A potential novel anthelmintic? The cysteine proteases show potent anthelmintic activity against cyathostomins in vitro

Anthelmintic resistance is a global problem and constitutes a major threat to the welfare of equids worldwide. The cyathostomins are the most numerous and pathogenic gastrointestinal nematode (GIN) of equids in the developed world. Cyathostomins show widespread resistance to 2 out of 3 of the major classes of anthelmintic and recently there are reports of reduced efficacy to the potent macrocyclic lactones (MLs). None of the 3 novel classes of anthelmintic that have emerged in the last decade are licensed for use in equids. The cysteine proteases (CPs) are plant proteins that have shown potent activity against GINs in vivo in sheep and pigs.


This study aimed to evaluate the anthelmintic effect of the CP papain on cyathostomins in vitro using the egg hatch assay (EHA) and larval migration inhibition test (LMIT).


Samples of cyathostomin eggs and third stage larvae were collected and cultured from a population of equids that have recently shown reduced ML efficacy in vivo. The EHA and LMIT were performed on repeated samples with increasing concentrations of papain. Dose–response curves were plotted and PROBIT analysis performed on the data to give EC-50 values (concentration that gives 50% of the maximal response).


Papain caused a dose dependent inhibition of both egg hatch and larval migration. The EC-50 values were 2 μmol/l and 100 μmol/l in the EHA and LMIT respectively, indicating a more potent effect on egg hatch.


The CP papain shows potent anthelmintic activity against cyathostomins in vitro. Good evidence of anthelmintic effect against GINs in other host species is supportive of its potential use in equids. Further work is indicated to evaluate safety and in vivo efficacy.

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