Oral vaccination against enteric red mouth disease in rainbow trout (Oncorhynchus mykiss Walbaum). The effect of vaccine dose rate on protection against the disease

Vaccines based on Yersinia ruckeri serotype O1 were incorporated into feed and administered to four groups of rainbow trout (Oncorhynchus mykiss) weig hing 17 g. Groups of fish were fed different dosages of the vaccines by var ying the feeding period from 3 to 6 days. After 500 degree days the fish we re challenged (intraperitoneally) with different isolates of Y. ruckeri ser otype O1 in triplicates. The mortality in unvaccinated controls ranged from 52 to 86%. A significant effect of dosage response was observed: groups fe d two or more vaccine units per fish achieved relative percentage survival (RPS) between 56 and 67%. Good cross protection between different biotypes of Y. ruckeri serotype O1, and no differences in protection between isolate s from different geographical areas in Europe were observed. The correct do sage of oral vaccine against Y. ruckeri gave sufficient protection against enteric red mouth disease.