Redmouth disease is caused by the infection of the enteric bacterium Yersinia ruckeri. The disease was first noticed in rainbow trout Oncorhynchus mykiss in the USA in 1950s and has since been observed in salmonids and some other fishes in many countries. Y. ruckeri can be classified into different biotypes, serotypes, or genotypes, according to biological properties such as motility or enzyme activity, antigenicity, or gene sequences. In particular, O-antigen, which is one of the markers for serotyping, is an important antigen for the vaccination for of Y. ruckeri. In Japan, redmouth disease is listed as one of the notifiable diseases of aquatic animals by the government to prevent occurrences or spreading of the disease. This review describes topics necessary important for the diagnosis and control of the disease.
