Streptococcal infections of farmed fish

Résumé

The genus Streptococcus is large and complex, accommodating a wide range of Gram positive bacteria. Only a few biotypes have been isolated from fish and the most pathogenic are those belonging to D serogroup, otherwise known as the Enterococci. Streptococcal septicaemia was first among cultured rainbow trout (Onchorynchus mykiss) in Japan in 1958. Since then, the disease has appeared sporadically or in epizootics among cultured or wild fish both in freshwater and marine environments all over the world. Among the freshwater species infected, rainbow trout (Onchorynchus mykiss) and tilapia (Oreochromis niloticus) are the most important species. Clinical signs vary among species of affected fish. The most common symptoms are loss of appetite, erratic swimming, darkening of body colour, eye lesions, external haemorrhagic lesions and ulcerations. The standard system for identification is based on the antigenicity of the carbohydrate moiety of the cell wall, according to a scheme devised by Lancefìeld in the 1930's. The American freshwater isolates obtained by Robinson & Meyer (1966), Plumb et al. (1974), Rasheed & Plumb (1984) were all typed Lancefield's group Β serotype in contrast to almost all of the much more frequent isolates from marine fish, which are usually untypable Enter ο co ecus-like strains. The Streptococcus strains isolated from yellowtail (Seriola sp), ayu (Plecoglosus altivelis) and flounder (Rhombosolea. sp) have similar biochemical reactions to Streptococcus iniae, a species isolated from dolphins by Pier & Madin (1976), which is defined in Bergey's Manual as a separate species. Kusuda et al.(1991) have analysed numerous strains from yellowtail infections in mariculture and in eels and have concluded, on the basis of DNA/DNA hybridization studies and biochemistry that a specific condition, caused by such streptococci, exists and that these bacteria were closest to Enterococci in characteristics and that infectious coccosis of marine fish, as seen in Japan, should be called "Enterococcal infection". Enterococci released from diseased fish seem to be the main source of infection. It has been shown that these bacteria remain in seawater and particularly in the mud around farms for a long time. Higher numbers of microorganisms exist in the seawater during summer months in contrast to the winter months when the bacterial load is higher in the mud (Kitao et al. 1979). Ghittino & Prearo (1992) have shown that in freshwater fish the outbreak of the disease was related to an organic pollution of the rivers supplying fish farms, associated with high water temperature (21-22°C). Wild fish can also be infected and spread the bacteria. It is generally believed that the infection is horizontal with infection occurring from direct contact with infected fish or contaminated fish food (Robinson & Meyer, 1996). The mechanisms of enterococcal infections pathogenicity are not yet fully understood.