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Parasites in Horses: Small StrongylesBy Kentucky Equine Research Staff · July 31, 2017

Internal parasites remain important health concerns for horses in all corners of the globe. In younger horses, roundworms (Parascaris spp.) can cause colic, ill thrift, and dull hair coats. Older horses, however, are primarily infested with cyathostomins, known more commonly as small strongyles. In fact, almost 100% of horses, regardless of how well they are managed, have cyathostomins.

While you may think that there is only one “type” of small strongyles, a recent study* indicated that there are actually 50 different species of cyathostimins capable of infecting horses. More importantly, an individual horse can be parasitized with as many as 15 different species at a time.

Why does this matter, you ask? After all, you can just dash off to the local tack shop or even shop online to purchase a dewormer and poof! Problem solved, right?


Three major classes of dewormers (anthelmintics) for horses currently exist, and cyathostomins affecting horses have already developed resistance to two of the three: benzimidazoles (e.g., fenbendazole) and pyrantel. This leaves the macrocytic lactones (e.g., moxidectin) as the proverbial “last man standing.” Experts believe that resistance will also develop against this latter class, leaving owners and veterinarians with no effective chemical dewormers for treating this ubiquitous class of parasite.

Knowing which species of cyathostomins infect horses could provide more information about their life cycle and how they develop resistance to chemical dewormers.

“In lieu of relying on chemical dewormers to single-handedly control internal parasites in horses, owners are encouraged to implement other control measures such as pasture management,” advised Catherine Whitehouse, M.S., a nutrition advisor for Kentucky Equine Research (KER).

She added, “While there are no proven ‘natural’ means of effectively deworming horses, ensuring optimal nutrition and health certainly helps horses build natural immunity to parasites.”

To ensure your horse’s diet supports maximal health, start a conversation with a KER nutrition advisor here.

“In addition, KER offers two products, the marine-derived fish oil product EO•3 and Nano•E, a water-soluble vitamin E product, that help boost your horse’s immune system,” Whitehouse advised.

*Bredtmann, C.M., J. Krücken, J. Murugaiyan, et al. 2017. Nematode species identification-current status, challenges and future perspectives for cyathostomins. Frontiers in Cellular and Infectious Microbiology. 28;7:283.