Massachusetts Tackles Listeria and Other Foodborne Bacteria Through Teamwork

In June 2007, MDPH’s State Laboratory Institute used pulsed-field gel electrophoresis (PFGE) to identify the “DNA fingerprint” of a novel strain of Listeria monocytogenes in a patient with listeriosis. This strain had neither been previously identified in Massachusetts nor detected by PulseNet. PulseNet is a national network of public health and regulatory laboratories that submit results of PFGE subtyping (the bacterial DNA fingerprints) to a central database where isolates can be compared across states. The state laboratory staff looked for identical matches of the novel Listeria strain and identified it in three additional Massachusetts patients. Using questionnaires developed for the Listeria Initiative, MDPH epidemiologists identified pasteurized milk from a local Massachusetts dairy as a possible source of contamination. A sample of milk provided by one patient confirmed the milk was indeed contaminated with the same strain. This finding directed investigators to the outbreak’s source—a local dairy.

Environmental experts from MDPH’s Food Protection Program conducted a full environmental inspection, which included milk sampling of the local dairy plant. This led to a voluntarily closure of the facility and recall of its dairy products. The team then collected more than 100 additional milk samples and environmental swabs for testing, including swabs from two floor drains and one from the bottle washer. The state laboratory detected the outbreak Listeria strain in eight milk samples and one swab from the floor drain. It was concluded that since records indicated that pasteurization methods at the dairy were adequate, contamination of the product likely occurred after pasteurization.

On February 1, 2008, the milk processing facility was permanently closed.

Massachusetts’ quick detection, coordinated approach, and thorough response saved lives. This unusual Listeria outbreak is one example of teamwork used to tackle food-related emergencies. To continue the fight against foodborne illnesses, MDPH has also

• Enhanced food laboratory capacity at the state level
• Expanded emphasis on food testing within Massachusetts
• Changed state regulations to require clinical laboratories throughout Massachusetts to submit all Listeria isolates to the state laboratory for PFGE testing
• Increased the number of food samples submitted for state laboratory testing three-fold from nearly 100 specimens to 300 specimens annually
• Increased the percentage of Listeria isolates uploaded and cases reported to national surveillance systems four-fold during 2007–2011