Influence of storage temperature on infectious hematopoietic necrosis virus detection by cell culture isolation and RT-PCR methods

The detection of infectious hematopoietic necrosis virus (IHNV) in infected rainbow trout Oncorhynchus mykiss and in cell culture supernatants stored under different conditions was studied. IHNV-positive fish visceral organ homogenates and cell culture supernatants were incubated at 4 and 25 degrees C. Virus titre was measured by virus isolation on epithelioma papulosum cyprini (EPC) cells and the IHNV RNA was detected by RT-PCR and semi-nested RT-PCR. The influence of repeated freezing and thawing on the virus isolation from organ homogenates and from cell culture supernatants was studied as well. It was possible to isolate the virus from IHNV-positive organ material during the 3 d of incubation at 4 degrees C but, only on the first day of incubation at 25 degrees C. Viral RNA could be amplified during the incubation period of 35 d at 4 degrees C but only during 8 d of incubation at 25 degrees C. In IHNV-infected cell culture supernatant stored at 4 degrees C, it was possible to detect virus for 36 and 16 d in supernatant stored at 25 degrees C. Viral RNA could be followed by using molecular methods during the entire experimental period of 123 d. Each cycle of freezing and thawing of samples resulted in a reduction of IHNV titre in the suspension of visceral organs, while the virus titre in cell culture supernatant remained almost the same following 33 freezing-thawing cycles. The present results show that rapid laboratory processing and storage of potentially virus-containing tissue samples as well as the use of different detection methods are very important for efficient IHNV diagnosis.