Last fall’s outbreak of Listeria traced to cantaloupes from Jensen Farms in Colorado grew into one of the deadliest in U.S. history, causing at least 146 illnesses and 32 deaths. But as with any outbreak, health officials can never say for certain that the contaminated product did not sicken, or even kill, more than those counted.
Now, just as Colorado cantaloupes return to store shelves for the new growing season, the foodborne pathogen tracking network known as PulseNet may have connected the cantaloupe outbreak to a listeriosis death in Montana.
The connection was made when PulseNetdiscovered that a clinical sample of Listeria from a 75-year-old Bozeman, Montana man who died in January was indistinguishable from a rare genetic fingerprint of Listeria found on a cantaloupe from an outbreak victim’s home. PulseNet compares pathogen samples across the U.S. using a DNA mapping technique called pulsed field gel electrophoresis, or PFGE.
Benjamin Silk, Ph.D., epidemiologist for the U.S. Centers for Disease Control and Prevention, told Food Safety News that the Bozeman victim was now considered part of the outbreak, bringing the official case count to 147 people in 28 states. The CDC has stopped short of attributing the man’s death to the outbreak until health officials in Montana complete an investigation into his infection and other health complications.
“The high number of deaths associated with this outbreak have caused them to be a focal point, but it’s hard to say whether an infection caused death when it happens weeks or months after infection, especially in elderly victims,” Silk said. “The medical risk factors that lead to susceptibility for Listeria infection can also independently be linked to risk of death.”
Either way, Silk said the CDC is certain that the man suffered a Listeria infection from eating Jensen Farms cantaloupes months prior to his death. One other case in Montana has been known since early in the outbreak investigation.
The Montana death had not been linked to the cantaloupe outbreak before now because the victim’s specific Listeria infection was not associated with PFGE pattern combinations from any other outbreak victims, food samples or samples found at Jensen Farms. Following months of investigation, health officials and the public were under the impression that Jensen Farms’ cantaloupes had transmitted four different PFGE combinations of Listeria to victims, and his combination was not one of those.
Now, officials know that the number of outbreak combinations is more likely five. This development came to light after a sample taken by the Colorado Department of Health from the home of a Colorado man sickened in the outbreak tested positive for four different combinations: Three known to be associated with the outbreak, and one combination not before seen in any other victims or samples. In fact, it didn’t match any combinations in Colorado’s database at all.
As it turned out, the Montana death resulted from the same fifth combination as was found in the Colorado cantaloupe sample. But while the cantaloupe sample was taken in September 2011 and the Montana man died in January 2012, information about the fifth combination from the cantaloupe sample was not uploaded to PulseNet until June 18 of this year.
Food Safety News learned of this connection through epidemiologist Patti Waller, who works for food safety law firm Marler Clark, which underwrites Food Safety News. The Colorado sample came from the household of a Marler Clark client who is part of a group of victims suing Jensen Farms and its distributors for medical expenses and damages resulting from their illnesses.
Waller assisted the Colorado Department of Health and CDC in ensuring that the genetic fingerprint of this fifth Listeria combination was uploaded to the network and compared to other samples.
“This situation is yet another example of what a revolution PulseNet has been for outbreak investigations,” Silk said. “It’s been a revolution in the sense that we can now instantly compare illnesses separated by geography and time — it just wasn’t possible before.”
Note (July 16, 2012): This article has been updated to include information and quotations gathered from an interview with Benjamin Silk that occurred just after initial publication.© Food Safety News