A new species of nucleo-cytoplasmic large DNA virus (NCLDV) associated with mortalities in Manitoba lake sturgeon Acipenser fulvescens.

A newly discovered virus, Namao virus, associated with morbidity and mortality, was detected among juvenile lake sturgeon Acipenser fulvescens being propagated by a conservation stocking program for this endangered species in Manitoba, Canada. The outbreaks resulted in cumulative mortalities of 62 to 99.6% among progeny of wild Winnipeg River or Nelson River lake sturgeon and occurred at 2 geographically separate facilities. Namao virus was detected in almost 94% of the moribund or dead lake sturgeon according to a conventional polymerase chain reaction (cPCR) test that is based upon amplification of a 219 bp fragment of the virus major capsid protein (MCP). The virus itself was large (242 to 282 nm) and icosahedral-shaped with 2 capsids and a condensed bar-shaped core. It was found in virus factories within the host cell cytoplasm and displayed a tropism for the integument. Namao virus caused cellular changes characterized by enlarged eosinophilic epithelial cells in the gills and skin. Samples suspected of containing Namao virus did not have cytopathic effects on primary lake sturgeon or established white sturgeon cell lines. However, viral nucleic acid was detected in the former after prolonged incubation periods. Using primers designed from conserved regions of the MCP from NCLDVs, an estimated 95 to 96% of the Namao virus MCP open reading frame was captured. Phylogenetic analysis using the MCP of Namao virus and 27 other NCLDVs suggested that Namao virus and white sturgeon iridovirus share a common evolutionary past and might be members of the family Mimiviridae or a new, as yet unrecognized, virus family.