Location: Cool and Cold Water Aquaculture Research
Title: Acute mortality, bacterial load and pathology of select lines of adult rainbow trout challenged with Weissella sp. NC36 Authors
Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 23, 2013
Publication Date: N/A
Interpretive Summary: Selective breeding for disease resistance offers new opportunities for improving fish welfare in aquaculture. At our location, we have produced a line of rainbow trout that has increased resistance against bacterial cold water disease (BCWD). The response of this line to unrelated pathogenic microorganisms is unknown. In this study, we compared the susceptibility of the BCWD resistant line, a BCWD susceptible line, a selection-control line and a commercially available trout line following challenge with an unrelated bacterial pathogen. We utilized Weisella sp. NC36 in these studies, as this pathogen was isolated from a severe disease outbreak in North Carolina, and future farm trials are planned at this site. In our experiments, we infected rainbow trout lines by injecting Weissella sp. NC36 and monitored clinical signs of disease, survival and body weight over 9 days. Survival of the BCWD resistant line did not differ compared to the control, susceptible or commercial line. However, a higher body weight at the time of challenge was found to correlate with an increased risk of death. Infected fish showed clinical signs of inflammation and bleeding around the eye, bleeding in the brain, and inflammation around the heart. The amounts of bacteria in the spleen and brain of infected fish did not differ between the BCWD resistant and susceptible line fish. Taken together, these findings demonstrate that breeding for BCWD resistance did not alter (either positively or negatively) the ability to fight off experimental infection by Weissella sp. NC36. Additionally, this study is important as it describes pathology associated with experimental Weisselosis and demonstrates for the first time that this disease can occur at a water temperature (12.7 deg C) relevant to a large segment of the rainbow trout aquaculture industry.
Technical Abstract: A challenge of genetic improvement for disease resistance in fish is to understand specificity of resistance and whether selection for one pathogen alters the response to unrelated pathogenic microorganisms. Adult rainbow trout Oncorhynchus mykiss that had been bred for differential susceptibility to Flavobacterium psychrophilum, designated ARS-Fp-R (resistant), ARS-Fp-S (susceptible), and ARS-Fp-C (control line), and a pool of commercial stock fish, were intraperitoneally challenged with Weissella sp. NC36. Clinical signs, survival, and innate mechanisms affecting disease resistance were monitored over 9 days. Acute disease signs included exophthalmia associated with retro bulbar inflammation and hemorrhage, cerebral hemorrhage, and mild to moderate granulomatous pericarditis. The ARS-Fp-R line did not demonstrate significant survival differences over a 9-day period compared to the ARS-Fp-C and -S lines (P is greater than or equal to 0.09), indicating that during the acute phase of disease, the resistance factors that limit BCWD do not confer cross protection against Weissella sp. NC36. The linear effect of body weight at challenge was statistically significant, as each 10-gram increase in body weight increased the hazard of death by 1% (P equals 0.02). Bacterial loads on day 3, assessed by splenic and cerebral CFU counts, did not differ between ARS-Fp-R and ARS-Fp-S fish and there was no correlation between CFU counts and body weight. These findings help elucidate the specificity of disease resistance in selectively-bred lines and contribute to our understanding of disease caused by Weissella sp., a recently described pathogen of rainbow trout aquaculture.