Boston, Jan. 17 - As part of its on-going investigation of a Listeria monocytogenes (Listeria) outbreak linked to contaminated milk, the Massachusetts Department of Public Health (MDPH) said today that additional samples of milk products and several environmental samples taken at the Whittier Farms milk processing plant have tested positive for Listeria.
The processing plant, located in Shrewsbury, Massachusetts remains closed and will not re-open until cleared to do so by the MDPH Food Protection Program and U.S. Food and Drug Administration (FDA) Regional Milk Specialists.
One environmental swab, one sample of skim milk and seven flavored milk samples tested positive for the same strain of Listeria found in four of the five cases linked to this outbreak. No sample was available to test in the fifth case linked to the outbreak — a 31 year-old pregnant woman from Middlesex County who drank Whittier milk products and was diagnosed with listeriosis, with a positive culture for the bacteria in September. The woman delivered a healthy child, and mother and child are well.
More than 100 environmental and milk samples were taken from the processing plant as part of the investigation. A total of twelve milk samples and four environmental samples tested positive for varying strains of Listeria contamination (see summary below).
The findings do not pinpoint where the contamination of the milk occurred, but they do suggest that Listeria bacteria colonized somewhere in the processing plant and that the bacteria entered the milk products at some point during the production process. Records indicate that the plant’s equipment met federal standards for time, temperature and flow for effective pasteurization, however, pasteurization at the processing plant will be further examined.
The presence of Listeria in the physical plant of the facility is consistent with contamination occurring during post-pasteurizing processing and bottling. One theory under consideration by health officials is that cleaning activities at the plant may have unintentionally caused contamination of the processing equipment allowing bacteria to enter the finished milk products. Three of the four positive environmental tests for Listeria were collected from sections of the plant that are considered part of the post-pasteurization areas of the facility.
Public health investigators are also closely scrutinizing the lab results from Whittier milk products to determine why most of the Listeria positive milk samples were found in flavored milk products. Health officials theorize that the sugar content of the flavored milk products provided an environment that enhanced bacterial growth.
The Listeria outbreak linked to milk products produced at Whittier Farms represents the first such foodborne outbreak connected to a Massachusetts food processing plant in more than 20 years. Five cases have been linked to the outbreak including three elderly men and two pregnant women. The three elderly men have died.
No new cases have been identified as part of the outbreak investigation.
Health officials stressed the risk to public health remains low because Whittier Farms halted milk processing operations immediately after being informed by the MDPH about the link to the current outbreak. They also emphasized that an outbreak of Listeria in pasteurized milk products is extremely rare; the source of the outbreak appears to be confined to the Whittier Farms milk processing plant; and they believe the general milk supply is safe.
The MDPH staff continue to work with Whittier Farms on the investigation and will work with the Farm’s management on a recovery plan for the Shrewsbury processing plant. Milk processing will not take place until efforts to eradicate the bacteria at the plant are complete, and testing has confirmed that milk products produced at the facility are safe.
Massachusetts public health officials also continue to work with other agencies, including U.S. Food and Drug Administration and the Massachusetts Department of Agricultural Resources to complete the investigation and review whether changes are needed in current state regulations or in the federally mandated inspection and testing processes for dairy farms and plants.