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.

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The number of reports increased substantially in July when the peak of the outbreak appears to have occurred. From May 1 to October 15, 2010, 2010, a total of 3,182 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 1,369 total illnesses during this same period. This means there are approximately 1,813 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 evaluating advanced molecular methodologies to see if they help distinguish between outbreak-related cases and sporadic (or background) cases.