presence in horses. Fecal egg counts can identify if one horse in your stable is more susceptible to internal parasites than others, and it can also highlight if particular worm species are becoming resistant to the deworming products that you are using.

The benefit of fecal egg counts is that you may have to deworm your horse less often, or even not at all during the course of a year. Since dewormers are chemicals, reducing the frequency that a horse is dewormed can be a health advantage.

Selective deworming also incorporates consideration of the seasons when internal parasites are most active. Deworming efforts should be concentrated during the times when transmission is most likely, and avoided during the parasite “off season.” The “off season” is usually the winter in Northern temperature zones, and the summer in Southern temperature zones.

A Note About Young Horses

While fecal egg counts are relatively accurate in adult horses, they are not as reliable when performed in younger horses and foals. Ascarid worms can accumulate in foals without fecal egg counts detecting the levels, and in cases of extreme worm buildup, they can even cause small intestine impactions. When working with foals and young horses, a rotational deworming program is still recommended.

Your Next Steps

Ultimately, you should develop a deworming program that works for your barn. Your vet can help you to establish a deworming program considering the risk factors of the types of horses you have and your pasture maintenance program. With an ideal program in place, you’ll deworm your horses less frequently while helping to ensure that the deworming products you use stay effective.