Abstract
This study compared the results of reverse transcription‐polymerase chain reaction (RT‐PCR) and traditional virus isolation on cell culture in detection of viral haemorrhagic septicaemia virus (VHSV) and infectious haematopoietic necrosis virus (IHNV). RT‐PCR was used for 172 tissue sample pools (total of 859 fish) originating from a field survey on the occurrence of VHSV and IHNV in farmed and wild salmonids in Switzerland. These samples represented all sites with fish that were either identified as virus‐positive by means of virus isolation (three sites, four positive tissue sample pools) and/or demonstrated positive anti‐VHSV‐antibody titres (83 sites, 121 positive blood samples) in a serum plaque neutralization test (SPNT). The RT‐PCR technique confirmed the four VHSV‐positive tissue sample pools detected by virus isolation and additionally identified one VHSV‐positive sample that showed positive anti‐VHSV‐AB titres, but was negative in virus isolation. With IHNV, RT‐PCR detected two positive samples not identified by virus isolation while in these fish the SPNT result had been questionable. One of the IHNV‐positive samples represents the first detection of IHNV‐RNA in wild brown trout in Switzerland. Compared to SPNT, the RT‐PCR method detected, as with virus isolation, a much lower number of positive cases; reasons for this discrepancy are discussed. Our results indicate that RT‐PCR can not only be successfully applied in field surveys, but may also be slightly more sensitive than virus isolation. However, in a titration experiment under laboratory conditions, the sensitivity of RT‐PCR was not significantly higher when compared with virus isolation.