Deworming Horses: Predatory Fungi May Help

Due primarily to the indiscriminate use of dewormers over the past several decades, widespread resistance to chemical deworming products has become a major concern for horse owners.

In the wake of that resistance, horses are once again susceptible to parasite-related diseases, such as colic and diarrhea. Researchers dedicated to the development of novel deworming products and strategies struggle to fill the void created by the misuse of chemical dewormers. One recent study* suggests efforts by researchers may prove fruitful, and the fruits of their labor could be fungi.

“As we know, horses become infected with internal parasites by ingesting eggs laid by adult internal parasites living within their gastrointestinal tracts. Eggs shed in the feces populate the environment where horses graze and are subsequently ingested, ultimately growing into adult parasites,” explained Kathleen Crandell, Ph.D., an equine nutritionist with Kentucky Equine Research (KER).

It is a proverbial vicious circle, and one that was only temporarily interrupted by the use of chemical dewormers when those products were still effective. Now, without reliable chemical dewormers, parasitologists are considering ways to effectively use fungi to fight parasites.

Using data from previous studies, Hernández and colleagues knew that some forms of fungal spores were parasitic to certain parasites. Spores from the fungi Duddingtonia flagrans and Monacrosporium thaumassium proved especially interesting. They also knew that those fungi were hardy, capable of surviving the hot, humid conditions required for making pelleted feeds.

To determine whether the spores of D. flagrans and M. thaumassium embedded in pelleted feeds could effectively deworm horses, the researchers manufactured a pelleted feed with fungal spores. Fecal egg count reduction tests and egg reappearance times, among other parameters, were measured to determine efficacy.

Feeding fungal spores to horses resulted in a prolonged depression of the egg reappearance rate, or the length of time that it took for horses to begin shedding parasite eggs in their feces. Once eggs reappeared in feces, horses that received the spore-embedded feed had lower numbers of parasite eggs per gram of feces than horses fed the control diet lacking the fungi spores for the 56 days of the study. Finally, the horses did not react to the fungal spores and did not become “sensitized,” meaning no immune response occurred after feeding fungal spores to the horses.

Overall, the study authors concluded that feeding horses with pelleted feeds and fungal spores appears to be a useful means of controlling some parasites.

“This study also highlights the importance of pasture management in the control of internal parasites rather than relying solely on a chemical or fungi to single-handedly control equine internal parasites,” Crandell added.

Article reprinted courtesy of Kentucky Equine Research (KER). Visit  for the latest in equine nutrition and management, and subscribe to The Weekly Feed to receive these articles directly (