Susceptibility of pike Esox lucius to VHSV and IHNV and potential transmission to rainbow trout Oncorhynchus mykiss.

ABSTRACT

Determining the origin of recurrent outbreaks of fish diseases occurring on fish farms is essential for disease prevention and control measures. In this study, we investigated the potential reservoir role of wild fish species living near salmonid farms which were regularly found to be positive for viral hemorrhagic septicemia virus (VHSV). In addition to VHSV, infectious hematopoietic necrosis virus (IHNV) was also isolated from several pike Esox lucius samples collected from a pond near the salmonid farms of interest. All isolates of VHSV and IHNV analyzed had 100% identical partial glycoprotein gene sequences. VHSV pike strain OO128-25 belonged to the Ia genotype and shared 99.1 to 99.5% nucleotide identity with strains recently isolated from the farms. IHNV pike strain OO121-8, European genotype, appeared to be different from strains from France characterized since the first isolation in 1987. Isolates representative of both viral species were highly virulent in rainbow trout Oncorhynchus mykiss. OO128-25 induced 65% mortality in pike fingerlings, whereas only weak mortality was observed with OO121-8, despite characteristic symptoms in infected fish. High levels of specific antibodies to VHSV and IHNV were detected in adult pike in the absence of clinical signs. Infection of rainbow trout in contact with experimentally VHSV- or IHNV-infected pike fingerlings indicates possible horizontal transmission. These results suggest that pike could act as a reservoir for VHSV and IHNV in the wild, providing additional evidence to explain viral persistence and resurgence in certain areas.