Cases of listeriosis, the food-borne bacterial disease that kills one of every five of its victims, are not as isolated as once believed. Using DNA evidence to track bacterial strains, a Cornell University food scientist and his collaborators have concluded that nearly one-third of the 2,500 U.S. cases annually might occur in geographic clusters at generally the same time.

Writing in the latest issue of Emerging Infectious Diseases Journal (June 2003), Martin Wiedmann, Cornell assistant professor of food science, describes examining bacterial samples from listeriosis victims from the New York State Department of Health obtained between 1996 and 2000. By matching strains of the bacterium using DNA fingerprint methods, Wiedmann and his colleagues were able to show connections between cases of listeriosis in the state. Previously it was believed that the vast majority of these cases occurred in isolation.

Through PulseNet, the national molecular subtyping network for food-borne disease surveillance operated by the Centers for Disease Control and Prevention (CDC) in Atlanta, a larger number of human listeriosis infections are likely to be recognized as occurring in clusters, making it easier to link them to specific food sources, says Wiedmann.

The bacteriumListeria monocytogenes is found in soil and water and can contaminate vegetables and infect animals. The bacterium has been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in processed foods that become contaminated after processing, such as soft cheeses and cold cuts.

"Most cases of listeriosis are thought to be sporadic, but public health officials who encounter it should be looking for clusters," says Wiedmann. "If we are on the lookout for clusters, then perhaps we can stop an outbreak after a few cases and save people who otherwise might have died during the outbreak. Traditional epidemiological surveillance alone may not detect common source outbreaks particularly over a large geographic area, but with the technology we have today, it is much easier to prevent outbreaks from going beyond a few cases," he says.Wiedmann, together with researchers from the New York State Department of Health and from the Cornell College of Veterinary Medicine, generated the genetic fingerprints from L. monocytogenes , which had been obtained from the blood or other bodily fluids of victims of the disease and collected by the state agency. The researchers used two DNA typing methods to define the L. monocytogenes strains. Analysis of the DNA typing by statistical methods provided evidence for regional and time-consistent clustering of infections caused by specific subtypes.

Each of the two methods of examining bacterial DNA has its advantage. Ribotyping, which was conducted at Cornell, is fast, automated, standardized and makes DNA fingerprints easily comparable. Pulsed-field gel electrophoresis, which was performed by the state Department of Health, is slower, more labor intensive but provides substantially more DNA detail, enabling scientists to define more genetic fingerprints and strains. "The two methods are complementary," says Wiedmann.

The ability ofL. monocytogenes to kill is greater than that of the bacteria E. coli O157: H7, Salmonella and Campylobacter . Listeriosis kills about 20 percent of victims in the United States, while E. coli O157: H7 and Salmonella kill 0.8 percent; Campylobacter kills 0.1 percent of victims. People with cancer, diabetes and kidney disease are particularly vulnerable to listeriosis, according to the CDC. Also vulnerable are the elderly, pregnant women and infants.

The research article, "Molecular Subtyping to Detect Human Listeriosis Clusters," also was authored by Brian D. Sauders, Cornell doctoral student in food science; Dale Morse, Nellie Dumas, Julia A. Kiehlbauch and Jonathan Hibbs of the New York State Department of Health; Esther Fortes, Cornell food science technician; and Ynte Schukken, Cornell associate professor of veterinary medicine.

The research was funded, in part, by the CDC, the U.S. Department of Agriculture and the National Institutes of Health. The Emerging Infectious Diseases Journal http://www.cdc.gov/ncidod/EID/vol9no6/02-0702.htm.