The Effects of Lethality and Infectivity on Emergence and Displacement Events of a Salmonid Virus in the Pacific Northwest

Infectious hematopoietic necrosis virus (IHNV) is a significant disease of salmonids. In the Pacific Northwest it regularly causes epidemics in hatcheries, and also infrequently in wild fish. Strains of the virus have arisen and become dominant in specific geographic regions, only to be displaced by another strain that then becomes dominant in turn. We have investigated four strains of IHNV that have been involved in displacement events in the Columbia River Basin and coastal Washington State. In an effort to understand what drives the displacements, we have tested these four IHNV strains for lethality in four different populations of steelhead trout, and for infectivity in two of these fish populations, to investigate the effects of host on the functioning of the virus. We found a remarkably consistent pattern of variation in the lethality of the four strains within each host population, indicating increasing viral virulence as a possible factor involved in displacement events. Conversely, the infectivity data imply that ability to infect is not a driving force behind these events. By understanding the differences and similarities between different strains of the virus, we are able to get a clearer picture of how IHNV ecology functions in the Pacific Northwest.