FDA: Salmonella strain in pistachios matches illness isolate

Apr 15, 2009 (CIDRAP News) – The US Food and Drug Administration (FDA) has identified the Salmonella strain found in early April at Setton Farms' California processing facility and said its genetic fingerprint matches the strain in at least one illness, in a child who suffered gastroenteritis after reportedly eating the firm's pistachios.

The FDA said in its latest online investigation update that three environmental samples and one finished-product sample obtained during the plant inspection all had the same genetic fingerprint for a strain of Salmonella enterica serotype Montevideo. On Apr 6, the FDA had said it found Salmonella in critical areas of the Terra Bella, Calif., facility, along with the potential for cross-contamination between raw and roasted pistachios.

The FDA said it submitted DNA fingerprints of the Salmonella Montevideo found in the plant to the US Centers for Disease Control and Prevention (CDC) PulseNet database and that some of them match the fingerprints of Salmonella strains from recently ill patients. Health officials have also isolated the same strain from a stool sample of a child who got sick after eating pistachios that came from Setton Farms. The FDA said the same Salmonella fingerprint also matches a number of other clinical isolates in PulseNet.

"However, it is important to recognize that when a patient’s isolate has a relatively common DNA fingerprint pattern (such as this one) that matches that of a food isolate, it does not necessarily follow that the patient’s illness was related to that food," the FDA cautioned.

CDC investigators are investigating whether other patients who were infected with the same Salmonella strain were exposed to Setton Farms pistachios, the FDA said.

Craig Hedberg, PhD, a foodborne disease expert at the University of Minnesota, told CIDRAP News that a wave of illnesses will almost certainly be linked to the contaminated pistachios. However, the timeline of the pistachio recall and illness findings create an unusual situation for epidemiologists, he said.

"The common way we identify the source of outbreaks is to first identify the outbreak and then conduct a study to associate the outbreak with a common food source," Hedberg said. "When you start with a priori knowledge of the common source, it is not possible to confirm it using the same type of epidemiological study." He predicted that a number of cases will be identified, but that a variety of arguments will surface about whether they reflect an outbreak or just unrelated cases in people who happened to have eaten pistachios.

Case-control studies hinge on selecting cases based on disease status rather than exposure status, Hedberg said. "Since in this investigation we will be starting with the exposure, a major principle of epidemiology will be violated, and the results fatally biased," he said. Instead, investigators could identify cohorts of exposed and unexposed people to compare illness rates, but it's difficult to identify an exposed group without referring to illness status. "That approach also falls apart, from a theoretical perspective," Hedberg noted.

Setton Farms recalled certain lots of its pistachios in late March after Kraft Foods identified four Salmonella strains, including Montevideo isolates, in pistachios supplied by Setton Farms. The company expanded its recall on Apr 6 after the FDA announced that investigators had found Salmonella in its California plant. So far, companies have recalled 486 products made with Setton Farms pistachios.

Until the FDA's latest update, no illnesses had been linked to the recall, though the agency had said earlier that it had received two consumer complaints, one from the East Coast and one from the West Coast.

See also:

FDA pistachio recall update

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