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During recent years, there has been a reversal of historically declining trends in the United States, and by 1991, the number of new cases of tuberculosis reported annually had increased 18%  (1, 2) compared to 1983. Also, seven of 15 Western European Countries (Denmark, Ireland, Italy, Netherlands, Norway, Spain, and Switzerland) have shown an increase in TB cases (3).  Meanwhile, New Zealand and Australia have presented a slight increase, Canada has showed a stagnation of notifications and rates over the last years (4). Argentina has a similar trend  than the rest of the world.  Although its rate of incidence did not decrease to the low levels reached in developed countries, the incidence of TB declined until 1987. Then, it was stationary until 1992 when it began to increase. For 1993, it was 42/100.000. Regarding the rate of mortality related to TB, its trend has declined with some oscillations until 1992, when it increased.  The 15-40 age class was the group most affected by TB, mainly in the City of Buenos Aires. The rate of notification in Buenos Aires  increased from 23.9 to 32.7/ 100,000 in the period 1991-1993 (5).

One explanation of those increases could the effect of TB community outbreaks . These are not uncommon. Could public transportation be responsible for these outbreaks? In 1992, Bloom (6) suggested: "...there is not a simple answer to that question. TB infection is a stochastic process and there appears to be no threshold of organism required to produce infection. The main determinants of individual risk are the concentration of organism-bearing particles exhaled by the source, their aerodynamic characteristics, the rate of ventilation, and the duration of exposure....". Through a simple model, which includes a stochastic process , we estimated that about 28% of active TB cases in the Buenos Aires Metropolitan Area could originate from cases infected in buses. The characteristics of the public transportation in the Metropolitan Area of Buenos Aires  and the risk factors  associated to the epidemiology of tuberculosis in our city can favour the spread of tuberculosis in Buenos Aires.

The transmission of air-borne communicable diseases requires at least an infectious individual, a certain number of susceptible individuals and a closed environment. These characteristics are common in public transportation, especially in big cities. Taking Tuberculosis as an example, we have developed a model  that includes the concept of heterogeneous proportional mixing, and allows us to analyze the effect of transmission in public transportation. We analyzed the effect of the time of travel of individuals of  different socio-economic groups, considering the number of contacts both in public transportation and in home environments. We carried out a sensitivity analysis  for 1-year and 10-years runs, in terms of the number of latent and infectious individuals.

References

 1. Centers for Disease Control (1992). Tuberculosis Statistics in the United States, 1990. U.S Department of Health and Human Services.

2. American Thoracic Society (1992). Control of tuberculosis in the United States. Am Rev Resp Dis; 146:1623-1633.

3. Raviglione MC, Sudre P, Rieder HL, Spinaci S, Kochi A. (1993). Secular trends of tuberculosis in western Europe. Bull WHO, 71:297-306

4. Snider DE, Raviglione M, Kochi A. Global Burden of Tuberculosis. (1994). In: Bloom B (ed). Tuberculosis Pathogenesis and Control. ASM, Washington, USA, pp3-11

5. Instituto Nacional de. Epidemiologia (Argentina) (1993). Notificaciones de la tuberculosis.

6. Bloom BR y Murray CJL (1992). Tuberculosis: Comentary on a Reemergent Killer. Science, 277: 1055-1064.

 


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