Dynamics of viral hemorrhagic septicemia, viral erythrocytic necrosis and ichthyophoniasis in confined juvenile Pacific herring Clupea pallasii

Abstract

Capture of wild, juvenile herring Clupea pallasii from Puget Sound (Washington, USA) and confinement in laboratory tanks resulted in outbreaks of viral hemorrhagic septicemia (VHS), viral erythrocytic necrosis (VEN) and ichthyophoniasis; however, the timing and progression of the 3 diseases differed. The VHS epidemic occurred first, characterized by an initially low infection prevalence that increased quickly with confinement time, peaking at 93 to 98% after confinement for 6 d, then decreasing to negligible levels after 20 d. The VHS outbreak was followed by a VEN epidemic that, within 12 d of confinement, progressed from undetectable levels to 100% infection prevalence with > 90% of erythrocytes demonstrating inclusions. The VEN epidemic persisted for 54 d, after which the study was terminated, and was characterized by severe blood dyscrasias including reduction of mean hematocrit from 42 to 6% and replacement of mature erythrocytes with circulating erythroblasts and ghost cells. All fish with ichthyophoniasis at capture died within the first 3 wk of confinement, probably as a result of the multiple stressors associated with capture, transport, confinement, and progression of concomitant viral diseases. The results illustrate the differences in disease ecology and possible synergistic effects of pathogens affecting marine fish and highlight the difficulty in ascribing a single causation to outbreaks of disease among populations of wild fishes.KEY WORDS: Disease · Fish · Viral hemorrhagic septicemia · Viral erythrocytic necrosis · Ichthyophonus · Pacific herring · Wild fish Resale or republication not permitted without written consent of the publisherDis Aquat Org 70: [201][202][203][204][205][206][207][208] 2006 Sardinops sagax biomass from 165 000 to 37 000 tonnes in 1995 and from 147 000 to 36 000 tonnes in 1998 (Jones et al. 1997, Whittington et al. 1997, Gaughan et al. 2000, Ward et al. 2001. Although mortality was associated with herpesvirus-like particles in the gills of affected sardines (Hyatt et al. 1997, Jones et al. 1997, Gaughan et al. 2000, transmission studies and attempts to fulfill Koch's postulates were unsuccessful (Bernoth 2002).In Puget Sound, Washington (USA), several pathogenic agents, including viral hemorrhagic septicemia virus (Hershberger et al. 1999, Kocan et al. 2001, erythrocytic necrosis virus (MacMillan & Mulcahy 1979) and Ichthyophonus hoferi (Kocan et al. 1999, Hershberger et al. 2002 are enzootic in populations of Pacific herring Clupea pallasii. Viral hemorrhagic septicemia virus (VHSV) is grouped with several other commercially and ecologically important fish viruses in the Novirhabdovirus genus of the family Rhabdoviridae (Walker et al. 2000). North American isolates belong to VHSV Genogroup IV and have been recovered almost exclusively from marine or anadromous fishes, similar to reports for the European Genogroups Ib, II and III (Snow et al. 2004). Juvenile and newly recruited Pacific herring are highly susceptible to VHSV (Kocan et al. 1997), and applica...

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).…”

“…To investigate the duration of viral persistence in Pacific herring, subsamples were removed from the VHSVexposed and control colonies 224 d post-exposure and transferred to smaller (260 l) triplicate tanks (n = 30 herring tank -1 ), where they were held for 14 d, a more stressful condition that has been effective at amplifying low levels of VHS in herring and other species (Kocan et al 2001, Hershberger et al 2006. Mortalities were collected daily, and tissues (brain, kidney, spleen) from all mortalities were pooled by replicate tank.…”

Kinetics of viral shedding provide insights into the epidemiology of viral hemorrhagic septicemia in Pacific herring

“…These fish actively shed virus because detectable titers of waterborne VHSV can be readily found soon after placing newly captured wild herring into static transport tanks, and VHS epizootics routinely occur following the stress of capture, transfer and confinement of wild cohorts in laboratory aquaria or marine net pens (Hershberger et al 1999, 2001, 2006, Kocan et al 2001a). During endemic conditions, free-ranging Pacific herring appear to experience relatively low infection pressures due to their highly migratory behavior and the dilution of virus shed by a small number of individuals in a large water mass.…”

Identification of the major capsid protein of erythrocytic necrosis virus (ENV) and development of quantitative real-time PCR assays for quantification of ENV DNA

“…The transient delay in inclusion body formation in our progression study was consistent with previous laboratory challenge studies and confinement studies in Pacific herring, which found detectable erythrocytic inclusion bodies appearing in a low percentage of cells 4-6 days after initiation of the epizootic. 9,12 Although DSe and DSp of blood smear analysis was similar to the qPCR assays, it should be noted that technical difficulties were encountered with the analysis of some blood films, where cell lysis occurred after inadvertent contact with seawater. We were not able to confidently analyze 20% of blood smear slides from the ENV-infected fish.…”