Early Kinetics of Infectious Hematopoietic Necrosis Virus (IHNV) Infection in Rainbow Trout

Abstract.—A series of experiments was carried out with infectious hematopoietic necrosis virus (IHNV; 193-110 isolate) in rainbow trout Oncorhynchus mykiss (weight,;1.2 g) to determine the duration of the patent period and the timing of onset of the infectious periods. We first attempted to transmit IHNV to recipient fish from infected rainbow trout 2–3 d after they had been exposed. No infection transfer occurred despite high titers (104.79 to 104.91 plaque-forming units 5–8 d postexposure (dpe). To determine the number of secondary cases produced by one infectious individual, we exposed approximately 50 rainbow trout (weight,;1.5 g) in each of seven replicate tanks to a donor fish that had been infected with virus by bath exposure 3 d earlier. The prevalence of infection in recipient fish rose from 0.84 % at 2 dpe to 7.9 % at 6 dpe. Maximum incidence (22 cases) occurred between 2 and 4 dpe. No disease-specific mortalities occurred in recipient fish during the experiment. The titer of virus in both recipient and donor fish increased from 2 to 4 dpe. There was a positive correlation between the level of infection among donors and prevalence values among recipient fish (r2 5 0.60). The level of challenge by one infectious fish under the conditions provided was enough for infection transfer from sick cohabitant to susceptible fish but was not enough for initiation of a full-scale epizootic among recipients. Infectious hematopoietic necrosis virus (IHNV), a rhabdovirus, causes a disease (IHN) character-ized by extensive necrosis of hematopoietic tissues in early life stages of economically important sal-monids. Enzootic in the Northwest Pacific of Nort