Occurrence of Yersinia ruckeri infection in farmed whitefish, Coregonus peled Gmelin and Coregonus muksun Pallas, and Atlantic salmon, Salmo salar L., in northern Finland

Abstract

Yersinia ruckeri, the aetiological agent of enteric redmouth disease (ERM), is now generally recognized to be distributed worldwide. The pathology and epizootiology of the disease was first described by Rucker (1966) and although rainbow trout, Salmo gairdneri Richardson, has been the most frequently affected species, all salmonids are now considered to be potential hosts for the organism (McDaniel 1979). The present paper reports the first confirmed occurrence of Y. ruckeri infection in farmed whitefish and Atlantic salmon, Salmo salar L., in Finland, with the known host range being extended to include the whitefish Coregonus peled Gmelin and Coregonus muksun Pallas . The outbreaks of infection involved two fish farms on the Oulujoki and Iijoki rivers in northern Finland (Fig. 1). Infection in whitefish Yersinia ruckeri infection was first identified in July 1982 in a mixed population of 3500 C. peled and C. muksun broodstock fish held in a single 1000-m 2 earthen pond at farm A. The organism was isolated from only one of four diseased C. muksun examined bacteriologically at this time and mortalities during the summer months were light (0 .5-1 .0%) for both species. The following year Y. ruckeri was isolated from the internal organs of both C. peled and-C. muksun on a number of occasions during the period from June to August, but again mortalities remained at a low level (0 .5—F5%) throughout. In 1984 infection was confirmed in C. peled but not C. muksun broodstock (Fig. 2). The low levels of mortality associated with the Y. ruckeri infection are similar to those recorded for diseased brook trout, Salvelinus fontinalis (Mitchill), and landlocked Atlantic salmon in Australia where a Y. ruckeri-like organism was isolated (Llewellyn 1980) and in an ERM outbreak in rainbow trout in Canada (Wobeser 1973). Diseased whitefish showed loss of scales, haemorrhages on the body surface and paired fin bases, and petechiation in the peritoneum and visceral fat. Some affected fish. also exhibited severe inflammation of the distal portion of the intestine and vent. Saprolegnia