Association between Plasma Antibody Response and Protection in Rainbow Trout Oncorhynchus mykiss Immersion Vaccinated against Yersinia ruckeri

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

Determining vaccination frequency in farmed Rrinbow trout using <em>Vibrio anguillarum</em> O1 specific serum antibody measurements

BACKGROUND: Despite vaccination with a commercial vaccine with a documented protective effect against Vibrio anguillarum O1 disease outbreaks caused by this bacterium have been registered among rainbow trout at Danish fish farms. The present study examined specific serum antibody levels as a valid marker for assessing vaccination status in a fish population. For this purpose a highly sensitive enzyme-linked immunosorbent assay (ELISA) was developed and used to evaluate sera from farmed rainbow trout vaccinated against V. anguillarum O1. STUDY DESIGN: Immune sera from rainbow trout immunised with an experimental vaccine based on inactivated V. anguillarum O1 bacterin in Freund’s incomplete adjuvant were used for ELISA optimisation. Subsequently, sera from farmed rainbow trout vaccinated with a commercial vaccine against V. anguillarum were analysed with the ELISA. The measured serum antibody levels were compared with the vaccine status of the fish (vaccinated/unvaccinated) as evaluated through visual examination. RESULTS: Repeated immunisation with the experimental vaccine lead to increasing levels of specific serum antibodies in the vaccinated rainbow trout. The farmed rainbow trout responded with high antibody levels to a single injection with the commercial vaccine. However, the diversity in responses was more pronounced in the farmed fish. Primary visual examinations for vaccine status in rainbow trout from the commercial farm revealed a large pool of unvaccinated specimens (vaccination failure rate = 20%) among the otherwise vaccinated fish. Through serum analyses using the ELISA in a blinded set-up it was possible to separate samples collected from the farmed rainbow trout into vaccinated and unvaccinated fish. CONCLUSIONS: Much attention has been devoted to development of new and more effective vaccines. Here we present a case from a Danish rainbow trout farm indicating that attention should also be directed to the vaccination procedure in order to secure high vaccination frequencies necessary for optimal protection with a reported effective vaccine

Boganmeldelser

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Flow chart of the experimental setup.

<p>A total of 800 rainbow trout were divided into four groups each containing 200 fish. One group was immersion vaccinated with the experimental bacterin vaccine. Two groups were immersion-vaccinated with the commercial AquaVac™ ERM. One of these groups received an oral booster vaccination with AquaVac™ ERM Oral vet 16 weeks post vaccination. All vaccines were diluted 1∶10 in water, and the fish were immersed for 5 minutes. The control group was sham-immersion vaccinated in pure water. A subsample of 25 rainbow trout from each group were given a challenge with <i>Y. ruckeri</i> 8 and 26 weeks post vaccination to monitor protection. Ten fish from each group were killed and used for plasma sampling 4, 8, 12, 16, and 26 weeks post vaccination.</p

Bactericidal effect of rainbow trout plasma.

<p>The present experiment shows that plasma from sham vaccinated rainbow trout contains factors that are able to kill a significant amount of <i>Y. ruckeri</i> (A, p<0.01 and B) p<0.05. Further it is shown that plasma obtained from immersion vaccinated trout kill a significant higher amount of <i>Y. ruckeri</i> than plasma from sham-vaccinated trout. Bars represent mean values + SD values. * Depicts statistical significance between groups (*<i>P</i><0.05; **<i>P</i><0.01; ***<i>P</i><0.001).</p

Descriptive data for 100 commercially farmed rainbow trout.

<p>Condition factor: CF = (W/L³)×100; where W = weight (g) and L = total length (cm) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049672#pone.0049672-Kheyrandish1" target="_blank">[18]</a>.</p><p>Speilberg score is a measure of vaccine-induced side effects.</p