Vaccination of rainbow trout, Salmo gairdneri Richardson, at low temperatures and the long-term persistence of protection

Experiments showed that the vaccination of rainbow trout, Salmo gairdneri , against vibriosis in spring, before the occurrence of temperatures at which outbreaks of the disease become frequent, is a worthwhile procedure. The persistence of protection through winter conditions was also studied. A formalised vaccine of a strain of Vibrio anguillarum , highly pathogenic for rainbow trout, was administered by intraperitoneal injection (i.p.), by direct immersion (d.i.) and perorally. Results showed that despite the low temperature regime at the time of vaccination almost complete protection is obtained with the i.p. treatment and 47% with the d.i.; protection by peroral administration is ineffective. In the case of fry and small fish, i.p. is not a realistic commercial practice but the less effective d.i. treatment may confer worthwhile protection against natural infection in a farmed population. Vaccination early in the season, repeated annually, is recommended.