By now, many horse owners have likely heard that rotational deworming is outdated. As a result, many are working with veterinarians to use fecal egg counts (FEC) to determine customized deworming schedules. As more horse owners turn away from rotational deworming, different questions arise around timing.

There are two main parasites—bots and tapeworms—that are targets for fall or winter dewormings:

  • Bots—Bots are the larvae of the botfly, which are common in barns and pastures where horses live. Owners often identify botfly eggs on their horse’s hair, especially on the legs. Deworming with ivermectin yearly, during late fall or early winter, is recommended as a “cleanout treatment” for bots, which will help decrease transmission in the next season.
  • Tapeworm—Adult tapeworms are fairly common and live in horses’ intestines; however, they are difficult to diagnose. Horse owners will rarely see tapeworm segments in manure, unlike some other parasites, and there is no reliable diagnostic test for active tapeworm infection.But because tapeworms have potential to cause disease like colic, a properly timed tapeworm treatment is beneficial. In most U.S. areas, deworming with praziquantel in combination with ivermectin is recommended during the late fall or winter. Cold weather means the end of tapeworm transmission, and that timing for treatment helps diminish additional transmission the following grazing season.

“Parasite control is an important part of any horse health program,” says Hoyt Cheramie, DVM, MS, DACVS, manager of Merial’s Large Animal Veterinary Services. “Choosing the right deworming product and the right timing for it is key.”