The outbreak investigation into the Listeria outbreak traced to the consumption of Whittier Farms milk products revealed contamination in several areas of the milk plant, according to reports from the Boston Herald and the Metro West Daily News

The Massachusetts Department of Public Health announced today:

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.

Listeria is the common name for the pathogenic or disease-causing bacterium known as Listeria monocytogenes. It is a foodborne illness that when ingested causes an infection known as listeriosis (Cossart & Bierne, 2001). Approximately 2,500 illnesses and 500 deaths are attributed to listeriosis in the United States annually (CDC, 2005).

Listeria is ubiquitous in the environment, and can be isolated from wild and domestic animals, birds, insects, soil, wastewater, and vegetation. The bacterium easily comes into contact with farm animals as it has been found to be present in grazing areas, stale water, and poorly prepared animal feed. In addition to being present in the environment, Listeria can live in the intestines of humans, animals and birds for long periods of time without causing infection. Because Listeria is present in nearly every environment – including in some food processing facilities – numerous opportunities for contamination exist during the food production process (Cossart & Bierne, 2001).