Salmonella bacteria in a petri dish. (AP Photo/CDC)

One of the biggest hurdles to halting foodborne illness outbreaks is spotting the source of the problem -- and spotting it quickly. More often than not, by the time authorities recognize an outbreak of salmonella, listeria or any of the other pathogens that sicken an estimated 48 million Americans each year, it already has had time to spread.

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But in recent years, academic researchers and public health officials in New York and Chicago, increasingly have experimented with ways to turn social media platforms such as Twitter and business review sites such as Yelp into early warning systems.

"What makes this useful is the fact that we can get information that's not actually going to public health departments. When people get sick, they usually don't report that," said Elaine Nsoesie, an assistant professor of global health at the University of Washington, who along with colleagues at Boston Children's Hospital and Harvard Medical School has been mining Twitter data for evidence of outbreaks.

Nsoesie said traditional surveillance systems allow health officials to investigate cases only after an individual shows up at a hospital or reports an illness to authorities. But she said the research shows that people often write about their bad experiences on Yelp or complain of their symptoms on Twitter, and that those posts tend to mirror outbreak reports issued by the Centers for Disease Control and Prevention.

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Researchers have created keywords (i.e., "food poisoning," "puking," and "diarrhea") as well as particular phrases (i.e., "I went to a restaurant") that are likely to flag posts about foodborne illness. From there, they can determine which cases offer potential signs of a real problem.

"If we can see people actually reporting that they're sick, you could get an early warning that there's an outbreak happening," Nsoesie said. "That's the goal."

The method has yet to prove itself on a broad scale, but some health departments around the country have embraced social media as one potential tool in helping investigators track down the source of outbreaks.

In 2013, officials at the Chicago Department of Public Health, realizing that people suffering from food poisoning often posted on social media about it rather than calling 311 to report the problem, built a program to mine Twitter for complaints about food-related illnesses.

Staff members monitor the potential cases and reach out to the ones that appear relevant, including a link to a form where residents can provide more information about their experience and flag restaurants that might be sickening people:

Between March 2013 and January 2014, the effort identified 2,241 "food poisoning" tweets originating from Chicago and its neighboring suburbs, according to findings published last summer by the CDC. Of those, staff members identified 270 describing specific complaints of foodborne illness.

The nearly 200 reports filed with FoodBorne Chicago, the group overseeing the effort, prompted 133 restaurant inspections. More than 90 percent of those inspections found at least one health violation. But nearly a quarter of the inspections turned up a "critical" violation, such as food not stored at appropriate temperatures.

"It's the real-time nature of this that's great," said Raed Mansour, the project lead for FoodBorne Chicago, recalling a moment last year when the Twitter program spotted three tweets within a single hour about the same restaurant. "We were able to mobilize our inspectors right then and there. It's hard to say we prevented an outbreak, but we at least prevented further illnesses."

In New York, officials at the city's Department of Health and Mental Hygiene have worked with researchers at Columbia University to scan reviews on Yelp for unreported cases of foodborne illness. Between July 2012 and March 2013, officials used a software program to analyze nearly 300,000 restaurant reviews and identified 893 that warranted further investigation by epidemiologists.

[How New York used Yelp to find unreported cases of food poisoning]

Ultimately, investigators interviewed dozens of Yelp reviewers and unearthed several previously unreported restaurant-related outbreaks in the city, involving dishes such as shrimp and lobster cannelloni and macaroni and cheese spring rolls. The three restaurants involved faced multiple food-handling violations, according to a CDC report.

Officials in New York and Chicago could end up borrowing each other's creations in the future. Nsoesie, the University of Washington researcher, said other cities such as St. Louis and Boston are exploring similar efforts.

Public health officials are quick to acknowledge that using social media to detect foodborne illness outbreaks is no panacea. It can be time-consuming, with unpredictable results. But when it works, it can give investigators what they need most: a head start.

"We're really encouraging places to look at these innovative methods," said Ian Williams, chief of the CDC's Outbreak Response and Prevention Branch. "It's one tool. It's not going to solve everything, but it can be an important piece. We want to find ways to solve these outbreaks as fast as we can."