Terpenic compounds possess anthelmintic and immunomodulatory properties with potential for controlling equine cyathostomin infections.

The emergence of anthelmintic-resistant parasite isolates necessitates alternative control strategies. The use of dietary additives with anti-parasitic or immunomodulatory activities has been proposed as a solution, but whether such additives can simultaneously exert dual bioactivities within this context has rarely been explored. Here, we evaluated whether selected terpenes could exert dual bioactivities (anti-parasitic and immunomodulatory) with a view to their use as food additives that can limit strongyle infections in horses. In vitro tests with cyathostomin larval development, larval migration, and immune modulation showed that cinnamaldehyde and carvacrol possessed high activity, and nemabiome analysis revealed that both compounds modulated cyathostomin community structure. Moreover, they also displayed significant anti-inflammatory activity in equine mononuclear cells and modulated gene expression induced by parasite antigen in monocytes, including an upregulation of an antioxidant defense network. However, no effects of cinnamaldehyde were observed on parasite egg excretion or host blood cell profiles during a 28 day in vivo feeding study. These data suggest that whilst terpenes have broad-acting bioactivities, gaps between in vitro properties and in vivo efficacy need to be overcome to realize their potential to improve host resistance to parasites.