Listeria Outbreak in Massachusetts Milk

It’s not always raw milk that's at fault.

Yesterday, the Massachusetts Department of Public Health announced that four cases of listeriosis (infection due to Listeria monocytogenes) had been traced to pasteurized milk produced by Whittier Farms of Shrewsbury, MA. The 4 victims included 3 elderly people and one pregnant woman. Two of the four have died.

Whittier Farms has voluntarily suspended all operations until the source of the Listeria contamination can be found and removed.

In an earlier post, I stated that one of the reasons to avoid drinking raw milk or eating dairy products made from unpasteurized milk was the risk of contracting Listeria monocytogenes. But what, you might well ask, is this heat-sensitive pathogen doing in pasteurized milk? There are several possibilities, all of which I am sure will be thoroughly checked by the State inspectors and by the company.

1. Faulty pasteurization. An equipment malfunction could result in underpasteurization, either by reducing the length of time the raw milk is heated, or by lowering the temperature at which it is pasteurized. This could allow Listeria monocytogenes to survive, along with other bacteria. But, according to a report in yesterday’s Boston Globe, the pasteurizer was working correctly.

2. Ingredients added after pasteurization. Adding an ingredient, such as a vitamin supplement or a flavoring, to the milk after it has been pasteurized could be a source of the Listeria monocytogenes.

3. Cross-contamination. A faulty pipe connection or a leak in a valve, a heat exchanger, or a pipe might allow raw milk to re-contaminate the pasteurized milk before the milk is bottled.

4. Inadequate sanitizing of processing and packaging equipment. Inadequate cleaning and sanitizing of the pipes and valves through which the pasteurized milk travels could allow Listeria monocytogenes to build up in the system and contaminate the pasteurized milk.

5. Inadequate cleaning and sanitizing of bottles. Whittier Farms is one of the few dairies that still use glass bottles. These bottles must be washed and sanitized before filling.

Listeria monocytogenes usually causes nothing more than mild symptoms in healthy adults, but can produce a severe, even deadly, illness in young children, the elderly, the immunocompromised, pregnant women and their unborn children.