Effects of fish size and route of infection on virulence of a Danish Yersinia ruckeri O1 biotype 2 strain in rainbow trout ( Oncorhynchus mykiss )
2019
The comparative virulence of Yersinia ruckeri serotype O1 biotype 2 strain 07111224 from a Danish outbreak of
enteric redmouth disease in rainbow trout ( Oncorhynchus mykiss ) was investigated in different sizes of rainbow
trout (5, 19, 25 and 52 g) using a panel of challenge methods (bath, intraperitoneal (IP) injection, anal intubation
and cohabitation).
The results show that Y. ruckeri 07111224 is virulent enough to successfully cause mortality in 5–52 g
rainbow trout through three different infection routes (bath, IP injection and anal intubation), and strongly
suggests that susceptibility to Y. ruckeri infection is dependent on fish size. Bath challenge (10 8 CFU/ml) caused
79% mortality in 5 g fish, while the mortality in 19–52 g fish was approximately 26%. IP injection challenges
(10 5 –10 6 CFU/individual) resulted in high mortalites in fish of all sizes. Anal intubation challenges (109 CFU/
individual) induced 82% mortality in 5 g fish and decreased with increasing fish weight. Finally, cohabitation
challenges induced 45% mortality in 5 g test fish when test fish were cohabitated 1:1 with shedder fish, while
mortalities in test fish of five other challenge groups were lower (5–25%). Statistical models were fit to the data
from each challenge model. The model parameter predictions suggests statistically significant effects of fish
weight for bath, IP and anal intubation challenges. Furthermore, they highlight the importance of the ratio of
shedders to test fish in cohabitation challenges and of the challenge doses, in particular for the IP challenge
model.
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