Photograph courtesy Samuel Stacey at World Fish
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A customer buys farmed tilapia from a market in Cairo, Egypt.
Photograph courtesy Samuel Stacey at World Fish

Disease Detective: Tracing Infection Back to Your Seafood

At first, there was just one patient, an older woman of Chinese descent who lived in New York City.

She went to a doctor in January because she had puzzling sores on her hands: inflamed red bumps, a half-inch in diameter, that were firm and hot and hurt when she pressed on them. The doctor prescribed antibiotics. The sores did not go away. Then a second woman came in; then a third, and a fourth. They all had the same sores; some of the nodules had broken open into deep ulcers, and some of the ulcers scored a line up the women’s wrists, as though their forearms had been stitched by a giant sewing machine.

The women were not related, and did not live in the same place, but they had one thing in common: They bought food for their families at markets that catered to Asian customers. When she heard that, the doctor realized she might know what the women’s infection was—but she also realized that the problem likely stretched far beyond her patients. She took samples of the women’s wounds, and sent them to a lab to see if bacteria could be grown from them. Then she called New York City’s health department, and asked to speak to their infectious-disease division.

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An outbreak of Mycobacterium marinum, skin and soft tissue infections of the hands and arms, has been identified in New York City’s Chinese-speaking communities. Photograph courtesy Elaine F. Kung

At the division’s offices, Dr. Kari Yacisin was about seven months into her job as a disease detective. Yacisin is an Epidemic Intelligence Service Officer, a member of an elite corps that the US Centers for Disease Control and Prevention assembles every year from young physicians and scientists who have just finished residency or just received their Ph.D. The officers serve two years, in Atlanta or in state health agencies, on a promise to go anywhere to investigate any outbreak. Sometimes the outbreaks are routine: church-supper food poisoning, pollution-fueled asthma clusters, cruise-ship vomiting. Sometimes they are rare.

This was going to be one of the rare ones.

Yacisin trained as an infectious-diseases doctor, and the symptoms the other physician described on the phone rang a chime in her memory too. She listened to the patients’ shared characteristics: middle-aged women, home cooks, did their own shopping, bought groceries at Asian markets. There are many such markets near the health department’s marble-and-brass building; it sits just three blocks south of Canal St., Chinatown’s historic border. They are raucous and multi-lingual and crowded, selling dry goods and condiments, gadgets and implements, produce and meat, and fish. The produce is extremely fresh. The meat is, too. And the fish are beyond fresh: They are still alive, thrashing in tanks for customers to choose.

Yacisin wondered about the fish. There were records in the medical literature of skin infections related to fish. The infection was caused by a bacterium called Mycobacterium marinum. It was a distant relative of the tuberculosis bacterium and, like tuberculosis, was slow to show symptoms in humans and difficult to grow in a lab for diagnosis. M. marinum infections tended to occur in people who raise fish in large aquariums, people who collect shellfish—and people who farm fish as a crop.

Among the fish in the market tanks were farmed fish.

Solving the Case

Piecing together the puzzle of an outbreak is slow, painstaking work. It involves phone calls, computer research, visits, more phone calls — and (the part that never shows up in the movies about disease detectives) composing long questionnaires that aim to narrow down where infections originated and how people were exposed to them.

Yacisin and her New York colleagues worked patiently through the painstaking parts. They scoured the city for similar cases. They interviewed doctors who described the patients’ symptoms; none had died, and none were disabled, but some had needed surgery to repair the deepest ulcers, and all were mystified and worried. They interviewed the patients about what they ate, where they shopped, what they bought, what they did with it when they got it home.

In the end, they identified 88 people, across the five boroughs of New York, who had had the same skin infections. They were not all Asian; they did not live in the same neighborhood; they did not shop at the same place. But they all bought live fish, which they cleaned and prepared at home. And most of them, as the lab finally confirmed, were infected with M. marinum. (Some tests, Yacisin told me, are still pending.)

Here is where the outbreak gets interesting. The patients did not buy the same kind of fish;  according to the questionnaires, there were at least 10 species. But the one that was most common was a farmed fish, tilapia—the fourth-most popular seafood, and most common farmed fish, in the United States.

“Tilapia is pretty spiny,” Yacisin said. “From what people have told us, it is pretty to easy to injure oneself preparing it.”

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An investigator takes water samples from a fish tank in New York. Photograph courtesy Claudius Wickham, NYC Dept of Health and Mental Hygiene

That doesn’t mean, though, that tilapia is to blame for this outbreak. The bacterium could have originated at a fish farm — either one growing tilapia, or raising a different species — and propagated through the system that transports live fish in trucks. It could have originated in a truck, infecting a previously clean load of fish that then spread the bacteria to the holding tanks at fish wholesalers. It seems unlikely, given the health department’s data, that the infection originated at one market, because customers of many markets got sick; or one distributor, because those many markets didn’t buy from the same one.

Yacisin and her colleagues are conducting a follow-up study that matches the infected patients with people who are as similar as possible but did not develop infections, to see what other factors they may be able to identify.

“We still don’t know why this outbreak happened,” she told me. “We don’t know if there was something about the patients themselves. We don’t know if it was a bacterial issue, or was it an environment issue or a distribution issue. It is an ongoing investigation.”

How to Prevent Infection

It’s possible they may never find out. The jurisdiction of the New York City department ends at the city’s borders, and none of the implicated fish originated in the city; they all were caught or farmed far away. Plus, the trade in fish isn’t a human health issue; so the US Department of Agriculture and the Food and Drug Administration have joined the hunt.

But what may frustrate all of those investigatory partners is that the New York City fish did not possess what is called “traceability”—a verified chain of custody, beginning at the boat or farm, that testifies to its species and place of origin, locations where it was handled and processed, and identity of the various businesses that got it to your plate. Very little of the fish we eat in the US does, actually, whether it originated in US waters or farms or was caught or grown far away. That’s why investigations routinely discover counterfeits in restaurants and at fishmongers—and will likely be why those city, state and federal investigators will never discover the actual source of the bacterium that made 88 New Yorkers sick.

And that’s worth thinking about, the next time you go to buy fish or order it, whether what you’re purchasing is cooked, or frozen, or something you are taking home raw. (If it’s raw, as the New York fish were: Wear gloves to protect your hands.) It shouldn’t take infections and surgeries to alert us that we don’t know as much as we should about the fish we eat. And until we do, we won’t know the ways we are at risk.

(Kari Yacisin, M.D., M.Sc., presented the results of her investigation at the 2014 EIS Conference hosted by the CDC in Atlanta. The full cite is: Yacisin KA, Leung YI, Li L, Hanson H, Weiss D, Blau D and Ackelsberg J. “Mycobacterium marinum Infections Associated with Handling Fish from Chinese Markets — New York City, 2013.”)