Assessing the Potential for Salmonella Growth in Rehydrated Dry Dog Food in a Simulated Home Environment

Introduction: Recent outbreaks of human and canine salmonellosis linked to contamination of dry dog food and pet treats has stimulated interest in better characterizing long term survival of Salmonella enterica in these products. In developing an exposure assessment, it became obvious that a potentially important factor is wetting of the product by dog owners. Apparently, rehydrated pet food can remain at room temperature for substantial periods before being consumed or discarded and any substantial amplification during this period could increase exposure to both pet and owner. 

Purpose: This study investigated the ability of S. enterica to grow in rehydrated dry dog food if held for extended periods at temperatures that would be encountered in the home.

Methods: Eight brands of dry dog food were rehydrated in measured amounts to 20, 35 and 50% moisture and inoculated with a cocktail of Salmonella (~10⁵ CFU/g). Initial characterization examined triplicate samples incubated for 72h at 18°C, 22°C, and 28°C to simulate home environments at various seasons. Brands that supported the greatest growth were subsequently used to characterize growth kinetics.

Results: Dog food brand, moisture content, and temperature influenced pathogen growth/survival patterns.  For example, at 35% moisture/28°C, 4 of 8 (50%) brands supported increases up to 3.39 log CFU/g, while levels in other four declined. Rehydration to 20% moisture generally did not support growth, while rehydration to 50%/28°C showed increases up to 4.55 log CFU/g in 5 of 8 brands. Kinetic studies at 35%/ 28°C showed growth with a calculated lag phase of 4.99h, exponential growth rate of 0.039 log cycles/ h, Tmax  = 68h and Ymax = 7.77 log CFU/g.

Significance: The data provided allows rehydration of dry dog food by pet owners to be factored into the exposure assessment currently being developed for dry dog food. Observed differences among brands suggest this will be a source of substantial uncertainty and variability.