The emergent disease infectious hematopoietic necrosis virus (IHNV) is the most significant disease threat to rainbow trout (Oncorhynchus mykiss) aquaculture. A lethal disease endemic to the Pacific Northwest region of North America, IHNV has expanded its geographic range and genetic diversity alongside trout aquaculture. Aquaculture settings are theorized to constitute novel selective pressures and may drive viral diseases towards more virulent phenotypes. Past work in the system has elucidated IHNV phylogenetics, but less empirical data is available on the phenotypic differences between IHNV genotypes. Viral shedding is the only way to non-invasively measure viral replication. Shedding rate is also used as a proxy for transmission rate. Current research seeks to understand how viral traits of virulence, shedding, and transmission interact and shape viral evolution in aquaculture. It is widely accepted that a diversity of virulence phenotypes exist for IHNV, but it is unknown how viral shedding is linked with virulence. Quantifying post-exposure shedding through time is the best way to approximate viral fitness since shed virions indicate both viral replication as well as transmission potential. Quantifying shedding profiles of different viral genotypes and comparing them to virulence profiles will allow comparison of evolutionary trajectories between different lineages through time, and enable building epidemiological models to better manage fish health.
In this study, we quantified the shedding profiles of 15 IHNV genotypes in rainbow trout. The genotypes represent two major genogroups of virus, pre- and post-host jump to rainbow trout, as well as five decades of evolution. Groups of 15-20 fish were exposed to IHNV via bath immersion. Following exposure, fish were randomly distributed to individual tanks. Daily water samples taken during the first week to record peak shedding, then periodically for one month to measure shedding duration. Water samples were analyzed RT-qPCR for virus quantification. For all viruses, shed load peaked on day 2, but magnitude of M versus U genogroup varied (Figure 1). This is the first work examining the shedding kinetics of this breadth of viruses through time and across IHNV lineages. Results indicate viruses are host species specific and correlated with virulence, which suggests virulence and fitness are linked across host jumps.