ABSTRACT: Cryptobia salmositica infects both rainbow trout Oncorhynchus mykiss and brook charr Salvelinus fontinalis, but it does not cause disease (e.g. anaemia) in brook charr. The natural antiprotease, alpha2-macroglobulin (alpha2-M), was found in the blood of rainbow trout and brook charr, but it had much higher activity in brook charr. Under in vivo conditions, alpha2-M of trout and charr were neutralized by C. salmositica extracellular protease. The onset and persistence of anaemia in infected rainbow trout were related to low levels of alpha2-M. Infected brook charr did not develop anaemia as alpha2-M remained high during the infection. Secreted metallo-protease of C. salmositica was detected in the blood of infected rainbow trout when its alpha2-M was low and the fish was anaemic; however, metallo-protease was not detected in infected brook charr. We suggest that host alpha2-M plays an important role in defense against cryptobiosis by inhibiting the activity of metallo-protease secreted by the pathogenic C. salmositica.

KEY WORDS: alpha2-macroglobulin · Oncorhynchus mykiss · Salvelinus fontinalis · Cryptobia salmositica · Protease

Published in DAO Vol. 29, No. 1 (1997) on April 24
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1997