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

—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) characterized by extensive necrosis of hematopoietic tissues in early life stages of economically important salmonids. Enzootic in the Northwest Pacific of North America (Parisot et al. 1965; Amend 1975), it has also been detected in other parts of the world, including Taiwan (Chen et al. 1985), Belgium (Hill 1992), Italy (Bovo et al. 1987), France (Laurencin 1987), and Japan (Sano et al. 1977). Early phases of an IHNV epizootic have not been evaluated. Understanding disease initiation and factors that could affect spread of virus in a susceptible population would be an essential tool in preventing and controlling IHN disease. In particular, using cohabitation method and only a single infectious individual would imitate conditions in nature where serious IHN epizootics occur in wild salmonids. For example, in a natural epizootic of IHNV, Fraser River System, British Columbia, * Corresponding author: [email protected] 1 Present address: Karadeniz Technical University, Sürmene Deniz Bilimleri Fakültesi, 61530 Camburnu, Trabzon, Turkey. Received August 26, 2003; accepted June 18, 2004 Canada, an estimated 8.3 million of 16.8 million sockeye fry died of IHN disease within days of leaving the spawning channel (Traxler and Rankin 1989). Various management strategies, such as decreasing density or increasing flow rate, could significantly decrease infection spread; sometimes, perhaps, practices such as holding fish to determine the level of prevalence in closed enclosures could unintentionally exacerbate spread of infection and consequent mortality. In this study, we examined the early stages of IHNV infection in rainbow trout Oncorhynchus mykiss. Our goal was to use the cohabitation method to examine the initiation of infection in a susceptible population of rainbow trout by a single infectious fish. The results obtained have implications in understanding early infection spread in populations and in developing control strategies for wild and cultured fishes.