In July 2010, CDC identified a nationwide sustained increase in the number of Salmonella Enteritidis isolates with PFGE pattern JEGX01.0004 uploaded to PulseNet, the national subtyping network made up of state and local public health laboratories and federal food regulatory laboratories that performs molecular surveillance of foodborne infections. This increase began in May 2010 and is evident in the epidemic curve, or epi curve. The number of reports increased substantially in July when the peak of the outbreak appears to have occured. From May 1 to August 31, 2010, a total of 2,508 illnesses were reported. However, some cases from this period have not been reported yet, and some of these cases may not be related to this outbreak. Based on the previous 5 years of reports to PulseNet, we would expect approximately 1,039 total illnesses during this same period. This means there are approximately 1,469 reported illnesses that are likely to be associated with this outbreak. Many states have reported increases of this pattern since May. Because of the large number of expected cases during this period, standard methods of molecular subtyping alone are not sufficient to determine which reported cases might be outbreak-associated. CDC is currently conducting testing using advanced molecular methodologies to help distinguish between outbreak-related cases and sporadic (or background) cases.
Illnesses that occurred after July 29, 2010 might not yet be reported due to the time it takes between when a person becomes ill and when the illness is reported. This takes an average of 2 to 3 weeks for Salmonella.
Epidemiologic investigations conducted by public health officials in 10 states since April have identified 29 restaurants or event clusters where more than one ill person with the outbreak strain has eaten. Data from these investigations suggest that shell eggs are a likely source of infections in many of these restaurants or event clusters. Wright County Egg, in Galt, Iowa, was an egg supplier in 15 of these 29 restaurants or event clusters; three are clusters that have been recently reported, but occurred earlier in the outbreak. Traceback investigations are ongoing for several of these clusters. A formal traceback was conducted by state partners in California, Colorado, and Minnesota, in collaboration with FDA and CDC, to find a common source of shell eggs. Wright County Egg in Iowa was found as the common source of the shell eggs associated with three of the clusters. Through traceback and FDA investigational findings, Hillandale Farms of Iowa, Inc., was identified as another potential source of contaminated shell eggs contributing to this outbreak. FDA is nearing completion of initial investigations at both of these firms in Iowa. The investigations involve sampling, records review and looking for potential sources of contamination, such as feed. FDA’s inspectional observations, in addition to sample results, indicate substantial potential for Salmonella to have persisted in the environment and to have contaminated eggs.