The spread of tuberculosis in a remote group of Brazilian Indians has revealed clues that epidemics play a role in the evolution of the body's immune system. In tomorrow's issue of the Proceedings of the National Academy of Sciences, researchers report that more than half of the Yanomami people who had been vaccinated against TB do not produce a regular immune response to the tuberculosis bacterium. This suggests, the researchers say, that because previous generations of Yanomami had never been exposed to TB, their immune systems had not evolved a mechanism for mounting an immune response to it.
"For hundreds of years in Europe, people were selected to be TB resistant," says Alexandra Sousa, lead author and postdoc at the Albert Einstein College of Medicine in New York City. "Those that didn't survive had no descendants." The Yanomami Indians, isolated from the outside world until the 1920s, she says, are in a situation similar to the first time Europeans were exposed to the disease.
In 1992, after doctors started reporting cases of tuberculosis among the Yanomami, Sousa and researchers from Brazil and France traveled into the Amazonian forest to study the group--a 5-day trip by boat from Manaus, Brazil. "It was the first time we've had a chance to see how an epidemic affects people who have never been exposed to [a contagion] before," says Sousa.
They found that tuberculosis, first spread to the Indians by gold miners in the 1970s and 1980s, had infected 6% of the tribe--an incidence 100 times higher than surrounding areas, the researchers say. Despite having been vaccinated against the disease in 1989, which was 3 years before Sousa and her colleagues examined them, 58% of the Indians had a weakened or nonexistent immune reaction in skin tests that measure cell response to the tuberculosis bacterium. And only 29% had antibodies that could activate T cells--a necessary part of a strong immune response. The researchers conclude that exposure to previous generations is needed before the immune system can learn to mount a fully protective response.
The team hasn't been able to return to the Amazon to continue studying the epidemic, because the Brazilian government closed the Yanomami Reservation to outsiders from 1993 until last year. "Now we might be able to go back to the reservation to find out more," says Sousa. Burton Andersen, chief of infectious diseases at the University of Illinois, Chicago, agrees that further research into the nature of the Yanomami's lack of an immune response would bolster the scientists' evidence: "It's certainly possible they [the Yanomami] never evolved any special genetic background to protect themselves."