EPIDEMIOLOGY OF EPIZOOTIC HEMATOPOIETIC NECROSIS VIRUS (EHNV) INFECTION IN FARMED RAINBOW-TROUT, ONCORHYNCHUS-MYKISS (WALBAUM) - FINDINGS BASED ON VIRUS ISOLATION, ANTIGEN CAPTURE ELISA AND SEROLOGY

The epidemiology of epizootic haematopoietic necrosis virus (EHNV) inf ection was studied in farmed rainbow trout, Oncorhynchus mykiss (Walba um). Estimates of mortality during five outbreaks on a commercial farm from 1986 to 1992 ranged from 0.033 to 0.2% per day and total mortali ty did not exceed 3-4% in any outbreak. Affected fish were 0+ and less than 125 mm forklength. Clinical signs were non-specific, and laborat ory examination was required to confirm the diagnosis. At the height o f the outbreak in 1992, EHNV was demonstrated by virus isolation and a ntigen capture ELISA in 89% of clinically affected fish and 51% of dea d fish, while the prevalence of infection in apparently healthy in-con tact fish was 4%. Two and 4 months later the virus was not detected in a group of apparently healthy fish that had been affected earlier. An tibodies specific for EHNV were not found in rainbow trout from the in fected farm; however, strong humoral responses were detected by ELISA in two immunized fish, indicating that the virus was immunogenic. Thes e data suggested that EHNV was poorly infective but highly virulent in rainbow trout. Clinical EHNV infection was positively correlated with high rearing density and a low rate of water exchange, and therefore, with presumed poor water quality. Water temperature, which ranged fro m 11 to 17-degrees-C during outbreaks, did not appear to determine the incidence of clinical infection. EHNV infection in farmed rainbow tro ut was preceded by infection in free-living redfin perch, Perca fluvia tilis L., in the water catchment, but it was uncertain whether this re presented the source of infection for rainbow trout.