Bugged by Foreign Cuisine

One common experience that tourists encounter while traveling far from home is gut-wrenching diarrhea. In some developing countries, it’s so common that it’s picked up geographic eponyms, like Montezuma’s revenge in Mexico or Delhi belly on the Indian subcontinent.

Mexican cuisine typically offers diners tabletop condiments–from spicy chili liquids to diced-veggie salsas and guacamole–to customize the heat and piquancy of their meals. Cyberphoto

Rates of disease can be amazingly high. On average, 40 percent of U.S. visitors to Mexico develop the runs. Meanwhile, some 50 to 70 percent of Europeans visiting such high-risk destinations as India and Kenya develop diarrhea during a 2-week stay, according to a 2000 report by an international group of researchers.

Now, data emerge confirming what seems obvious in retrospect: Unrefrigerated condiments can serve as a major reservoir of the bacteria responsible for travelers’ diarrhea.

Herbert L. DuPont, chief of internal medicine at St. Luke’s Episcopal Hospital in Houston, enjoys Mexican food. Indeed, this infectious disease expert makes regular trips south of the border. Each time, he announces cheerfully, “I gain weight–and never get sick.”

However, he’s spent much of the past 2 decades probing why so many others do.

In 1980, he and his colleagues at the University of Texas Medical School in Houston correlated diarrhea incidence with the dining places of 130 U.S. summer-school students during their first 3 weeks in Guadalajara, Mexico. Stool samples before the trip confirmed they had been disease-free.

The good news, that early study found, was that for the students who preventatively downed bismuth subsalicylate, the active ingredient in that familiar, pink, over-the-counter liquid marketed for settling upset stomachs, the diarrhea incidence was a mere 14 to 30 percent.

Among the other students, cramping and frequent bathroom trips proved more common. Greater than half of those who ate from street vendors–even just occasionally–developed runny stools. The percentage was a little smaller for those who ate 20 percent or fewer of their meals at restaurants or a school cafeteria. For those eating out more often, more than 60 percent developed diarrhea. Fecal samples taken within a day of experiencing four or more watery stools confirmed the presence of food-poisoning germs.

DuPont suspects that meals prepared at home proved safer because the cooks took hygiene more seriously with foods they’d be eating themselves or providing to their kin.

In its most recent study, DuPont’s team indicates why dining in Mexican restaurants can be so problematic. The researchers took samples of 71 sauces from 36 popular Guadalajara restaurants and another 25 from 12 Houston establishments. All were eateries specializing in Mexican cuisine that weren’t part of restaurant chains.

In the June 18 Annals of Internal Medicine, the scientists report that although most of the sauces harbored bacteria, only those collected south of the border hosted those associated with food poisoning and diarrheal disease.

Cultural differences

At each restaurant, DuPont’s group had picked up a well-mixed tablespoon-size portion of one or more condiments that had been served in open dishes. These are items that gringos know as green sauce, red sauce, guacamole, and pico de gallo.

In the lab, the team cultured, counted, and identified any microbes growing in the sauces. The only class of food-poisoning agents that turned up was Escherichia coli. Most bacteria in this gut-dwelling family do not cause disease.

Overall, 66 percent of the sauces from Guadalajara and 40 percent of those collected in Houston bore E. coli. Moreover, where these bugs occurred, counts in sauces from Mexican restaurants were invariably higher. Whereas populations in Houston sauces grew in the lab into 1 to 10 colonies, those from south of the border often provided more than 1,000 colonies–and occasionally as many as 80,000.

But the most important difference, the new data show, is that while none of the Houston sauces carried pathogens, the Mexican condiments frequently did.

Sickening E. coli come in two forms: enterotoxigenic and enteraggregative. Their machinery for wreaking disease is entirely different. Indeed, DuPont says, although the symptoms are the same, “they cause two different diseases.”

Members of the first–and far better known–group of the illness-inducing agents produce a poison or two. When the intestinal tract absorbs such a poison, it begins leaking fluids. Voila, cramping diarrhea.

Enteraggregative E. coli, by contrast, attach to the lining of the gut. At each binding site, they produce damage that induces inflammation. It’s “that inflammation which leads to a watery outpouring” that a diner will later experience as diarrhea, DuPont explains. In the mid1990s, his team was the first to recognize enteraggregative E. coli as a new and distinct group of food-poisoning agents. Last year, the team also showed that these germs are ubiquitous globally and account for a large share of travelers’ diarrhea.

In their latest paper, DuPont and his colleagues report that of Guadalajara sauces hosting E. coli, 9 percent were contaminated with enterotoxigenic types, and 44 percent contained the enteraggregative bugs.

To eat well

The prevalence and high concentrations of pathogens in the Guadalajara sauces aren’t surprising, DuPont says. It’s a matter of sanitation.

Houston restaurants invariably deliver their condiments to diners fresh from the kitchen. Most are either cool to the touch, having just come from the refrigerator, or are steaming hot. In Mexico, those condiments sat on tables all day. Not only were they not refrigerated–or heated to sterilizing temperatures–but they may have been touched by the fingers of successive diners.

If there was any surprise, he says, it was that even the acidic, spicy sauces proved hospitable to bacteria. DuPont had assumed that if a hot sauce could burn a diner’s stomach, bacteria would shun it. His new data now demonstrate that even such condiments can host dangerous concentrations of germs.

However, DuPont cautions would-be tourists not to interpret these findings as a reason to eschew the charms–and cuisine–of foreign destinations.

He says the trick to eating safely in Mexico and many other lands where hygiene isn’t up to U.S. standards is to restrict menu choices to four categories:

Foods that are piping hot. When DuPont receives fish or another course that is only lukewarm, he returns it to the kitchen for reheating in the microwave until it’s sizzling.

Foods that are dry, such as bread.

Foods with an especially high sugar content, such as syrups, jellies, and honey.

Foods that are very acidic, like citrus fruit, or that can be peeled at the table immediately before consumption.

DuPont says that it’s helpful to explain these requirements to a waiter before ordering. He finds most can get into the spirit of helping gringos eat well.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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