Assessing the potential for Salmonella growth in rehydrated dry dog food

Background: A substantial percentage of dog owners add water to dry dog food to increase its palatability. The recent association of Salmonella contamination of dry pet foods with salmonellosis cases in both dogs and their owners has generated a need to determine the ability of Salmonella to grow in eight commercial brands of rehydrated dry dog food. Results: Eight brands of commercial dry dog food were rehydrated to 20, 35 and 50% added moisture, inoculated with two S. enterica strains (~10 CFU/g) and incubated for 72 h at 18 °C, 22 °C, or 28 °C. Dog food brand, moisture content, and temperature affected pathogen growth/survival patterns. Rehydration to 20% moisture did not support growth of S. enterica, and in general there was a 0.5–2.0 Log decline. At 35% moisture and 28 °C, 4 of 8 brands supported up to 3.4 Log(CFU/g) of growth, while Salmonella levels declined in three brands, and remained unchanged in one. Rehydration to 50% moisture at 28 °C supported increases of up to 4.6 Log(CFU/g) in 5 of 8 brands. Growth kinetics determinations with two of the brands that supported growth had calculated lag times, generation times, and maximum population densities of 4.4 and 2.2 h, 1.4 and 10.8 h, and 7.3 and 6.9 Log(CFU/g) when rehydrated to 35% moisture and held at 30 °C. Conclusions: Results of this study establish that the rehydration of dry dog food with sufficient amounts of water may support the growth of S. enterica. Based on the most rapid observed lag times, growth of Salmonella, if present, in rehydrated dog food could be avoided by discarding or refrigerating uneaten portions within 2–3 h of rehydration. These data allow accidental or intentional rehydration of dry dog food to be factored into predictive microbiology models and exposure assessments.