Abstract
A key hallmark of the vertebrate adaptive immune system is the generation of antigen-specific antibodies from B cells. Fish are the most primitive gnathostomes (jawed vertebrates) possessing an adaptive immune system. Vaccination of rainbow trout against enteric redmouth disease (ERM) by immersion in Yersinia ruckeri bacterin confers a high degree of protection to the fish. The immune mechanisms responsible for protection may comprise both cellular and humoral elements but the role of specific immunoglobulins in this system has been questioned and not previously described. The present study demonstrates significant increase in plasma antibody titers following immersion vaccination and significantly reduced mortality during Y. ruckeri challenge.Rainbow trout were immersion-vaccinated, using either a commercial ERM vaccine (AquaVac™ ERM vet) or an experimental Y. ruckeri bacterin. Half of the trout vaccinated with AquaVac™ ERM vet received an oral booster (AquaVac™ ERM Oral vet). Sub-groups of the fish from each group were subsequently exposed to 1x109 CFU Y. ruckeri/ml either eight or twenty-six weeks post vaccination (wpv). All vaccinated groups showed 0% mortality when challenged, which was highly significant compared to the non-vaccinated controls (40 and 28% mortality eight and twenty-six weeks post vaccination (wpv), respectively) (P<0.0001). Plasma samples from all groups of vaccinated fish were taken 0, 4, 8, 12, 16 and 26 wpv. and Y. ruckeri specific IgM antibody levels were measured with ELISA. A significant increase in titers was recorded in vaccinated fish, which also showed a reduced bacteremia during challenge. In vitro plasma studies showed a significantly increased bactericidal effect of fresh plasma from vaccinated fish indicating that plasma proteins may play a role in protection of vaccinated rainbow trout.
Highlights
Yersinia ruckeri is the aetiological agent of enteric red mouth (ERM) disease or yersiniosis, affecting salmonids in general and rainbow trout in particular [1,2]
It has been reported that antibodies in rainbow trout only in few cases are associated with protection following immersion vaccination [12] and protection induced by i.p. injection of Y. ruckeri bacterin does not seem to be due to agglutinating antibodies [13,14]
Detection of Y. ruckeri specific IgM antibodies in plasma The Y. ruckeri specific IgM antibody titers were significantly higher 26 weeks post-vaccination compared to week 4 postvaccination
Summary
Yersinia ruckeri is the aetiological agent of enteric red mouth (ERM) disease or yersiniosis, affecting salmonids in general and rainbow trout in particular [1,2]. Protective immunity in rainbow trout against ERM induced by immersion vaccination using formalin killed Y. ruckeri has been known since 1976. Y. ruckeri bacterin can be administrated by intraperitoneal (i.p.) injection, immersion and oral administration [7] and the obtained protective immunity is superior with i.p. injection followed by immersion, and oral administration [7] The explanation for this observation might be that the protective effect of the bacterin seem to be dependent on the amount of bacterin uptake in the rainbow trout [8]. It has been reported that antibodies in rainbow trout only in few cases are associated with protection following immersion vaccination [12] and protection induced by i.p. injection of Y. ruckeri bacterin does not seem to be due to agglutinating antibodies [13,14]. The effect of an oral booster vaccination following a primary immersion vaccination was evaluated
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