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

The course of a VHS (viral haemorrhagic septicaemia) 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 electrofishing and the presence of anti-VHS virus antibody in the serum was determined by counter-current immunoelectrophoresis. There were fluctuations in the prevalence of antibodies to VHS virus in several distinct populations of brown trout varying from 6 to 67%. In one population, the prevalence of antibody was found to be 4 1 % in the first year of examination. 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 considerable 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 ev~dence of the presence of virus. Rainbow trout Oncorhynchus mykiss in aquaculture are assumed to be the main host for viral haemorrhagic septicaemia virus (VHS-V), a virus that has caused devastating infections in both rainbow trout fry and adults. A review of the disease has been given by Liversidge et al. (1985). In spite of good hygienic conditions, VHS outbreaks occur in farms which are supplied with stream water. Very often an outbreak of disease correlates with such factors as a drop in water temperature, water pollution, or other stress situations such as high stocking densities. These circumstances have been discussed m an earlier publication (Enzmann & Konrad 1984). In the course of an investigation examining the health condition of brown trout in a stream that supplies 2 commercial fish farms rearing rainbow trout, sera of brown trout were tested for antiO Inter-Research/Printed in Germany bodies against VHS-V (Enzmann & Konrad 1985). On these farms severe VHS outbreaks had occurred about 6 mo prior to the first sampling of the brown trout. In previous reports brown trout were described to be sensitive to serotypes He and 23.75 of VHS-V and to be refractory to serotype F1 ( J ~ r g e n s e n 1972, J ~ r g e n s e n 1980, de Kinkelin & Le Berre 1977). The detection of antibodies against serotype F1 in brown trout initiated the present epizootiological studies on VHS among the feral brown trout. We were particularly interested in following the time course of antibody development and disappearance, and we did so over a period of several years. Materials and methods. Cells and virus strains: The RTG-2 (rainbow trout gonad) cell line, purchased from Flow, Bonn, Germany, was used for virus propagation. VHS-V strain F1 was obtained from Dr P. E. V. Jorgensen, State Veterinary Serum Laboratory, Aarhus, Denmark. The virus strain was cloned by 8 end-point dilutions in RTG-2 cells. For production of antigen, virus was grown in RTG-2 cells. Virus isolated from infected rainbow trout after VHS outbreaks in trout farms was determined to belong to the F1 serotype using the neutralization test: it was strongly neutralized with antiserum prepared against the F1 serotype but only feebly neutralized with antisera prepared against other VHS serotypes. Antibody detection assay: Antibodies to VHS-V were detected by means of counter-current immunoelectrophoresis in 1 % agarose gels with high electroendosmosis using barbiturate buffer (pH 8.2; 0.05M) as described earlier (Deuter & Enzmann 1986). The tests were performed using a potential difference of 5 V cm-'. The distance between the centres of the wells was 7 mm. Sera (15 p1) were applied to loading wells which