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Food poisoning cases linked by DNA fingerprints.

The killer grabbing headlines across the United States was one that even the resourceful minds of public health "detectives" couldn't track down.

But ARS researchers at the National Animal Disease Center in Ames, Iowa, managed to "fingerprint" the culprit implicated in the three worst cases of food poisoning known to occur in North America.

Just 2 years ago, a 6-year-old child from northern California was diagnosed with meningitis, an infection of the membranes surrounding the brain and spinal cord. Medical specialists suspected his infection was caused by food poisoning.

Could the culprit be Listeria monocytogenes, the same bacterium responsible for 142 listeriosis cases and 48 deaths during the 1985 Southern California epidemic involving Mexican-style cheese?

The child had eaten such a Mexican-style soft cheese, and state health authorities wanted to know if bacterial isolates matched isolates from the cheese and from the factory where the cheese was made.

Enter ARS microbiologist Irene V. Wesley and co-researchers at the NADC. They were the only scientists in the country using restriction enzyme analysis to check the isolates involving L. monocytogenes from these three outbreaks.

Says Wesley, "We |fingerprinted' isolates from each of the three big listeriosis outbreaks--Canada (1981), New England (1983), and California (1985). Most isolates recovered from each of the outbreaks showed a DNA pattern characteristic of that episode.

"Now we can refer to the New England DNA pattern, the California DNA pattern, and so on."

The method is simple and can be used by lab technicians and public health safety workers to track the spread of Listeria from the food-making environment to the food product and on to the human patient.

In analyzing the Listeria isolated in the northern California case, the researchers found that the isolates recovered from the patient, the suspect cheese, and the cheese factory all matched.

"Not only were they identical to each other, but the DNA pattern from that isolated case matched the profile of L. monocytogenes recovered from the 1985 southern California epidemic," adds Wesley.

The victims of the 1985 California epidemic were mainly pregnant Hispanic women. Wesley says the likely source of contamination was raw milk that had become mixed with pasteurized milk during the cheese-making process.

Two years earlier, in 1983, milk from a major grocery chain was believed to be implicated in 17 deaths in Massachusetts. This incident prompted the U.S. Food and Drug Administration (FDA) to ask NADC researchers to assist in a study of the pasteurization process and its effectiveness in controlling Listeria.

In this joint project, NADC researchers established a herd of Listeria-infected dairy cows in Ames, Iowa. Listeria-contaminated raw milk from the herd was shipped to the FDA laboratory in Cincinnati, Ohio, for pasteurization. FDA scientists confirmed that pasteurization kills L. monocytogenes in milk.

Wesley examined 14 strains of L. monocytogenes from patients and raw milk recovered from the 1983 New England outbreak.

According to Wesley, it is important to know that "Not all strains of Listeria are deadly. The DNA fingerprints of Listeria that we examined indicated that milk was not responsible for the fatal infections during the 1983 epidemic." But to this day, the cause of that outbreak remains a mystery.

The last of the three major epidemics Wesley studied occurred in 1981. Listeria was traced to coleslaw served in Nova Scotia that caused 41 cases of listeriosis and 14 deaths. Epidemiologists speculated that the cabbage had been grown in fields fertilized with manure from Listeria-infected sheep.

Finding and Identifying Harmful Strains

DNA fingerprinting can be completed in less than a week. And time is critical when dealing with potentially contaminated food products.

"The sooner a suspect pathogen is found, the faster food processors can recall products and prevent their widespread distribution throughout the country," says Wesley.

L. monocytogenes kills one out of three people in whom clinical disease becomes evident. Found in soft cheeses, milk, delicatessen food, and undercooked meat, poultry, and seafood, L. monocytogenes sickens about 1,850 Americans yearly, according to a recent report from the Centers for Disease Control (CDC) in Atlanta, Georgia.

Where does this killer bacterium lurk? In addition to isolating it from both raw and processed foods, scientists have also found it in the soil, in vegetation, and in animals.

"But finding Listeria in foods doesn't always mean people will get infected and develop listeriosis. In fact, some people may have a natural immunity to infection," says Wesley.

"Most healthy people," she says, "needn't be alarmed about Listeria food poisoning because fatalities usually occur within special high-risk population groups."

These include the young, the elderly, pregnant women and the unborn, and people with weakened immune systems. In pregnant women, listeriosis causes intrauterine infections that can lead to miscarriages and stillbirths.

Listeriosis patients have mild, flulike symptoms that can start as soon as 48 to 72 hours--or as late as 4 to 8 weeks--after eating contaminated food. People feel like they're coming down with the flu, so they usually don't seek medical help.

"But once full-blown symptoms develop, listeriosis has a higher mortality rate (25-40 percent) than any other foodborne disease. The mortality rates for Salmonella and Campylobacter are less than 1 percent," says Wesley.

The CDC recently reported 165 confirmed cases of human listeriosis in California, Georgia, Tennessee, and Oklahoma between November 1988 and 1990.

To date, Wesley has participated in food safety studies with FDA, CDC, and USDA's Food Safety and Inspection Service. Also, various state departments of health have sought her expertise in solving isolated cases of Listeria food poisoning.--By Linda Cooke, ARS.

Irene V. Wesley is at the USDA-ARS, National Animal Disease Center, P.O. Box 70, Ames, IA 50010. Phone (515) 239-8291, fax number (515) 239-8458.

How Can You Protect Yourself and Your Family?

While you can't see, taste, or smell Listeria in food, a few simple, common sense food preparation precautions will help prevent all kinds of foodborne illnesses. They are:

Don't drink raw milk.

Wash hands thoroughly both before and after preparing foods.

Use a plastic, dishwasher-safe cutting board--not wood.

Wash countertops, cutting board, and any utensils that have come in contact with raw meat, vegetables, or seafood.

Don't prepare, cut, or carry raw meat, poultry, and seafood on the same surface or plate that you later use for cooked foods or other foods.

Don't let juices from raw meat drip on other foods.

Rinse and scrub raw vegetables thoroughly.

Cook meat, poultry, and seafood thoroughly to kill micro-organisms. USDA recommends that beef be cooked to an internal temperature of 160 [degrees] F at its thickest, poultry to 185 [degrees] F, and seafoods until they flake and easily break apart.

Refrigerate foods promptly after a shopping trip. Even though L. monocytogenes can grow while foods are refrigerated, it's wise to keep them cold for as long as possible before cooking or serving.

In What Foods Have L. Monocytogenes Bacteria Been Found?

Dairy Products: cheese, milk, ice cream, ice milk, novelty ice cream products (for example, ice cream bars) Meats: raw beef and poultry Processed Meats: hot dogs, luncheon meats, prosciutto Seafood: imported shrimp, imported and domestic crabmeat, mackerel, and pollack Vegetables: cabbage

Who Are the People in the Listeriosis High-Risk Group?

People in the following categories should get medical attention if they suspect an L. monocytogenes infection: Pregnant women Elderly Individuals with depressed immune systems caused by such conditions as: AIDS, alcoholism, cancer, cirrhosis, diabetes, drug abuse, ulcers, ulcerative colitis, or extended use of steroid medications
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Title Annotation:includes related article on how to protect against food poisoning
Author:Cooke, Linda
Publication:Agricultural Research
Date:Oct 1, 1992
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