Epizootiological studies on viral haemorrhagic septicaemia in brown trout Salmo trutta fario

Abstract

The course of a VHS (viral haemorrhagic septi-caemia) infection in brown trout Salmo trutta fario L. in a stream was followed for 4 yr using anti-VHS virus antibodies a s the indication of infection. Trout were caught by electro-fishing and the presence of anti-VHS virus antibody in the serum was determined by counter-current immunoelectro-phoresis. There were fluctuations in the prevalence of anti-bodies to VHS virus in several distinct populations of brown trout varying from 6 to 67%. In one population, the preva-lence of antibody was found to b e 4 1 % in the first year of ex-amination. In the following year the percentage Increased to 67%, then fell to 33% the next year and to 0% in the fourth year. Prevalence also varied with location in the stream. In one extreme case prevalence decreased from 37% to 10U/o, over a stretch of only 3 km. The antibody data indicate that the virus persisted in the brown trout populations for a consid-erable period because antlbody titres were prevalent in the trout dunng 3 successive years of sampling. In the fourth year, however, the virus appeared to have been largely eliminated from the trout as only a few fish in one population showed e v ~ d e n c e of the presence of virus.