Abstract

Our laboratory has been investigating a recurrent problem of episodic mortality in fry and juvenile koi carp. Die-offs in these age groups occurred sporadically on a farm where fish were grown in earthen ponds. Multiple lots of juvenile fish transported to our pathogen containment facility exhibited 98-99% mortality over a period of 3-7 days post transport. Moribund animals exhibited lethargy, signs of hypoxia, and gill pallor, while no evidence of parasitic or bacterial infection was consistently found on necropsies. Abnormal findings consisted of excessive proliferation and sloughing of epidermal cells from the gills and body surfaces of affected fish. Electron microscopy of affected epithelial cells demonstrated poxvirus-like virions (360 nm dia.) intracytoplasmically. The size, morphology and cellular location of the virus indicated it was not a herpesvirus, one viral agent known to induce systemic disease and mortality in juvenile koi (under experimental conditions only). The characteristics of the virus we observed are most similar to poxviruses; such a virus has been described and investigated in koi carp in Japan, but this is the first report of it in koi in the United States. While the source of the virus is unknown at this point, similar poxviruses occur in insects (entomopoxviruses), and it may be that this agent is capable of moving from predatory aquatic insects to juvenile koi in grow-out ponds. Investigations are in progress at our facility to compare the characteristics of the virus we observed with those of the Japanese isolates, and to examine possible modes of transmission of the agent between insects and fish in the pond environment.