Yersinia ruckeri septicaemia in experimentally infected carp (Cyprinus carpio L.) fingerlings

The presence of Yersinia ruckeri, the causal agent of enteric redmouth disease (ERM) in salmonids and a few other freshwater fish, has so far been reported from a variety of sources including the intestine of healthy carp. Since there are no data on the pathogenicity of this bacterium for carp, 15 fingerlings were experimentally infected by intraperitoneal injection of about 5 x 10(5) cells. Thirteen injected fish were moribund or died within 4 days with septicaemic lesions. Two survivors were sampled on Day 28 after infection. Yersinia ruckeri was reisolated from the internal organs of all experimental fish. By histopathological examination moribund fish had generalised bacteriaemia with inflammation, degeneration and necrotic foci in kidney, liver and spleen, corresponding to findings described previously in ERM of rainbow trout. Survivors of challenge on Day 28 had a chronic disease characterised by prominent peritonitis and enteritis, exhaustion of the erythroid, granuloid and lymphoid components in haematopoietic kidney tissue as well as focal degeneration and necrosis in organs. These data indicate a high sensitivity of carp to intraperitoneal infection with a relatively low dose of Y. ruckeri.