Larval Pacific herring, Clupea pallasii (Valenciennes), are highly susceptible to viral haemorrhagic septicaemia and survivors are partially protected after their metamorphosis to juveniles

Abstract

Pacific herring were susceptible to waterborne challenge with viral haemorrhagic septicaemia virus (VHSV) throughout their early life history stages, with significantly greater cumulative mortalities occurring among VHSV-exposed groups of 9-, 44-, 54- and 76-day-old larvae than among respective control groups. Similarly, among 89-day-1-year-old and 1+year old post-metamorphosed juveniles, cumulative mortality was significantly greater in VHSV-challenged groups than in respective control groups. Larval exposure to VHSV conferred partial protection to the survivors after their metamorphosis to juveniles as shown by significantly less cumulative mortalities among juvenile groups that survived a VHS epidemic as larvae than among groups that were previously naïve to VHSV. Magnitude of the protection, measured as relative per cent survival, was a direct function of larval age at first exposure and was probably a reflection of gradual developmental onset of immunocompetence. These results indicate the potential for easily overlooked VHS epizootics among wild larvae in regions where the virus is endemic and emphasize the importance of early life history stages of marine fish in influencing the ecological disease processes.

Chronic and persistent viral hemorrhagic septicemia virus infections in Pacific herring

“…In addition to acute manifestations of VHS in Pacific herring which are well-documented (Kocan et al 1997, Hershberger et al 2006, 2007, 2010a, we found that the disease can also manifest in this species as a chronic form that is characterized by viral persistence within infected hosts for extended periods with nearbackground levels of VHS-specific host mortality (Fig. 1, Table 1).…”

“…After surviving a sub acute outbreak of VHS, Pacific herring became solidly immune to the acute form of the disease (but not necessarily to re-infection), and the level of protection conferred was similar to that of herring surviving acute VHS manifestations (Kocan et al 2001, Hershberger et al 2007, 2010b. Although the immunological mechanisms of this adaptive response remain unknown, the resulting immunity following recovery from either the acute or chronic form of the disease has important implications in forecasting the potential for future VHS epizootics in wild populations.…”

“…Additionally, VHSV titers in the brain of infected Pacific herring were generally as high as or higher than those in kidney/spleen pools, a pattern of viral tropism that is similar to reports of chronic and neurotropic forms of VHS in European rainbow trout VHS (Smail 1999). These results support the recommendations of the OIE (2009) that standard procedures for determining VHSV prevalence in wild fish populations should include collection of brain tissue as well as samples from the heart, kidney, and spleen for viral assays.After surviving a sub acute outbreak of VHS, Pacific herring became solidly immune to the acute form of the disease (but not necessarily to re-infection), and the level of protection conferred was similar to that of herring surviving acute VHS manifestations (Kocan et al 2001, Hershberger et al 2007, 2010b. Although the immunological mechanisms of this adaptive response remain unknown, the resulting immunity following recovery from either the acute or chronic form of the disease has important implications in forecasting the potential for future VHS epizootics in wild populations.…”

“…Once exposed, laboratory stocks of Pacific herring typically experience acute disease followed by rapid mortality (Kocan et al 1997), and epizootics in the wild are likely initiated as a result of a combination of environmental stressors and concentration or amplification mechanisms associated with viral shedding from infected individuals within a school (Hershberger et al 2010a). Individuals that survive these epizootics become refractory to disease, indicating the involvement of a protective adaptive immune response (Kocan et al 2001, Hershberger et al 2007, 2010b). …”

Hypotheses concerning the decline and poor recovery of Pacific herring in Prince William Sound, Alaska

“…Recent studies suggest that VHS is primarily a disease of age-0 herring and that the disease is either rapidly resolved or fatal within 2 weeks following infection (Kocan et al 2001;Hershberger et al 2007;Elston and Meyers 2009). Fish surviving VHS acquire immunity.…”