Stone-Age Brazilian Tribe Infected With TB May Face Extinction

NEW YORK--An outbreak of tuberculosis (TB) in a stone-age Brazilian Indian tribe is producing mortality rates 100 times higher than were reported in New York City at the peak of the multidrug-resistant TB epidemic in 1994.

"Although an outbreak of TB in our communities is serious, when this disease emerges for the first time in populations that never have experienced it, the effects are devastating," said Alexandra De Sousa, MD, PhD, Howard Hughes Research Associate in the Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, NY.

Speaking at the Irvington Institute for Immunological Research, Dr. De Sousa detailed her studies of the Yanomami Indians, an Amazonian rain forest tribe, and the TB epidemic currently ravaging this population.

This group of Indians had remained unknown to outsiders until 1920, when explorers and missionaries first ventured deep into the Amazon jungle spanning regions of northern Brazil and southern Venezuela, Dr. De Sousa said.

"As the 20th century progressed, other outside groups went to the jungle where the Yanomami lived. With the recent influx of over 80,000 miners looking for gold in this region, TB was introduced to this population between 1985 and 1990," she said, adding that the epidemic was declared in 1992.

"Among the 9,400 Yanomami alive today, we documented 40 cases of active TB among the 625 people we studied," Dr. De Sousa said. This translates to a 6.4% prevalence or about 100 times higher than that observed in surrounding regions of Brazil. Similarly, the mortality rate among the Yanomami is 1.28%, about 100 times higher than the TB death rate recorded at the height of New York City’s multidrug-resistant epidemic.

The fact that this population has never before been exposed to the pathogen is key to why this epidemic has such high mortality rates, she said. Also, the nomadic lifestyle and 3-week hunts the men must participate in to obtain food make quarantining and long-term drug administration very difficult.

DNA Analysis

Detailed DNA analysis of the bacilli isolated from infected Indians revealed the presence of two genetic clusters, signifying that in this region, the epidemic was likely started by two infected people.

Furthermore, despite the fact that the Yanomami were vaccinated with the bacille Calmette-Guérin (BCG) vaccine 3 years prior to Dr. De Sousa’s studies, screening these Indians with the purified protein derivative (PPD) test showed that most Yanomami were negative for PPD responses regardless of whether they were previously vaccinated. Normally, the PPD test is positive if an individual was vaccinated with BCG or exposed to the TB pathogen.

"We found high rates of mortality and morbidity; low response rates to PPD; high titers of IgG class 2 and 4 antibodies; and a negative correlation between cellular (T-cell) and humoral (B-cell) responses in the Yanomami Indians," Dr. De Sousa said. Notably, the trends observed among the Yanomami epidemic parallel those identified in other TB epidemics in populations exposed to TB for the first time in their history, for example, the Alaskan Eskimos in 1893.

Epidemic Progressions

Moreover, historical data confirmed by mathematical modeling indicate that TB epidemics follow defined epidemio-logic progressions. "TB epidemics last about 300 years, with the highest mortality rates occurring at about 50 years and the highest morbidity at 150 years from the outset," she said. "The Yanomami epidemic is in its 20th year, so we won’t see a decline in mortality for approximately another 70 years."

The TB epidemic may be overshadowed by out-of-control slash and burn fires that are destroying the Amazon rain forest and threatening the existence of this tribe regardless of TB, she lamented. "Although TB is a killer," she said, "mankind has unleashed its own epidemic of destruction, taking with it the Amazon jungle and its inhabitants."