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New technology allows the Centers for Disease Control (CDC) to "fingerprint" a bacterium's DNA and trace it back to the source more quickly in hopes of heading off a potential epidemic.

One of the deadliest food-borne illness outbreaks in the U.S. happened in late 1998. In November of that year, the CDC received several reports of listeria infections and traced them back to what it suspected were the sources of the bacteria: hot dogs from Bil Mar Foods, the country's biggest distributor of packaged meats and a division of Sara Lee Corporation. The CDC then took its evidence to the USDA, which questioned whether it was enough to implicate Bil Mar. The USDA allowed Bil Mar to release a tepid consumer warning that had no mention of the deaths and serious illnesses. Consumers, meanwhile, continued to eat the tainted meats. Ultimately, the CDC's suspicions were confirmed; the tainted meat did in fact originate from Bil Mar. In the end, the CDC traced 21 deaths -- 15 adults and 6 children -- and more than 100 illnesses to the outbreak.

According to a March 2000 report issued by the CDC, the number of food-borne illness outbreaks reported to the CDC remained relatively steady between 1993 and 1997. In 1993, the same year that an E. coli outbreak at the Jack in the Box hamburger chain claimed the lives of four children, there were 489 food-borne illness outbreaks. Four years later, there were yet more -- 504 reported outbreaks. Overall, there were 2,751 outbreaks of food-borne disease between 1993 and 1997, resulting in more than 86,000 illnesses and 29 deaths. (See this chart for the number of outbreaks attributed to four of the most commonly found food-borne pathogens.)

In 1998, five years after the E. coli outbreak at Jack in the Box, the CDC finally completed work on PulseNet, a national computer network of health laboratories and its most potent weapon in the fight against outbreaks of food-borne illnesses. Madeline Drexler, author of Secret Agents: The Menace of Emerging Infections, says that PulseNet is the "public health version of the FBI's 10 Most Wanted List, except that it features thousands of microbial criminals." The laboratories in the network use a method called pulsed-field gel electrophoresis (PFGE) to identify a bacterium's DNA "fingerprint." The fingerprint is then entered into PulseNet's electronic database, where it is compared to other bacteria DNA. Lookalike patterns may indicate that samples hail from a common source.

While PulseNet has helped epidemiologists connect the dots between disparate cases and to identify smaller outbreaks, it is unclear whether it has led to a decrease in the number of actual cases of food-borne illnesses. According to CDC data, between 1996 and 2000, the incidence of campylobacter, listeria, and salmonella infections decreased in the reporting areas, whereas the incidence of E. coli O157:H7 increased. (See this chart for details.)

graph of number of reported food-borne outbreaks

SOURCE: "Surveillance for Foodborne Disease Outbreaks: United States, 1993-1997," CDC, March 17, 2000.

graph of incidence of diagnosed food-borne infections

SOURCE: "Preliminary FoodNet Data on the Incidence of Food-borne Illnesses Selected Sites, United States, 2000," from the CDC's Food-borne Diseases Active Surveillance Network (FoodNet), April 6, 2001.

  RELATED LINKS

· FRONTLINE interviews

For more on PulseNet and how the CDC traces pathogens, see FRONTLINE's interviews with Dr. Glenn Morris and Dr. Robert Tauxe.

· "Surveillance for Food-borne Illness Outbreaks: United States, 1993-1997"

· "A Killer in Our Food"

The Detroit Fre- Press's in-depth series on the listeria outbreak that was traced to Bil Mar Foods, with analysis of the USDA's culpability and background on Listeriosis.

· PulseNet

The CDC's network of laboratories across the nation that can "fingerprint" bacteria DNA.

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