Pulsed-Field Gel Electrophoresis (PFGE) Cluster Detection and Investigation in the United States

BACKGROUND:   State and local health departments conduct routine surveillance for lab-confirmed cases of Salmonella and STEC infection, which are reportable in every state. State public health laboratories perform tests to subtype these cases, including molecular analysis like PFGE. Once these cases are reported to the local health authority, public health practitioners investigate each case to assess the risk of ongoing transmission, gather information about possible sources of exposure, and provide education as indicated. We wanted to assess how HDs detect and investigate molecular-subtype clusters of Salmonella and Shiga-toxigenic E. coli (STEC) infections and to describe how state health departments collect, compile, and analyze food and animal exposure data from reported cases of Salmonella and STEC infection.

METHODS:   We surveyed each of 50 U.S. state health departments (HDs) and the District of Columbia (DC) online via SurveyMonkey.com. Questions focused on PFGE cluster definition, detection, and investigation. HDs were contacted to establish who the designated participant would be; participants entered answers to survey questions online. Questionnaire data was exported and analyzed using SPSS.

RESULTS:   100% of state health departments and the District of Columbia completed the survey. Over 50% of states reported having a standardized PFGE cluster definition in place; clusters were most frequently detected by state public health laboratorians. Nearly all sites (88%) reported using a standardized hypothesis-generating questionnaire to investigate PFGE clusters and many sites reported using a custom questionnaire. More than 60% of sites reported waiting until a PFGE cluster is identified by the state public health laboratory before they begin conducting hypothesis-generating interviews with cases. Almost half of sites reported that data from PFGE investigations is most commonly analyzed at CDC (and not locally).

CONCLUSIONS: Though CIFOR recommends that state health departments establish standardized PFGE cluster definitions, nearly half of sites reported they have not done this. Most states are waiting until PFGE clusters are identified to begin investigations; less than one third of the states are able to interview sporadic cases of Salmonella and STEC as they are reported and of these states, over half receive dedicated federal funding for this enhanced surveillance. When PFGE clusters are investigated locally, analysis is most frequently conducted at the federal level; many sites reported conducting local analysis as well (though not as frequently). States are locally entering data from PFGE investigations in a variety of ways; some sites are aggregating these data to estimate background food exposures among cases.