3 February 1999

Take The Bus And Die

The crowded metropolitan bus system in Buenos Aires could be responsible for 30 per cent of new cases of tuberculosis in the city, a new study shows. According to a Cornell University biomathematician, taking public transportation "is a considerable component of transmission and probable evolution of the disease."

Tuberculosis has been on the increase around the world since 1985 after recessing in incidence for several years. It now results in 8 million deaths a year. The leading causes of infection are thought to include global movements of people, urban crowding, poverty and the HIV virus, which appears to accelerate tuberculosis infection in HIV-positive individuals.

Speaking during a session on the mathematics of epidemics and disease at the annual meeting of the American Association for the Advancement of Science in California, Carlos Castillo-Chavez, Cornell professor of biometry and statistics, provided insights into the discovery that crowded urban buses can act as incubators for tuberculosis.

Castillo-Chavez noted that there is "a lot of evidence" that tuberculosis is spread by airplane travel in which close proximity of passengers provides a source of possible infection. In fact, he said, "living in a global economy," and the associated international travel, "might even accelerate the transmission of tuberculosis."

Until now, there have been few studies of increased contact rates due to the use of public transportation in large urban areas such as Buenos Aires and Mexico City. Castillo-Chavez said that he and his colleagues in the United States, Argentina and Mexico concluded from their study of Buenos Aires neighborhoods that "a person who takes a bus regularly is likely to have more contacts and have a greater chance of infection."

Argentina, Castillo-Chavez said, has a tuberculosis incidence rate of 42 people per 100,000 of population, but in the inner city of Buenos Aires, the incidence rate shoots up to more than 160 people per 100,000.

Buenos Aires is one of the most crowded cities in the world, with a population of about 12 million. Castillo-Chavez cited a recent study showing that about 18 million trips are made daily by train, subways, taxi, private car and on the area's 295 bus lines. The bus system, he said, daily carries 9.2 million passengers, or 82 per cent of the movement in public transportation and 50 per cent of the area's total transportation trips.

Four main factors, he noted, are essential to an outbreak of tuberculosis: A large proportion of susceptible individuals, a disseminator of the microorganism Mycobacterium tuberculosis, overcrowding and a lack of ventilation. "All are present in a big city like Buenos Aires," he said.

Researchers at the Universidad de Belgrano in Buenos Aires attempted to calculate the probability of being infected by the tuberculosis bacillus on a bus by keeping track of the numbers of people taking a bus in a particular neighborhood and how long they spent on the bus, Castillo-Chavez reported at the AAAS meeting. It was found that on the average, 100 people an hour entered and left the bus. From this they calculated that for every one hour of travel there was one tuberculosis infection for every 1 000 travelers.

Bus travel, the Buenos Aires researchers concluded, could be responsible for about 30 percent of new cases of tuberculosis.

"This is an important computation about the role of tuberculosis," Castillo-Chavez said. Computer simulations indicate that "you get more disease in poor neighborhoods where more people take the bus; you can actually see that the contribution by bus takers is very critical," he added.

The Cornell researcher noted that with the increased use of public transportation, particularly by the disadvantaged, "we create more opportunites for opportunistic infections."

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