Australian plants show anthelmintic activity toward equine cyathostomins in vitro

Anthelmintic resistance in gastrointestinal parasites of horses is an increasing problem, particularly in cyathostomins, and there is a need to find alternative means for the control of these parasites. We screened crude extracts from 37 species of Australian native plants for their anthelmintic activity in vitro against cyathostomin larvae (development from egg to third larval stage), with the aim of identifying those species that may be suitable for incorporation into sustainable parasite management programs. Water extracts from seven species, namely Acacia baileyana, Acacia melanoxylon, Acacia podalyriifolia, Alectryon oleifolius, Duboisia hopwoodii, Eucalyptus gomphocephala and Santalum spicatum completely inhibited larval development (100% inhibition compared to the control), while another 10 species caused 90% inhibition at the initial screening concentration of 1400 μg of extractable solids/mL. The seven most potent extracts produced IC50 values (concentration of extract which resulted in a 50% inhibition of development) in the range 30.9-196 μg/mL. Fourteen extracts were incubated with polyvinylpolypyrrolidone (PVPP) before the assays, which removed the anthelmintic activity from 12 of these extracts, indicating that tannins were likely to be the bioactive compound responsible for the effect, while in two species, i.e. A. melanoxylon and D. hopwoodii, compounds other than tannins were likely to be responsible for their anthelmintic action. Our results suggest that a number of Australian native plants have significant anthelmintic activity against cyathostomin larval development in vitro. There is potential for these plants to be used as part of sustainable parasite control programs in horses, although more research is needed to identify the compounds responsible for the anthelmintic effects and confirm their activity in vivo.