Prevalence of Viral Erythrocytic Necrosis in Pacific Herring and Epizootics in Skagit Bay, Puget Sound, Washington

Abstract

Epizootics of viral erythrocytic necrosis (VEN) occurred among juvenile Pacific herring Clupea pallasii in Skagit Bay, Puget Sound, Washington, during 2005-2007 and were characterized by high prevalences and intensities of cytoplasmic inclusion bodies within circulating erythrocytes. The prevalence of VEN peaked at 67% during the first epizootic in October 2005 and waned to 0% by August 2006. A second VEN epizootic occurred throughout the summer of 2007; this was characterized by disease initiation and perpetuation in the age-1, 2006 year-class, followed by involvement of the age-0, 2007 year-class shortly after the latter's metamorphosis to the juvenile stage. The disease was detected in other populations of juvenile Pacific herring throughout Puget Sound and Prince William Sound, Alaska, where the prevalences and intensities typically did not correspond to those observed in Skagit Bay. The persistence and recurrence of VEN epizootics indicate that the disease is probably common among juvenile Pacific herring throughout the eastern North Pacific Ocean, and although population-level impacts probably occur they are typically covert and not easily detected.

Infectious Diseases of Fishes in the Salish Sea

“…Viral erythyocytic necrosis (VEN) is a disease that can lead to severe anemia in many marine fish species worldwide. The disease, caused by a fish iridovirus, occurs in the Salish Sea, where it affects populations of Pacific Herring (Hershberger et al 2009) and the seawater stages of Pink (O. gorbuscha) and Chum (O. keta) Salmon (Evelyn and Traxler 1978). The extent and severity of VEN in the Salish Sea are likely underestimated due to the absence of a coordinated marine fish health surveillance program in the region and lack of sensitive or standardized diagnostic procedures for the disease.…”

“…Viral erythrocytic necrosis typically occurs at low intensity and prevalence among populations of Pacific Herring in the Salish Sea; however, poorly understood host and environmental factors can produce an increase in severity leading to large outbreaks accompanied by fish kills. Epizootics are reported most frequently in juvenile Herring cohorts (Meyers et al 1986;Hershberger et al 2009); however, low-intensity infections are often seen in adults (Hershberger et al 2009, and unpublished data). Correlations with environmental variables during periods of epizootics led to the hypothesis that cool temperatures-possibly accompanied by stress from exposure to low salinity water-were associated with the increased severity of VEN (Meyers et al 1986).…”

“…Correlations with environmental variables during periods of epizootics led to the hypothesis that cool temperatures-possibly accompanied by stress from exposure to low salinity water-were associated with the increased severity of VEN (Meyers et al 1986). Additionally, the proximity of immunologically naïve juvenile cohorts to fish carrying low-level infections also represents a likely risk factor (Hershberger et al 2009). Epizootics can be easily induced after confinement of free-ranging Herring into net pens or laboratory tanks (Traxler and Bell 1988;Hershberger et al 2006), likely as a result of viral shedding from a few infected individuals that, when combined with limited water exchange in confined situations, results in increased infection pressure among susceptible cohorts.…”

Principles underlying the epizootiology of viral hemorrhagic septicemia in Pacific herring and other fishes throughout the North Pacific Ocean

“…While most are generally considered secondary pathogens of negligible importance as proximate mortality factors, others (including VHSV, Ichthyophonus sp., and ENV) are primary pathogens of Pacific herring and can cause direct host mortality (Kocan et al 1997Hershberger et al 2009). Recurring epizootics of VHS have caused fish kills in Pacific herring and other forage fishes .…”

Molecular identification of erythrocytic necrosis virus (ENV) from the blood of Pacific herring (Clupea pallasii)

“…Natural hosts from the Pacific coast of North America include coho Oncorhynchus kisutch, Chinook O. tshawytscha, chum O. keta, pink O. gorbuscha salmon and Pacific herring Clupea pallasii (Evelyn and Traxler, 1978;MacMillan and Mulcahy, 1979;Rohovec and Amandi, 1981;Hershberger et al, 2013). Disease outbreaks periodically occur in populations of Pacific herring, particularly juveniles, throughout coastal regions of the NE Pacific Ocean in both cultured and wild fish (Meyers et al, 1986;Hershberger et al, 2006Hershberger et al, , 2009. Laboratory challenges of chum salmon, pink salmon, and Pacific herring with blood or tissue homogenates from VENaffected fish have shown these species to be highly susceptible to infection and have reproduced essential features of the disease (Evelyn and Traxler, 1978;MacMillan and Mulcahy, 1979;Haney et al, 1992;Glenn et al, 2012).…”