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 activity have been proposed as solutions, but whether such additives can simultaneously exert dual bioactivities within this context has rarely been explored. Here, we evaluated whether selected terpenes could simultaneously exert dual bioactivities (anti-parasitic or 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. • Terpenic compounds have anti-parasitic activity against equine strongyles in vitro • These compounds also modulated parasite community structure in vitro • Both compounds exerted immunomodulatory effects in equine monocytes • Cinnamaldehyde did not lower parasite burdens during a 28 day grazing study • Further studies are needed to bridge the gap between in vitro and in vivo