North American Strain of Viral Hemorrhagic Septicemia Virus is Highly Pathogenic for Laboratory-Reared Pacific Herring

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...

Section: Discussionsupporting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

Chronic and persistent viral hemorrhagic septicemia virus infections in Pacific herring

Hershberger

Gregg²,

Grady³

et al. 2010

“…6) to concentrations capable of producing lethal infections among juvenile laboratory-reared, spec~fic pathogen-free herring (Kocan et al 1997). The lower concentrations in water samples collected from outside Pounds #1998-5 and #1998-6 may have resulted from samples being taken on the upstream slde of slight currents circulating through the pounds during tide changes.…”

Section: Discussionmentioning

confidence: 99%

Epizootiology of viral hemorrhagic septicemia virus in Pacific herring from the spawn-on-kelp fishery in Prince William Sound, Alaska, USA

Hershberger

Kocan²,

Elder³

et al. 1999

Both the prevalence and tissue titer of viral hemorrhagic septicemia virus (VHSV) increased in Pacific herring Clupea pallasi following their introduction into net pens [pounds) used in the closed pound spawn-on-kelp (SOK) fishery in Prince William Sound, Alaska. VHSV was also found in water samples from inside and outside the SOK pounds after herring had been confined for several days; however, water samples taken near wild free-ranging, spawning hernng either failed to test positive or tested weakly positive for virus. Little or no virus was found in tissue samples from freeranging, spawning herring captured from the vicinity of the pounds, nor did the prevalence of VHSV increase following spawning as it did in impounded herring. The data indicated that increased preval e n c e~ of VHSV were correlated with confinement of herring for the closed pound SOK fishery and that infection was spread within the pounds through waterborne exposure to virus particles originating from impounded fish. In addition, pounds containing predominantly young fish had higher prevalences of VHSV, suggesting that older fish may be partially immune, perhaps as a result of previous infection with the virus. Operation of SOK pounds during spawning seasons in which young herring predominate may amplify the disease and possibly exacerbate the population fluctuations observed in cvlld herring stocks.

“…Among the 16 organisms that occurred in Pacific herring Clupea pallasii at >10% prevalence (Table 1), only Ichthyophonus hoferi and VHSV met all 4 criteria for significance at the population level (Kocan et al 1997, Marty et al 2003. VHSV occurred sporadically in fish sampled in the spring, but it was never cultured in any of the 710 fish sampled in the fall (Table 1).…”

Section: Significant Lesions and Pathogensmentioning

confidence: 99%

Failure of population recovery in relation to disease in Pacific herring

Marty

Hulson²,

Miller³

et al. 2010

Following an estimated 60% decline in population abundance in early 1993, recovery of the Pacific herring Clupea pallasii population of Prince William Sound, Alaska, USA, has been impaired by disease. Comprehensive epidemiological study from 1994 through 2002 validated an age-structured assessment (ASA) model of disease and population abundance; from 2003 to 2006, the impact of disease was modeled by analyzing only 2 lesions: ulcers and white foci in the heart. The ASA model identified increased natural mortality since 1993 that can be explained by (1) epidemics associated with ulcers (prevalence about 3%) and the North American strain of viral hemorrhagic septicemia virus (VHSV Type IVa; prevalence up to 14%) in 1994 and 1998 and (2) relatively high prevalence of the mesomycetozoean Ichthyophonus hoferi from 1994 through 2006, including epidemics with the greatest sample prevalence in 2001 (38%, by histopathology) and 2005 (51%, estimated histopathology prevalence). Fourteen other parasites occurred at prevalence >10%, but none were considered significant contributors to fish mortality. We predict that if natural mortality after 1994 had returned to background levels that best fit the model from 1980 to 1992 (0.25 yr -1 ), population biomass in 2006 would have been 3 times the best estimate, despite relatively poor recruitment since 1994. In conclusion, disease information can be used to explain and predict changes in populations that have confounded traditional fisheries assessment.KEY WORDS: Pacific herring · Clupea pallasii · Viral hemorrhagic septicemia virus · VHSV Type IVa · Ulcers · Ichthyophonus hoferi · Age-structured assessment model · Population-level response Resale or republication not permitted without written consent of the publisherDis Aquat Org 90: [1][2][3][4][5][6][7][8][9][10][11][12][13][14] 2010 are 3 to 5 yr old, live for about 12 yr, and weigh up to about 250 g. Mature fish spawn every year, but for reasons probably related to environmental conditions (Williams & Quinn 2000), strong year-classes recruit into the adult population only about once every 4 yr. The most accurate biomass estimates are made in early spring, when fish aggregations move to shallow waters to spawn. After spawning, fish disperse and do not reaggregate near spawning areas until late fall. Feeding is minimal during the winter (Foy & Norcross 2001).Four years after the March 1989 'Exxon Valdez' oil spill in Prince William Sound, about 60% of the Pacific herring population died during the winter of 1992/1993. Death was attributed to poor food availability in 1992, which resulted in poor fish condition the following winter (Pearson et al. 1999, Elston & Meyers 2009). The weakened fish had 2 types of lesions or pathogens that are commonly associated with population stress: (1) cutaneous ulcers and (2) the North American strain of viral hemorrhagic septicemia virus (VHSV Type IVa) (Meyers et al. 1994, Pearson et al. 1999, Carls et al. 2002). An alternative hypothesis attributes a greater proportion of th...