Persistence of viral RNA in fish infected with VHSV-IVb at 15 °C and then moved to warmer temperatures after the onset of disease

Smallmouth bass, Micropterus dolomieu Lacepede, bluegill, Lepomis macrochirus Rafinesque (coppernose strain), koi carp, Cyprinus carpio L., and channel catfish Ictalurus punctatus (Rafinesque), were infected by intraperitoneal injection with viral haemorrhagic septicaemia virus genotype IVb (VHSV-IVb) at 15 degrees C. When clinical signs of disease developed, one-third of the fish was moved to 20 degrees C and one-third to 25 degrees C. Mortality in challenged fish at all three temperatures ranged from 25 to 45% in smallmouth bass and from 70 to 90% in bluegill. No koi carp or channel catfish died during the study. Viral copy numbers detected by quantitative real-time reverse transcriptase PCR (qrt-RTPCR) in fish dying at 20 and 25 degrees C decreased over time. In survivors of the challenge, viral copy numbers were higher in the more susceptible species (smallmouth bass and bluegill) than in the more VHSV-IVb disease-resistant species (koi carp and channel catfish). In fish surviving 28 days post-infection, prevalence of infection was 66-100% depending on species and temperature, and VHSV-IVb was detected at 103105 copies mu g-1 host RNA. Our results show that qrt-RTPCR is a useful tool to investigate fish kills even 28 days after temperatures are elevated above those known to be permissive for VHSV replication.