Abstract

This short communication reports two outbreaks of a disease in farmed rainbow trout during December 1986 caused by a virus morphologically indistinguishable from the iridovirus causing epizootic haematopoietic necrosis in redfin perch. The first case was in a group of 6-month-old fish and the second case was in 18-month-old fish. Moribund fish exhibited darkening, ataxia, reduced feeding activity and skin ulcers with a sharply defined border on the lateral body surface. Morbidity approached 100% in the 6 -month-old fish but mortality was only slightly above normal levels at about 0.05% per day. Internally, affected fish showed swelling and protrusion of the kidney, and a pale swollen spleen. Histopathological examination revealed focal hepatocellular necrosis centred on or adjacent to an hepatic artery or vein and there was focal to diffuse necrosis of the splenic red and white pulp and, to a lesser extent, necrosis of the renal haematopoietic tissue. A virus was isolated in RTG-2 cells which electron microscopy revealed to be morphologically indistinguishable from the EHN iridovirus. Experimental infection trials confirmed the pathogenicity of the virus for rainbow trout.