- ITHACA, N.Y. -- The crowded metropolitan bus system in Buenos Aires
could be responsible for 30 percent 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
- 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 Anaheim, Calif., last week, 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 percent
of the movement in public transportation and 50 percent of the area's total
- 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."
- Castillo-Chavez' study was carried out
in conjunction with Angel Capurro and Juan Aparicio from the Universidad