Chronic and persistent viral hemorrhagic septicemia virus infections in Pacific herring

Abstract

Chronic viral hemorrhagic septicemia virus (VHSV) infections were established in a laboratory stock of Pacific herring Clupea pallasii held in a large-volume tank supplied with pathogenfree seawater at temperatures ranging from 6.8 to 11.6°C. The infections were characterized by viral persistence for extended periods and near-background levels of host mortality. Infectious virus was recovered from mortalities occurring up to 167 d post-exposure and was detected in normal-appearing herring for as long as 224 d following initial challenge. Geometric mean viral titers were generally as high as or higher in brain tissues than in pools of kidney and spleen tissues, with overall prevalence of infection being higher in the brain. Upon re-exposure to VHSV in a standard laboratory challenge, negligible mortality occurred among groups of herring that were either chronically infected or fully recovered, indicating that survival from chronic manifestations conferred protection against future disease. However, some survivors of chronic VHS infections were capable of replicating virus upon re-exposure. Demonstration of a chronic manifestation of VHSV infection among Pacific herring maintained at ambient seawater temperatures provides insights into the mechanisms by which the virus is maintained among populations of endemic hosts. KEY WORDS: Viral hemorrhagic septicemia · VHS · Pacific herring Resale or republication not permitted without written consent of the publisherDis Aquat Org 93: [43][44][45][46][47][48][49] 2010 tion. Recognizing the unlikelihood that this narrow R 0 range persists for extended periods in dynamic natural systems like the NE Pacific, it is hypothesized that alternative or complementary VHSV perpetuation strategies exist in wild populations of Pacific herring. Thus, while only acute manifestations of VHS have been reported, lower levels of mortality are difficult to observe in the ocean, and the occurrence of subacute forms of the disease could easily go unnoticed. Recognition of subacute forms of VHS in Pacific herring, analogous to those known to occur in cultured rainbow trout Oncorhynchus mykiss in Europe (Smail 1999), could offer an alternative means by which the virus may be maintained in populations without the need for exclusive reliance on transmission of acute disease within a narrow R 0 range.Here, we report the results of long-term laboratory studies of VHSV-exposed Pacific herring that resulted in atypical disease manifestations that were characterized by tempered and prolonged mortality. The purpose of this study was to describe the kinetics of these subacute VHS outbreaks and to determine whether survivors were protected against future infections or disease. MATERIALS AND METHODSEstablishment of subacute and persistent VHSV infections. Sub-acute and persistent VHSV infections were initiated among laboratory-reared, age 1+ yr, specific pathogen-free (SPF) Pacific herring (n = 3668). The fish were housed in a 2.5 m diameter (3200 l) circular tank provided with flowing, processe...

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