The Effect of Temperature Change on Viral Pathogenesis of Viral Hemorrhagic Septicemia Virus (VHSV) in Fathead Minnow (Pimephales promelas)

Viral hemorrhagic septicemia virus (VHSV) prevalence in Great Lakes fish populations varies seasonally, with greater prevalence during spawning seasons. Chronic infections appear to be associated with neural tissue, while acute forms target endothelial tissue. Given the potential immunosurpessive effect of water temperature variation in natural environments, I investigated the effect of a 5ºC temperature change on disease development due to VHSV infection in fathead minnow Pimephales promelas. Fish were exposed to VHSV genotype IVb and either a temperature increase from 10ºC to 15ºC, decrease from 20ºC to 15ºC, or a stable temperature of 15ºC. I evaluated prevalence of gross lesions and death, tested fish for VHSV via cell culture of pooled visceral tissue, and performed quantitative reverse transcription polymerase chain reaction (qRT-PCR) assays on brain tissue and pooled visceral tissue extracts. Fish that experienced temperature decrease had greater VHSV prevalence than those that experienced temperature increase, and more tested positive by qRTPCR of brain tissue than those that experienced no change, suggesting greater prevalence of infections. Visceral organ samples from fish that experienced either temperature change contained higher viral RNA N-gene copy numbers than those from fish that experienced stable temperature, while brain tissue samples contained similar viral RNA copy numbers among all groups. In fish that contained more viral RNA copies in viscera than brain, those fish that experienced either temperature change had a greater difference between the two quantities than those that experienced no temperature change. The severity of chronic infections was not affected by temperature change, but the severity of acute infections was increased in fish that experienced any temperature change. These results suggest that fish that encounter temperature changes, especially decreases, of 5ºC are at higher risk of contracting chronic VHSV infections and severe acute VHSV infections, helping to explain seasonal fish die-offs attributed to VHSV, especially in fish that encounter temperature change during the spawning season