Farm size, seining practices, and salt use: risk factors for Aeromonas hydrophila outbreaks in farm-raised catfish, Alabama, USA.

In freshwater aquaculture systems, Aeromonas hydrophila is usually considered to be an opportunistic pathogen most often associated with secondary bacterial infections. Since 2009, the U.S. catfish industry, especially in West Alabama, has been affected by mortality from a strain of A. hydrophila that has been acting as a primary pathogen. Tens of millions of pounds of catfish production have been lost as a consequence of this disease. This study used data from two whole-population farmer surveys to examine farm-level risk factors for two A. hydrophila outbreaks in foodsize Alabama catfish, one in 2009 (surveyed in 2010), and one in 2011 (surveyed in 2012). The response to the 2010 survey was 85% and the response to the 2012 survey was 82%. Univariate analyses were used to examine biologically plausible variables (farm size, pond stocking density, seine exposure, use of salt (NaCl) in ponds), and used categorical disease outcome and dependent variables. Farm size was included in bivariate analyses with the other variables, because it was a potential confounding variable. For both study years, the odds of an A. hydrophila outbreak were significantly greater for farms larger than the mean size (2009: mean=132 acres (53.4 hectares), odds ratio (OR)=8.2; 95% confidence interval (CI)=3.3-20.6, p<0.001; 2011: mean=116 acres (46.9 hectares), OR=5.3, CI=1.7-17.0, p=0.009). Compared with 42% of control farms, every case farm was seined by a commercial or processing plant seining crew in 2009. The bivariate analysis of the 2011 variable "average number of times each pond was seined per year" indicated that regardless of farm size, farms with ponds that were seined more than twice per year had a significantly greater odds of an A. hydrophila outbreak (OR=4.1, CI=1.2-14.4, p=0.02). For 2009, the results of the bivariate analyses of chloride concentrations indicated that farms that had chloride concentrations >135 ppm had a significantly lower odds of experiencing A. hydrophila outbreaks (OR=0.2, CI=0.05-0.6, p-value=0.004). To achieve economies of scale, catfish farmers raise fish on large farms at higher stocking densities, but this practice may result in increased susceptibility to disease outbreaks. Producers should prioritize implementing biosecurity measures such as improved seining practices and other management practices to protect fish grown at high population densities. Further work will determine what the detailed seining protocols should include, and whether the use of salt, and at what concentrations, reduces the risk of A. hydrophila outbreaks.