VIRUSES Aquareovirus I. Causative Agent and Disease associated with epizootic fish mortal- Aquareovirus is a genus in the virus ity producing severe hemorrhaging in family Reoviridae. These icosahedral fingerlings and yearlings resulting in up (60-80 nm) 11 segmented double- to 80% mortality. stranded RNA viruses (over 50) have been isolated from a variety of marine IV. Transmission and freshwater aquatic animals world- Transmission is horizontal via water wide including finfish, and bivalve mol- or from fish to fish. Isolates from bivalve luscs. Genetic analyses have identified 7 mollusks likely represent virus that has different genotypes or species (A-G) of been shed into the water column from a aquareoviruses. Most of these viruses fish host and then bioaccumulated into produce self-limiting infections of low shellfish tissues by filter feeding. pathogenicity and are not associated with extensive disease or mortality. V. Diagnosis Exceptions include isolates from 7 fish Detection of Aquareovirus is by species that have been associated with isolation of the virus in cultures of fish mortality, most notably the grass susceptible fish cell lines inoculated carp aquareovirus (G). The viral agents with infected tissue. The virus causes a are most often isolated from asymp- unique cytopathic effect (CPE) charac- tomatic adult carrier fish during routine terized by focal areas of cellular fusion screening examinations. (syncytia) and cytoplasmic destruction creating a vacuolated or foamy appear- II. Host Species ance. The exception is grass carp species In the Pacific Northwest states of G that produces a diffuse CPE. Presump- Washington, Oregon and California, tive identifications are made based on adult Chinook salmon appear to be the the typical CPE and are confirmed by most frequent species infected with serology, electron microscopy or poly- aquareovirus A or B. The virus has also merase chain reaction (PCR). been isolated from adult coho and chum salmon and steelhead. Rainbow trout VI. Prognosis for Host have been experimentally infected with The prognosis for the fish host is the virus resulting in mild hepatitis with good in the majority of cases where the no overt disease or mortality. In Alaska, virus is not a primary pathogen. There aquareoviruses have been isolated are no corrective therapies for viral from Chinook salmon (species B) and infections in fish except avoidance. geoduck clams (species A). VII. Human Health Signifcance III. Clinical Signs There are no human health concerns Fish naturally infected with aquareo- associated with aquareoviruses. viruses generally do not exhibit clinical signs of disease. Experimental infections can produce focal necrotic lesions in the livers of rainbow trout, chum salmon and bluegill fry. Other pathogenic exceptions include the grass carp species G that is 2 VIRUSES Large rounded plaques of syncytial cell CPE (arrow) of Aquareovirus in bluegill fry cells. Double capsid morphology of Aquareovirus particles (arrow) in negative stain; transmission electron microscopy, X 91,000. 3 .