Skip to main content

Rutgers – Ask the Expert – Deworming Question

Question: I have a question about deworming horses in New Jersey. I have been in upstate New York for four years and used a rotating schedule of Ivermectin and Pyrantel Pamoate every other month. I was wondering what the current research on deworming products is for New Jersey? Are there other dewormers that you would suggest for horses in New Jersey?

Answer: Deworming has been the subject of much research over the past few decades. As most people are probably aware, frequent rotational deworming practices have resulted in anthelmintic resistance to several internal parasites, such as small strongyles in the adult horse. A study in the southern United States ( identified that small strongyles have become 97.7% resistant to fenbendazole, 53.5% resistant to oxibendazole, and 40.5% resistant to pyrantel pamoate.

Many veterinarians are diverging from rotational deworming strategies and instead are recommending a targeted deworming program that addresses each individual horse’s parasite load and parasitic resistance to anthelmintics on that specific farm. Anthelmintic resistance on a farm can be identified by performing a Fecal Egg Count Reduction Test on a few horses.

Basically, a Fecal Egg Count (FEC) test is performed right before deworming (approximately two months after last deworming), and then again two weeks later. The percent reduction in eggs will identify if a horse’s parasites have developed resistance to the anthelmintic administered. This will need to be repeated for other anthelmintic classes to see which ones are still effective.

An FEC can identify if a horse is a high or low egg shedder (a measure of parasite load). Most horses fall into one of these categories and will not change throughout its lifetime. Low egg shedders do not pose a high risk of contaminating pastures; therefore they do not need to be dewormed as often as high shedders. Most veterinarians can perform FECs.

The guidelines concerning deworming practices are rapidly changing to combat the resistance problem. Specialists no longer recommend a cookie-cutter eight-week rotation for every farm. Your veterinarian can help you customize a deworming schedule that includes circumstantial details such as your climate, your horse’s egg shedding levels, and the level of resistance to certain anthelmintics on your specific farm.

If you would like to read more in-depth on this issue, please read the new AAEP (American Association of Equine Practitioners) Parasite Control Guidelines available here.

Dr. Carey Williams is the Extension Specialist in Equine Management for Rutgers Cooperative Research and Extension. Her research focus is in equine nutrition and exercise physiology. She coordinates “Ask the Expert,” a feature of the Equine Science Center website.

Medical Disclaimer
This material is for educational purposes only and is not intended to diagnose, prevent, or treat any illness. Any recommendations are not intended to replace the advice of your veterinarian. Any products mentioned are not intended to diagnose, treat, cure, or prevent disease. Mention or display of a trademark, proprietary product, or firm in text or figures does not constitute an endorsement by the Equine Science Center or Rutgers University and does not imply approval to the exclusion of other products or firms.

Got a question? Ask the Expert at