Virus-like particles in erythrocytes of coho salmon ( Oncorhynchus kisutch)

Virus-like particles were found in erythrocytes of coho salmon ( Oncorhynchus kisutch) reared in seawater in Northern California, U.S.A. Erythrocytes from these salmon had numerous rod-shaped cytoplasmic inclusions, ranging in size from 1 to 2 μm. These inclusions were pink to red following Lieshman-Giemsa staining and were present in all of the erythrocytes from several fish. An examination of these erythrocytes by electron microscopy showed numerous spherical virus-like particles. The particles had a mean diameter of 106 nm (s.d. = 2.7, n = 20) and were evident within membranebound vacuoles in the cytoplasm and between the inner and outer aspects of the nuclear envelope. The particles were spherical, sometimes found budding into the cytoplasmic vacuoles, and possessed regular surface projections or spikes on a tightly fitting external envelope. The consistent size and structure of these particles and their absence in normal erythrocytes suggests they may represent a new virus not previously recognized in salmonid erythrocytes.