Testing the effectiveness of a Yersinia vaccine in infected and chemically treated juvenile rainbow trout (Oncorhynchus mykiss)

Summary Formalised, high pH‐lysed bacterin was prepared from Yersinia ruckeri serotype O1 which was isolated from rainbow trout farms in Turkey after an epizootic outbreak. The vaccine was diluted (1:10) and applied in a one‐minute bath to 2.8–3.5g juveniles which were free from yersiniosis and which were kept both under controlled aquarium conditions (15 °C) and on‐site at the reported farms. Experimental challenges were carried out to determine the protection effect after exposing fish in a one‐minute bath to a concentration of 106 cfu ml‐1 of pathogenic Yersinia ruckeri serotype O1 at days 25, 76, and 125 following vaccination. RPS (relative percentage survival) at day 125 reached values of 83 and 85 in both the laboratory and field‐exposed fish, respectively. Bath vaccination was also applied to infected 3–3.5g fry at the end of the antimicrobial treatment period (day 11). Fish were treated with a sulphamethoxasole‐trimethoprime combination (ratio 5:1) via feed for a period of 10 days at a level of 50 mg kg‐1 d‐1. Calculated RPS values for day 25, 76, and 125 were found to be 94, 88 and 79 in field exposed fish, respectively. More than 3 million healthy‐looking fish (3–20 g weight) were vaccinated, once only, at a farm known as being ‘endemic’ for yersiniosis. No significant yersiniosis outbreak was identified in protected fish up to the end of the production cycle (11 months). However, two outbreaks of yersiniosis resulted into high mortalities (35 % and 25 %) on the farm under investigation while these fish remained unvaccinated (control fish). A booster vaccination after a 4 week interval was not successful and 20 % mortality was observed on the farm.