Experimental transmission of epizootic haematopoietic necrosis virus (EHNV) to adult redfin perch Perca fluviatilis and juvenile rainbow trout Oncorhynchus mykiss was undertaken at different water temperatures using intraperitoneal (IP) and bath inoculation. Redfin perch were highly susceptible to EHNV by both routes of infection. Bath inoculation with as few as 0.08 TCID50.mL-1 was lethal. The incubation period in redfin perch was about 11 days at a water temperature of 19-21 degrees C but was longer at colder temperatures and disease did not occur at temperatures below 12 degrees C. The longest incubation period recorded in redfin perch was 28 days. Rainbow trout were not susceptible to infection by bath inoculation but the disease was reproduced after IP inoculation with 10(5.6) TCID50 at water temperatures ranging from 8-21 degrees C. The incubation period was 3-10 days at 19-21 degrees C, but was up to 32 days at 8-10 degrees C. Persistent infection with EHNV was detected by virus isolation in a clinically unaffected rainbow trout after 63 days. The implications of these findings in the understanding of the epidemiology of EHNV infection are discussed.

The epidemiology of epizootic haematopoietic necrosis virus (EHNV) infection was studied in farmed rainbow trout, Oncorhynchus mykiss {Walbaum). 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 mortality did not exceed 3-4% in any outbreak. Affected fish were 0+ and less than 125mm forklength. Clinical signs were non-specific, and laboratory examination was required to confirm the diagnosis. At the height of the outbreak in 1992, EHNV was demonstrated by virus isolation and antigen capture ELISA in 89% of clinically affected fish and 51% of dead fish, while the prevalence of infection in apparently healthy in-contact 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. Antibodies specific for EHNV were not found in rainbow trout from the infected farm; however, strong humoral responses were detected by ELISA in two immunized fish, indicating that the virus was immunogenic. These 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 quahty. Water temperature, which ranged from 11 to 17°C during outbreaks, did not appear to determine the incidence of clinical infection. EHNV infection in farmed rainbow trout was preceded by infection in free-living redfin perch, Perca fluviatilis L., in the water catchment, but it was uncertain whether this represented the source of infection for rainbow trout.

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