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Cyathostomins, or small strongyles, are the most pervasive internal parasites of horses and the primary targets of adult deworming programs. The larval stages of this species, which become encysted in the large intestine’s mucosa, are what cause disease and clinical signs in horses. In large numbers they can cause a rare, life-threatening disease called larval cyathostominosis—a mass eruption of encysted larvae that leads to a profound inflammatory response in the horse’s gastrointestinal tract.
“Concerns have been raised that killing larvae while encysted within the mucosal walls could lead to adverse (e.g., inflammatory) reactions,” said Ashley E. Steuer, DVM, PhD, assistant professor of parasitology at Texas Tech University’s School of Veterinary Medicine, in Amarillo.