Brook charr, Salvelinus fontinalis (Mitchill), and cryptobiosis: a potential salmonid reservoir host for Cryptobia salmositica Katz, 1951

Abstract. Laboratory-raised Cryptobia -susceptible brook charr, Salvelinus fontinalis (Mitchill), and rainbow trout, Oncorhynchus mykiss (Walbaum), were vaccinated intraperitoneally with a live Cryptobia salmositica vaccine (250000 parasites per fish), and 4 weeks later were challenged with the pathogen (250000 parasites per fish). Unvaccinated and infected brook charr had high parasitaemias but no clinical signs of disease, while unvaccinated and infected rainbow trout had anaemia and general oedema. Vaccinated and challenged fish had very low parasitaemias compared to unvaccinated and infected brook charr and rainbow trout. Complement fixing antibodies were detected in vaccinated and challenged fish 2 weeks after challenge. Unvaccinated and infected brook charr had consistently higher litres of complement fixing antibody than unvaccinated and infected rainbow trout. Parasitaemias were lower in all fish in which titres of complement fixing antibody were high. In a second experiment, brook charr inoculated intraperitoneally or intramuscularly with 100000 C. salmositica per fish had high parasitaemias but no anaemia or other clinical signs. The results show that susceptible brook charr do not suffer from cryptobiosis and may serve as reservoir hosts for C. salmositica in areas where the disease is prevalent. Vaccination to reduce the parasitaemia when fish become infected may be a control strategy in these areas.