The bacteria killed two elderly men last year and also affected another two people, including a pregnant woman. DNA fingerprinting carried out by the State Laboratory Institute showed that the bacteria causing these infections came from a common source.
The Massachusetts Department of Public Health (MDPH) is investigating milk products from West Sutton-based Whittier Farms Dairy. On Wednesday it said in a statement that a source of contamination at the firm's processing plant had "not yet been identified".
But samples from Whittier Farms milk products collected from a patient's home and from the farm showed listeria contamination. The DNA fingerprints of the listeria found in the milk samples and in the patients were the same.
It is very rare to discover cases of listeria caused by germs with identical genetic profiles, said investigators, making the dairy a likely source of contamination.
"The pattern is very unique," Dr. Alfred DeMaria, the state's director of communicable disease control, told the Boston Globe. "It means there's an outbreak here. And it implies that the dairy is the common source."
While it is still not known how the bacteria contaminated Whittier's products, the investigation into the source of contamination is increasingly focused on the packaging process, officials of the Department of Public Health told the paper.
Whittier Farms has voluntarily suspended operations and distribution of its milk products until the source of contamination is identified and corrected. It has also contacted customers and distributors of its products.
The company sells milk products under brands including Whittier, Schultz, Balance Rock, Spring Brook, Model Dairy and Maple.
Listeriosis is a serious foodborne disease that can be life-threatening to certain individuals. Most cases of severe listeriosis occur in adults with weakened immune systems, the elderly, pregnant women and newborns. About 2,500 cases of listeriosis occur each year in the United States.
Twenty-two cases of listeriosis were reported to MDPH in Massachusetts in 2006.
In November 2007 the USDA announced a new Food Protection Plan, which FDA Commissioner Andrew von Eschenbach, MD, described it as a forward-oriented concept that combines use of science and information technology to identify potential hazards before they have an impact.
The plan has three strings to it, of which the first - the prevention of foodborne contamination - aims to promote increased corporate responsibility to prevent illness. It also seeks to identify and assess vulnerabilities and expand understanding and use of mitigation measures.
The second string to the plan is intervention at critical stages in the food supply chain, including focus inspections and risk-based sampling, risk-based surveillance, and better detection of signals that indicate contamination has occurred.
Finally the FDA is aiming to respond more rapidly to problems so as to reduce the impact, and improve its communication on risks to the public, industry, and other stakeholders.