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In mid-August, millions of Americans in Kentucky, Ohio, Virginia and West Virginia were advised to return a particular brand of ham salad. The salad had been distributed not only to grocery stores, but to restaurants and institutions as well. Not a single person had fallen ill from eating the dish. So what was all the fuss about?

The answer is that tests had shown that the salad contained a bacteria called Listeria monocytogenes (pronounced mono-sigh-taw-genees). If the name sounds familiar, it may be because it gained far wider attention in 1985, when a Mexican-style cheese in Southern California was linked to 142 cases, 78 of them fatal. A key point about Listeria bacteria is that while most of us can get rid of these microscopic creatures without ill effect, for certain other individuals they may pose a deadly threat.Here's the word from the USDA press release, advising the August recall: "Consumption of food contaminated with Listeria monocytogenes can cause listeriosis, a rare but potentially fatal disease. In general, healthy people are at little risk from listeriosis; most vulnerable are those with weakened immune systems - infants, pregnant women, the elderly and the chronically ill. Listeriosis is usually manifested as meningitis or meningoencephalitis, which affects tissues around the brain or spine, or septicemia, which is blood poisoning. Listeriosis can cause spontaneous abortions and stillbirths."

The mixed tones of reassurance and gravity in the message reflect the difficulties in understanding how to control the problem.

Listeriosis is not common, although it's true that significant numbers of infections-especially sporadic cases-probably go unidentified. It is believed that there are 1,700 serious cases annually, and that it contributes to approximately 450 deaths and 100 stillbirths a year. Unfortunately, developing measures to prevent illness presents a formidable challenge to food manufacturers and government agencies charged wth protecting the safety of the food supply. A major problem is that unlike many harmful bacteria, this one can continue to grow under refrigeration.

Listeria is not a new organism, nor are its potentially harmful effects on humans newly recognized. That knowledge has been around for over 50 years. What is new is that in recent years it has been identified as the organism in commercially produced foods consumed without further cooking that is responsible for clustered cases of foodborne illness.

Like numerous other microorganisms, it finds homes in many places, including dust, soil, water, sewage and decaying vegetation such as animal feed and silage. It causes illness in sheep and cattle, and has been isolated from 42 different wild and domestic animals, 17 types of birds, certain crustaceans, trout, ticks and flies. It has also been isolated from cucumbers, radishes, potatoes and cabbage. One outbreak several years ago was traced to cole slaw. It turned out that the farmer had fertilized his cabbage patch with untreated sheep manure.

Symptoms of the illness vary among those at particular risk. Pregnant women may display flu-like symptoms, with fever, headache and muscle pain. It may also be accompanied by lower back pain. The condition in infants who become ill shortly after birth is difficult to separate from other types of infection, although small tumors may appear on the skin. Infants who fall ill within a few days to a few weeks of birth are more likely to have meningitis. Non-pregnant adults and children most likely to be attacked include cancer patients, transplant recipients, individuals taking medications that suppress their immune systems, and the elderly. Definite diagnosis requires isolating the organism from a normally sterile site, such as the blood. Antibiotics are used in treatment.

Preventing the disease is no easy task. To start with, it is unclear why some people become ill. Studies show that some individuals who have the organism in their stools show no symptoms. And since the incubation period may be as long as several weeks, it is often difficult to identify the likely source. By the time the infection is diagnosed, the suspect food may no longer be available for testing.

Programs run both by the Food and Drug Administration and by the USDA to monitor foods for the presence of Listeria microorganisms are being upgraded. In 1986, the Centers for Disease Control launched an active surveillance program in several states aimed at better understanding the infection. By now, it has been established as a disease that should be reported to public health authorities.

It is also known that while an estimated 5 percent of milk carries the bacteria, the danger is destroyed by pasteurization. Consumers thus can take a first step toward reducing risk by avoiding raw milk. And early diagnosis and treatment can improve the outcome, especially in pregnant women, who should consult a doctor if typical symptoms occur.

Still, more work is needed to bring the problem under complete control. Meanwhile, most of us can comfort ourselves with the thought that the threat of infection is rare.