Investigators probing the source of a listeria outbreak said Thursday the strain that killed three people was found at a dairy processing plant in central Massachusetts.
But officials have not yet determined exactly where the milk was contaminated.
“We know that there’s a problem in that plant and we have connected the patients to the products to the plant, now it would be nice to know exactly how that happened, but that is part of the ongoing investigation,” said Dr. Alfred DeMaria, state director of communicable disease control.
Three elderly men have died since June after drinking bacteria-contaminated milk from the Whittier Farms plant in Shrewsbury, about 35 miles west of Boston. The same strain of listeria sickened a pregnant woman, who then miscarried. A second woman also was sickened after drinking milk from the plant.
No new cases have been identified and health officials say the public health risk is low.
The deadly strain was found in seven bottles and on the floor near the homogenizer at the Shrewsbury plant. Different listeria strains were found inside four other bottles of milk and at three different areas inside the plant — in a drain, in a bottle washer and in an empty unwashed bottle. More than 100 samples were taken.
It remains unclear how the milk was contaminated, but one theory is that processing equipment may have unintentionally been contaminated during cleaning, state officials said.
The processing plant, which distributes milk under various brand names to homes and stores across central Massachusetts and also operates a 500-acre farm in the town of Sutton, will remain closed during the investigation and until the bacteria is eradicated, officials said.
A call to Whittier Farms on Thursday was not immediately returned.
Listeriosis is a kind of food poisoning that can be dangerous to the elderly, newborns, pregnant women and people with chronic medical conditions. Symptoms include fever, headache, stiffness, nausea, abdominal pain and diarrhea.