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
Rj. Whittington et al., 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, Journal of fish diseases, 17(3), 1994, pp. 205-218
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.