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Pulsenet: Q & A on the Computer Network that Warned of Salmonella

U.S. health officials were first alerted to the current peanut-based salmonella outbreak through a government computer network called PulseNet. But the system relies on doctors testing patients and it takes interviews with patients to identify the food source. Here is an explanation of how this system works:

Q: What is PulseNet?

A: It's a national network of public health labs coordinated by the U.S. Centers for Disease Control and Prevention. When state and local health officials get lab results of people sick with food poisoning, they post information about cases on the WebBoard, the PulseNet listserv. State and national health officials use this information to look for patterns.

PulseNet gets its name from a lab technology called pulsed-field gel electrophoresis, which enables investigators to do DNA "fingerprinting" of the infection bacteria and its strain subtype.

It's somewhat like an FBI database that can compare fingerprints from an arrest record in California to fingerprints at the scene of a crime in Florida and determine if it's the same crook. It takes about two days to run a genetic fingerprint on a sample. Each year, PulseNet identifies more than 300 clusters of patients infected with salmonella, E. coli, shigella and listeria.

Q: How comprehensive is it?

A: This network can't catalog every case in an outbreak. Only some people who get sick go to doctors, and not all doctors run tests to confirm what infection the patient has. Health officials estimate that the actual number of illnesses in an outbreak may be 10 times higher than the lab network reports.

But by detecting an outbreak while it's still going on and then identifying the food that's spreading it, health officials believe they can prevent countless illnesses and some deaths.

Other countries have similar systems. The CDC routinely shares data with Canada, but not with other countries.

Q: How long does it take to identify a foodborne germ?

A: It generally takes two to four weeks from the time the first person in a cluster gets ill until the cluster is detected by PulseNet. In the case of the peanut butter outbreak, CDC first detected a national pattern in November, the month after substantial numbers of lab-tested illnesses first emerged.

CDC officials did not disclose the outbreak until January. They said it took more than a month for health officials to interview sick patients to determine that peanut butter was the food they all had in common. Then, a test from a peanut butter container in Minnesota found the outbreak strain of salmonella.

Q: Can't this process be sped up?

A: The PulseNet system itself is relatively rapid. But investigation timelines are driven by what patients and doctors do about illnesses, and how quickly health officials in individual states react.

"The PulseNet system is a great system. The problem is it's a 22nd century system resting on pillars of epidemiologic research that go back to the 1800s," said Bill Marler, a Seattle plaintiff's attorney who specializes in national food poisoning cases.

Such limitations may explain why Florida has had no lab-confirmed cases in the current salmonella outbreak. Unlike some other states, Florida does not require doctors and hospitals to send salmonella samples to a state lab for analysis and does not budget to handle such a workload. At least 270 samples that may be the outbreak strain have been collected, but only 25 were sent to a state lab for genetic fingerprinting.

Q: How long has this network been in place?

A: PulseNet was launched in 1996, but didn't have the participation of all states until 2002. The cost is shared by the CDC and states. CDC says it spends about $5.6 million annually, but has no figure for what states spend.

The origin of the system lies with a 1993 outbreak of E. coli food poisoning in the western United States. CDC successfully used DNA fingerprinting to nail the strain of E. coli O157:H7 that sickened hundreds and killed four children; it was traced to hamburger patties served at Jack in the Box restaurants. That's when the idea of a collaborative system among the states and federal government began to take shape.

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