Guest Editorial
Listeria monocytogenes re-emerges as a dairy industry pathogen

by Kathryn J. Boor, Ph.D.

Kathryn J. Boor, Ph.D. is a professor and chair of the Cornell University Food Science Department. She contributes this column exclusively for Cheese Market News®.

On Feb. 1, 2008, Wayne Whittier, owner of the Whittier Farms fluid milk processing plant in Shrewsbury, Mass., announced his family’s decision not to re-open their plant following the association of their products with a listeriosis outbreak that resulted in a miscarriage and the deaths of three elderly people.

Whittier Farms voluntarily suspended its operations following the Dec. 28, 2007, announcement by the Massachusetts Department of Public Health (MDPH) that four cases of listeriosis had been traced to consumption of its pasteurized milk. MDPH also announced that the plant would remained closed until it was cleared by the MDPH Food Protection Program and by regional milk specialists from FDA.

Listeria monocytogenes is destroyed by proper pasteurization procedures. The Whittier Farms plant was found to have met federal standards for effective pasteurization; therefore, officials speculated that product contamination likely resulted from post-pasteurization entry of L. monocytogenes into the milk, possibly during bottling. MDPH carried out an extensive investigation into the outbreak, testing more than 100 environmental and milk samples for the presence of L. monocytogenes. A total of 12 milk samples and four environmental samples tested positive for the presence of L. monocytogenes. Nine samples (one environmental swab, one sample of skim milk and seven flavored milk samples) collected from the plant tested positive for the same strain of L. monocytogenes that had been obtained from patients.

Despite the extensive investigation, however, the exact source and route of product contamination was not definitively established. In the face of the resulting uncertainty, the Whittier family’s decision to close the plant was based on the high cost of the comprehensive interventions that would have been necessary to obtain regulatory clearance for re-opening combined with the loss in revenue that already resulted from reduced retail sales.

The illnesses associated with the Massachusetts outbreak occurred between June and November 2007. Three elderly men who contracted listeriosis died. Two pregnant women also became ill. One woman had a miscarriage, while the other recovered and delivered a healthy baby.

Bacteria isolated from four of the human listeriosis cases were “fingerprinted” using Pulsed Field Gel Electrophoresis (PFGE). The results, which became available in the middle of December, showed that all four patients had been sickened by the same strain of L. monocytogenes, suggesting a common food source for these infections. Health officials initiated interviews with the patients and their families, with the goal of identifying the common food source. A health care worker learned that one patient had consumed unpasteurized cider and pasteurized milk, and suggested the analysis of these products. When the patient’s partially used containers of these beverages were tested for the presence of L. monocytogenes, the milk was found to be positive. The discovery of L. monocytogenes in an unopened Whittier Farms container that had been obtained from the dairy’s store in Shrewsbury, Mass., provided further evidence linking the outbreak with these fluid milk products.

Simultaneously, in an unrelated set of circumstances, health officials announced in mid-December 2007 that at least three cases of listeriosis had occurred among pregnant women in North Carolina. In two of the three confirmed listeriosis cases, the pregnancies ended in miscarriage. In the third case, a pregnant woman delivered early, but both mother and baby recovered. In a fourth probable, but unconfirmed, case of listeriosis, the pregnant woman lost her baby. All four women reported consuming soft Hispanic-style cheeses from various sources. While the cases were temporally clustered, different strains of L. monocytogenes were involved in the cases, suggesting that no single product was the source of these illnesses.

While most healthy people do not get severe forms of listeriosis, certain populations are at risk for serious illness. At-risk groups include fetuses of pregnant women, newborns, older adults and people with weakened immune systems, including AIDS and cancer patients. The more severe forms of listeriosis include symptoms of septicemia, meningitis and encephalitis. While listeriosis during pregnancy may cause a flu-like illness with fever and chills for the mother, it can lead to premature delivery, infection of the newborn or stillbirth. Early symptoms of listeriosis can include fever, muscle aches and occasionally gastrointestinal symptoms such as nausea or diarrhea. If the infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance or convulsions can occur. Illness can begin 2-8 weeks after eating contaminated food; as a result, it can be very difficult to track the infection back to source. The infectious dose of L. monocytogenes is not known, but appears to vary with the strain and susceptibility of the individual victim. In susceptible individuals, fewer than 1,000 total organisms may cause disease. In the United States, an estimated 2,500 persons become seriously ill with listeriosis each year, and, of these, approximately 25 percent die.

L. monocytogenes occurs widely in the environment and has been isolated from soil, dust, animal feed, water, sewage and plants. Improperly fermented silage can harbor L. monocytogenes. Many animals, including dairy cows, can carry L. monocytogenes in their intestines without becoming sick. This bacterium also has been shown to cause mastitis, although rarely. One survey of raw milk in the United States found a 4.2 percent contamination rate for L. monocytogenes. This organism has been isolated from red meats, poultry, seafood, vegetables and fruits, but outbreaks of listeriosis are generally associated with consumption of unpasteurized (raw) milk, contaminated soft cheeses and ready-to-eat meats. Current guidelines for people at high risk for listeriosis recommend avoiding consumption of soft cheeses such as Feta, Brie, Camembert, blue-veined cheeses or Mexican-style cheeses unless they are clearly labeled as made from pasteurized milk. To remain safe for consumption, pasteurized products must be protected from re-contamination from environmental sources.

The occasional presence of L. monocytogenes in raw milk suggests that this organism is introduced into dairy processing operations, likely on a routine basis. Therefore, to reduce risks associated with the presence of L. monocytogenes in the processing plant environment, re-examine your plant sanitation, HACCP and other strategies designed to control environmental bacterial contaminants. Implementation of an environmental sampling and testing program to assess the effectiveness of L. monocytogenes control measures may save your business ­— and your customers’ lives.

CMN

The views expressed by CMN’s guest columnists are their own opinions and do not necessarily reflect those of Cheese Market News®.