Histological and ultrastructural studies of abnormal lymphoid organs from a populat~on of cultured Penaeus vannamei (Crustacea: Decapoda) revealed the presence of a previously unreported virus infecting this group of marine invertebrate animals. Named LOW (lymphoid organ vacuolization virus), the virus was found in the cytoplasm of lymphoid organ cells which had a highly vacuolated cytoplasm and intracytoplasmic eosinophilic to pale basophilic, Feulgen negative, inclusion bodies. Many affected cells also possessed pyknotic or karyorrhectic nuclei. In some foci, affected lymphoid cells formed large multicellular spherical structures, termed spheroids, which lacked a central vessel. Icosahedral nucleocapsids averaging 30 nm in diameter were present in dense cytoplasmica ggregates, occasionally forming paracrystalline arrays, or a s single rows of particles thatwere closely associated with host cell membranes where they acquired their host-membrane-derived envelope. Purified enveloped virions had a buoyant density of 1.23 g ml-1 and a diameter of 52 to 54 nm, while nucleocapsids were icosahedral in shape. 30 to 31 nm in diameter, and exhibited a buoyant density of 1.32 g ml-1 Constitutive polypeptides had a molecular weight of 70. 60, 38 and 37 kDa. Based on its size, structure, and virogenesis, LOVV is considered to be a member of the Togaviridae.
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