Pathogenicity of Streptococcus ictaluri to channel catfish.

The pathogenicity of a Streptococcus ictaluri isolate in channel catfish Ictalurus punctatus at the fry (0.5 g), fingerling (15 g), and juvenile (55 g) stages was determined by experimental bath immersion and injection experiments. Channel catfish were exposed in 1-L immersion baths containing 10(8), 10(9), 10(10), 10(11) or 10(12) colony-forming units (cfu) of S. ictaluri. Fish were also injected intraperitoneally with 0.1 mL of bacterial solution for final doses of 10(4), 10(5), 10(6), 10(7), or 10(8) cfu of S. ictaluri per fish. Streptococcus ictaluri caused mortality in fry, fingerling, and juvenile channel catfish within 21 d postinfection. When mortalities were calculated based on size and challenge route, the cumulative percent mortalities were 11% for fry and 0% for fingerlings by the bath immersion route and 14% for fingerlings and 6% for juveniles by the injection route. Isolation of S. ictaluri from moribund and dead catfish was confirmed by the newly established BIOLOG profile (MicroLog3 system). The results indicate that channel catfish were only susceptible to high concentrations of S. ictaluri and that juvenile channel catfish were less susceptible, possibly explaining why little mortality has been attributed to S. ictaluri infection in catfish aquaculture.