Diseases of Wild and Cultured Shellfish in Alaska

BIVALVE mOLLUSc VIRUSES Aquabirnavirus

I. Causative Agent and Disease III. Clinical Signs Aquabirnavirus is a recent new Bivalve molluscs are generally genus in the virus family Birnaviridae. asymptomatic carriers and/or vectors of These unenveloped icosahedral (~60 these viruses. nm) viruses (over 200 isolates) contain a bi-segmented double stranded RNA IV. Transmission genome that encodes 5 proteins and Transmission is horizontal animal to have been isolated in cell culture from a animal via water. Isolates from bivalve variety of marine and freshwater fish and molluscs may also represent bioaccumu- shellfish species worldwide. There are lation from filter feeding after the virus three and possibly four serogroups (A, B, is shed into the water column from a C & D) comprising at least 16 serotypes nearby fish host. of aquabirnaviruses. Molecular testing has determined there are currently 7 V. Diagnosis genogroups. Several of these viruses oc- Detection of Aquabirnavirus is ac- curring in finfish cause disease (such as complished either by direct examination IPNV in salmonids) while those infect- of shellfish tissues with transmission ing molluscs are mostly apathogenic, al- electron microscopy (TEM) or by isolat- though some isolates have been reported ing the virus in cultures of susceptible to cause cell pathology or mortality in fish cell lines that have been inoculated stressed bivalves. Some of these viruses with contaminated or infected shellfish that are shed into the water column by tissue. Cytopathic effect (CPE) is gener- a fish host may be bioaccumulated by ally a nondescript diffuse thinning and nearby bivalve molluscs through the necrosis of infected cells. Identification filter feeding mechanism. These viral is based on serology and TEM or by agents in shellfish are most often isolated polymerase chain reaction (PCR). There from asymptomatic adult animals during are no established bivalve mollusc or routine virus screening examinations. crustacean cell lines for isolating viruses that may have a strict host specificity for II. Host Species shellfish, although there has been some Currently, aquabirnaviruses have success in culturing primary cell mono- been isolated from many different layers from certain shellfish species. fish and shellfish species including 32 families of finfish, 11 species of bivalve VI. Prognosis for Host molluscs and at least 4 species of crus- Bivalve molluscs are asymptomatic taceans. In Alaska, Aquabirnavirus carriers and/or vectors of these viruses. has been isolated in fish cell lines from asymptomatic adult littleneck clams VII. Human Health Significance and an aquabirnavirus-like agent has There are no zoonotic human health been observed by transmission electron concerns associated with aquabirnavi- microscopy in a male blue king crab ruses in shellfish. associated with an adenocarcinoma-like proliferative lesion (discussed in the crustacean section).

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 TEM of cultured bluegill fry cell with cytoplasmic aggregate of hexagonal- shaped aquabirna-like virus particles (arrow) isolated from littleneck clams

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CPE in CHSE-214 cells showing areas of cell rounding and necrosis (arrow) typical of Aquabirnavirus

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