Tracking down an emerging disease.

By examining geographic patterns of outbreaks of a disfiguring skin disease in tropical nations, scientists are finding tentative clues about how the ailment spreads.

Known as Buruli ulcer, the disease is caused by Mycobacterium ulcerans, a microbe from the same group that causes tuberculosis and leprosy. Early symptoms of the illness include nodules beneath the skin. If untreated, skin inflammation leads to open sores that can eat into bone, ultimately requiring amputation, says M. Eric Benbow, an ecologist at Michigan State University in East Lansing.

No one yet knows how the disease spreads. One theory holds that bacteria on sediment suspended in the abundant bodies of water in tropical regions may get into scratches or open wounds and trigger infections. Another theory points to insect bites as the means of microbial transmission.

In 2004, Benbow and his colleagues studied environmental conditions at 12 sites near Accra, Ghana, and conducted a survey of animals there that might be involved in the bacterium's life cycle. In six of those locales, cases of Buruli ulcer are widespread; at the others, the disease is absent or nearly so.

Areas where the disease is endemic typically have slow-flowing or stagnant water with high concentrations of dissolved minerals, analyses showed. Lab tests indicated that several species of fish, snails, and insects are sometimes infected with the M. ulcerates bacterium, says Benbow.

These results don't favor one hypothesis of disease transmission over another. Next, the researchers plan to examine whether organisms that host the bacterium actually transmit the disease or are merely infected.--S.P.