Activity of the antimicrobial polypeptide piscidin 2 against fish ectoparasites

The antiparasitic effects of piscidin 2, an antimicrobial polypeptide (AMPP) first isolated from mast cells of hybrid striped bass, were tested against three protistan ectoparasites of marine fish (the ciliates Cryptocaryon irritans and Trichodina sp., and the dinoflagellate Amyloodinium ocellatum) and one ciliate ectoparasite of freshwater fish (Ichthyophthirius multifiliis). I. multifiliis was the most susceptible parasite, with all theronts killed at 6.3 mu g mL(-1) piscidin 2. The most resistant parasite was Trichodina, where a few cells were killed at 12.5 mu g mL(-1), but several were still alive even at 100 mu g mL(-1). C. irritans was of intermediate sensitivity, with some theronts killed at 12.5 mu g mL(-1) and all killed at 25 mu g mL(-1). High parasite density apparently exhausted the piscidin 2 before it could attain its maximal effect, but surviving parasites were often visibly damaged. The lower efficacy of piscidin 2 against marine parasites compared with the freshwater ciliate might be related to the inhibitory effects of high sea water cation levels. The tissue concentration of piscidins estimated in healthy hybrid striped bass gill (40 mu g mL(-1)) suggests that piscidin 2 is lethal to the parasites tested at physiological concentrations and is thus an important component of innate defence in fish expressing this type of AMPP.