Leptospirosis may be the most common disease no one has ever heard of. The bug infects tens of millions of people every year in tropical countries, causing flulike symptoms and even death. But the risks are not equal for all: In places such as the Peruvian Amazon, people in more urban areas are far more likely to die from leptospirosis than those in rural areas. Now, scientists may have figured out why.
The Leptospira bacterium is transmitted to humans through water contaminated by animal urine. People get exposed either through open wounds or via the mouth, nose, or eyes. Although rural dwellers usually recover from the disease, some urbanites don't. In the city of Iquitos, in northern Peru, 10 to 50 of the 400,000 residents die of the disease every year, many more than succumb to malaria. Leptospirosis can be treated with antibiotics, but only if correctly diagnosed, which it often isn't.
In an effort to understand why severe leptospirosis is such an urban phenomenon in this part of the world, Joe Vinetz, an infectious disease expert at the University of California, San Diego, and colleagues have been studying the disease in and around Iquitos. The researchers collected water samples from gutters, puddles, and streams in two locations: an urban slum and a rural village. They also examined patients from both rural and urban areas suspected of contracting the disease.
The data suggest that Leptospira prefers the excitement of the big city. Water samples from the urban slum were crawling with 20 times more bacteria than those from the rural site, the team reports online this week in PLoS Medicine. Furthermore, the bacteria in the urban samples were more likely to be the most virulent form of the bug. Pathogens isolated from sick patients mirrored this trend: People from urban areas were most often infected with the most virulent type of bacteria, whereas rural residents were usually infected with less harmful Leptospira species.
Vinetz blames the gap on rats. The rodents tend to harbor a more virulent form of Leptospira than do rural animals, such as cattle and pigs, the team reports. And they're much more common in the city: "There are so many rats," says Vinetz, "the rats have to sleep in shifts."
Understanding where the Leptospira bug comes from is essential for the prevention of the disease, says Bruce Wilcox, a disease ecologist at the University of Hawaii at Manoa. This work, he says, "represents that new front of research." Vinetz believes the findings will be important for public health efforts. Reducing the rat population by cleaning up trash, for example, would significantly reduce transmission, he says.
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