Last Friday, CDC’s Morbidity and Mortality Weekly Report included a report on the listeriosis outbreak associated with Jensen Farms cantaloupe (the grower is recalling the melons; look for “Rocky Ford” on the label). So far, 84 cases have been confirmed in 19 states, and 15 of these victims have died.

The number of cases may continue to rise, because even though the contaminated cantaloupes are at or near the end of their shelf life, the illness has a long incubation period (usually 1-3 weeks, but as much as 70 days). People who ingested the bacteria in late September might still experience symptoms over the next week or two. Even so, the 15 deaths associated with this outbreak makes it the third-worst outbreak since CDC started regular outbreak tracking in the 1970s. The New York Times’ William Neuman reports on other deadly foodborne illness outbreaks:

The deadliest outbreak in the United States since then occurred in 1985, when a wave of listeria illness, linked to Mexican-style fresh cheese, swept through California. A federal database says 52 deaths were attributed to the outbreak, but news reports at the time put the number as high as 84.

The second deadliest outbreak was in 1998 and 1999, when there were at least 14 deaths and four miscarriages or stillbirths in a listeria outbreak linked to hot dogs and delicatessen meats. Some sources put the death toll in that outbreak as high as 21.

With the updated death toll on Tuesday, the Rocky Ford cantaloupe outbreak surpassed the 2008 deaths associated with salmonella-tainted peanuts and peanut butter produced by a Georgia company, the Peanut Corporation of America. That outbreak, which drew a large amount of news coverage, killed nine people and sickened more than 700.

Unlike other bacteria commonly associated with foodborne illnesses, Listeria grows well in the cold, so keeping food refrigerated isn’t sufficient for prevention. And as Maryn McKenna at Superbug points out, we tend to think of foods with outer rinds or peels as being safe bets, but bacteria can cling to cantaloupes’ rinds and enter the fruit on the knives used to cut the fruit open.

Investigators are still trying to figure out how the cantaloupes got contaminated in the first place. But their identification of cantaloupes as the likely culprit was also an important achievement that saved lives. That process took a lot of effort and expertise, as Jennifer Brown and Michael Booth report in the Denver Post. The first key step in the process was Colorado doctors and labs reporting to the state health department that they’d found listeria in patients. When state epidemiologist Alicia Cronquist learned of several cases of listeriosis within a few days, her suspicion of a foodborne illness outbreak launched an investigation. Brown and Booth describe that process:

What followed were two weeks of late-night brainstorming, blood samples rushed to the state lab, confiscations of half-eaten food from patients’ refrigerators and, eventually, state scientists’ shopping for grocery-store cantaloupe that led them to a single farm in Holly.

… As quickly as possible, for this outbreak, county health authorities armed with 15-page questionnaires interviewed patients and their families about what they ate and where they bought it. Did you eat at a cafeteria? Did you buy hummus? In the last four weeks, did you eat watermelon more than five times a week?

“The vast majority of people are elderly,” Cronquist said. “The average age is in the 80s and they are quite ill. Their family members are at their bedside, and we are asking them to remember food that they ate a month ago. They are actually very difficult interviews.”

Simultaneously, the bacteria in patients’ blood was isolated and sent to the state lab in a sunlit, low-slung brick building at Lowry. There, state microbiologist Hugh Maguire’s labs deconstructed the DNA profiles to see whether they had listeria strains in common.

Those profiles were uploaded into a Centers for Disease Control and Prevention database to determine whether others across the nation were infected with the same strain.

Other states were indeed seeing infections with the same strains, and departments of health from different jurisdictions were sending CDC information on their cases. Brown and Booth report that this database pointed Cronquist in the direction of cantaloupe. Then, health authorities started purchasing cantaloupes and interviewing patients about the melons they’d eaten. Information on patients’ cantaloupe purchases allowed investigators to follow the trail back to Jensen Farms.

As I’ve written before, the ability to identify the bacteria behind a foodborne-illness outbreak and trace it back to the farm isn’t something we should take for granted. It requires participation from healthcare providers, laboratories, local and state health departments, and the CDC. Surveillance has to be happening all the time, and health workers have to be ready to start interviewing patients as soon as an outbreak is identified. Investing in this kind of public-health infrastructure is a worthwhile of our tax dollars, but their payoffs aren’t always visible to taxpayers. I don’t know how many regular cantaloupe consumers have stopped to consider that they might’ve avoided a case of listeriosis – which could mean mild diarrhea, or severe illness and death – because epidemiologists from Colorado and elsewhere helped identify the source of this outbreak and spur a recall.

As states face financial pressure to cut their health department budgets, this is the kind of behind-the-scenes work that can end up getting sacrificed. Most people probably won’t notice the impact until they or a loved one is severely affected by a foodborne illness whose source remains a mystery.

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