Mortality and carrier status of bluegills exposed to viral hemorrhagic septicemia virus genotype IVb at different temperatures.

The emergence of the viral hemorrhagic septicemia virus (VHSV) genotype IVb (VHSV-IVb) in the Great Lakes of North America has led to concern that the virus might spread to natural fisheries and aquaculture in the southern USA. We exposed bluegills Lepomis macrochirus to VHSV-IVb by intraperitoneal injection at six temperatures from 10 degrees C to 30 degrees C and followed the disease course by quantitative real-time reverse transcriptase polymerase chain reaction (qrt-RT-PCR). Mortality of injected fish was 90% at 10 degrees C, 35% at 14 degrees C, and 10% at 18 degrees C; no mortality attributable to VHSV was observed at temperatures of 22-30 degrees C. In survivors tested at 21 d postchallenge, viral copies and prevalence determined by qrt-RT-PCR were inversely related to temperature, and VHSV-IVb could not be detected in fish held at temperatures above 22 degrees C. Similar results were obtained for bluegills that were exposed by cohabitation with the intraperitoneally injected fish. Acclimation of the fish to 12 degrees C after 21 d at higher temperatures did not appear to cause a re-emergence of the virus. Based on our findings, the temperature range of VHSV-IVb appears to be the same as published values for VHSV genotype I, which has an optimum of 9-12 degrees C and an upper limit of 18-20 degrees C.