Negligible risk associated with the movement of processed rainbow trout, Oncorhynchus mykiss (Walbaum), from an infectious haematopoietic necrosis virus (IHNV) endemic area

To assess the risk of transmission of infectious haematopoietic necrosis vi rus (IHNV) associated with the movement of processed rainbow trout, Oncorhy nchus mykiss, from an area where the virus is endemic, 240 freshly eviscera ted fish (225-500 g) exhibiting spinal curvature or spinal compression type s of deformities were tested for IHNV by virus isolation and polymerase cha in reaction (PCR) techniques. Commercially produced rainbow trout, approxim ately 1-year-old, that exhibited spinal deformities were considered to have had a high likelihood of having survived an outbreak of IHN. Serological a nalysis of fish exhibiting spinal curvature or spinal compression types of deformities for anti-IHNV antibodies resulted in 71 and 50% of the serum sa mples, respectively, with detectable neutralization activity suggesting pre vious infection with IHNV. A portion of the skin and muscle in the area of the deformity was collected, as well as brain tissue from each commercially processed fish. Tissue homogenates were tested for IHNV using the epitheli oma papulosum cyprini (EPC) cell line pretreated with polyethylene glycol a nd the chinook salmon embryo (CHSE-214) cell line using standard methods. N ested, reverse transcriptase (RT)-PCR for the detection of IHNV used the ce ntral 1231 by portion of the glycoprotein (G) gene. All brain and skin-musc le homogenates were negative for IHNV by virus isolation and by nested RT-P CR. To assess virus clearance experimentally, groups of 100 specific-pathog en-free (SPF) rainbow trout (mean weight, 100 g) were either intraperitonea lly injected with IHNV or sham infected and held separately in 385 L aquari a in SPF water at 15 degreesC. In the first study, a total of 33% of the in fected fish died. Ten days after the last mortality or 34 days after post-i nfection, six fish from each treatment were killed at weekly intervals for 6 weeks. Kidney and brain homogenates from individual fish were tested by v irus isolation and nested RT-PCR using the nucleoprotein (N) gene. All tiss ue homogenates were negative for IHNV. When this study was repeated, 54% of the infected fish died and beginning 34 days post-infection, six fish from each treatment were killed at biweekly intervals for 8 weeks, and kidney a nd brain homogenates from individual fish were tested by virus isolation an d quantitative real-time PCR utilizing primers and fluorescent labelled pro bes generated for the specific identification of the N and G genes of IHNV. One brain specimen from an IHN survivor obtained 34 days post-infection wa s positive by both virus isolation and quantitative PCR; however, all remai ning tissue specimens were negative over the 8-week period. Intraperitoneal injection of supernatants from pooled kidney or brain tissue obtained from control or IHN survivor fish into SPF rainbow trout (mean weight, 10 g) al so failed to establish virus infection. A strong humoral response was detec ted in the IHN survivors in both challenge studies and is suggested as a me chanism responsible for virus clearance. These results provide scientific i nformation that can be used to assess the risk associated with the movement of processed rainbow trout from an IHNV endemic area.