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Title: A prospective matched nested case-control study of bacterial gill disease outbreaks in Ontario, Canada government salmonid hatcheries

Author
item GOOD, CHRISTOPHER - Freshwater Institute
item THORBURN, MARGARET - University Of Guelph
item RIBBLE, CARL - University Of Calgary
item STEVENSON, ROSELYNN - University Of Guelph

Submitted to: Preventive Veterinary Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2010
Publication Date: 6/3/2010
Citation: Good, C., Thorburn, M., Ribble, C., Stevenson, R. 2010. A prospective matched nested case-control study of bacterial gill disease outbreaks in Ontario, Canada government salmonid hatcheries. Preventive Veterinary Medicine. 95:152-157.

Interpretive Summary: We investigated a potential rearing unit-level risk factors for bacterial gill disease (BGD) in Ontario Ministry of Natural Resources (OMNR) salmonid fish hatcheries. The OMNR raises a variety of indigenous and non-native salmonids propagated from wild and captive brookstocks for stocking the Great Lakes watershed, and employs a system of 10 fish culture stations located throughout the province to carry out these activities. The majority of losses in early rearing fish have been attributed to BGD (a disease associated with the causative opportunistic bacterial pathogen Flavobacterium branchiophilum), but little research (in Ontario and elsewhere) has been conducted to investigate potential rearing unit-level risk factors for BGD. Our findings indicate that rearing units experiencing confirmed outbreaks of BGD were significantly more likely to have, among other things, higher mortality levels in the weeks preceding outbreaks, and were being fed at higher rates than matched control tanks. These major findings are predictive rather than causative; however, their utility in identifying at-risk rearing units prior to observable BGD outbreaks should be considered by hatchery managers attempting to prevent BGD-associated mortalities.

Technical Abstract: Early-rearing salmonids in Ontario, Canada government fish hatcheries have been persistently affected by bacterial gill disease (BGD), and outbreaks at these locations have often been associated with high morbidity and mortality. The causative agent of BGD, Flavobacterium branchiophilum, is considered ubiquitous in fresh water. This paper summarizes a 14-month rearing unit-level prospective nested matched case-control investigation at six Ontario government hatcheries (raising a total of six different salmonid species) to identify, and quantify the effects of, important predictors of BGD outbreaks. Ongoing husbandry data were collected on all early-rearing (<9 months of age) fish tank-lots (“tank-lot” = a segment of a specific fish lot existing in a single hatchery tank for a particular period during the study time frame) at participating hatcheries, and all outbreaks of BGD were confirmed by light microscopy during the study period. Control tank-lots were selected at the end of the study and matched to individual cases based on time, hatchery, and species. Data were analyzed using logistic regression modeling, controlling for fish age. The final multivariable model indicated that affected tank-lots were significantly more likely to have had lower fish numbers, lower individual fish weights, higher mortality levels and higher feeding rates during the week preceding observed BGD outbreaks than were asymptomatic control tank-lots. Refinements in the observation and manipulation of these factors could therefore aid in the prevention of fish losses associated with observable BGD outbreaks. The predictive (as opposed to causal) nature of the identified factors needs to be considered, and further research is required to understand the relationships between these factors and BGD.