Tuberculosis Spreads Through Crowded City Buses, Cornell Researcher Reports

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

Speaking during a session on the mathematics of epidemics anddisease at the annual meeting of the American Association for theAdvancement of Science in Anaheim, Calif., last week, CarlosCastillo-Chavez, Cornell professor of biometry and statistics, providedinsights into the discovery that crowded urban buses can act as incubatorsfor tuberculosis.

Castillo-Chavez noted that there is "a lot of evidence" thattuberculosis is spread by airplane travel in which close proximity ofpassengers 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 ratesdue to the use of public transportation in large urban areas such as BuenosAires and Mexico City. Castillo-Chavez said that he and his colleagues inthe United States, Argentina and Mexico concluded from their study ofBuenos Aires neighborhoods that "a person who takes a bus regularly islikely to have more contacts and have a greater chance of infection."

Argentina, Castillo-Chavez said, has a tuberculosis incidence rateof 42 people per 100,000 of population, but in the inner city of BuenosAires, 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 apopulation of about 12 million. Castillo-Chavez cited a recent studyshowing 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, dailycarries 9.2 million passengers, or 82 percent of the movement in publictransportation and 50 percent of the area's total transportation trips.

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

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

Bus travel, the Buenos Aires researchers concluded, could beresponsible 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 moredisease in poor neighborhoods where more people take the bus; you canactually see that the contribution by bus takers is very critical," headded.

The Cornell researcher noted that with the increased use of publictransportation, particularly by the disadvantaged, "we create moreopportunites for opportunistic infections."

Castillo-Chavez' study was carried out in conjunction with AngelCapurro and Juan Aparicio from the Universidad de Belgrano.