ERM booster vaccination of rainbow trout using diluted bacterin: Field studies

Enteric Red Mouth Disease ERM caused by Yersinia ruckeri infection is associated with morbidity and mortality in salmonid farming but immersion vaccination of fry may confer some protection for a number of months. Revaccination of rainbow trout, even by use of diluted ERM immersion vaccine, can under laboratory conditions extend the protection period. The present field study investigated the applicability of the method under practical farming conditions (freshwater earth ponds supplied by stream water). Primary immersion vaccination of trout (3–4g) for 30s in Y. ruckeri bacterin (diluted 1:10) in April 2015 was followed 3months later (July 2015) by 1h bathing of rainbow trout in bacterin (diluted 1:650 or 1:1700) in order to evaluate if this time saving vaccination methodology can improve immunity and protection. Trout were subjected in farms to natural Y. ruckeri exposure in June and July and non-vaccinated and non-boostered fish were less protected as they exhibited increased mortality compared to booster vaccinated fish. Blood and tissue samples were recovered from one farm in the autumn. No significant antibody titer differences were detected between groups but immunohistochemical investigations showed elevated densities of IgM and IgT positive lymphocytes in spleen tissue of vaccinated fish. The limitations and interpretations of the field results are discussed in the light of other infections (parasitic and bacterial) recorded during this farm study.