National Tuberculosis Seminar, Hanoi, Vietnan, February, 1997
Global Tuberculosis Epidemiology and Vital Components in NTP
Tour Mori,M.D.
(Research Institute of Tuberculosis)
1. Global burden of tuberculosis
The epidemiological aspect of the tuberculosis problem is best expressed by the annual risk of infection, i.e., the chance of a non-infected person getting infection during a year. The value of this parameter for Vietnam is roughly estimated as about 1.5%, estimated some year ago, which is typical of the situation of the developing world.
In the developed countries it is less than 0.3%,e.g,0.05% in Japan and 0.001% in the Netherlands. The prevalence of infection as accumulation of the past infection is measured as age-specific rates. Figure 1 shows the age-specific prevalence curve of infection which indicates the diffrence in the past and present levels of risk of infection in both settings to which people have been exposed. In most developed countries, the majority of the infected persons are those aged more than 40 years,while in the developing countries younger generations predominate as a result of the high infection risk in the near past and age composition of the population.
The infected persons are at risk of tuberculosis diseases, at a rate of roughly 10% over the lifelong period.
This risk of clinical breakdown is different according to ages, with peakings at infancy and adolescence. The level of infection prevalence together with the age-compositon of the infected determines the incidence of tuberculosis and age-compositon of the newly occurring cases.Figure 2 shows the comparison of the age-specific incidence rate of tuberculosis for the developed and developing countries. In the developing country, Tanzania there is a peak of incidence in the younger ages, while in USA the peak is seen oldest age segment Age-composition also indicates that in the developing countries the tuberculosis hits the younger and most productivee generations of the society. Here is the implicaton of the tuberculosis problem that should be evaluated not merely with the number of deaths or cases, but also in terms of the comprehensive loss of the society.
A measure of foss of health, "DALYs" (disability-adjusted life-years) lost is proposed by the World Bank/WHO research group for that purpose.
The analysis of the global DALY-loss burden reveals that tuberculosis is one of the unduly neglected health problems for their size of DALY lost. Table 1 shows the results of estimated level and trend of incidence of tuberculosis according to the WHO regions. Globally tuberculosis is increasing toward the year 2005, not only in the developing areas, but in the industrialized countries.
This gloomy situation will be aggravated due to the spread of HV infection mainly to Asian countries.
According to the estimate cited above, in 1990 HIV accounts for 5% of newly occurring cases globally, which will be 14% by the year 2000. In southeast Asia the HIV-attributable risk is only 2% in 1990,but will be 15% in 2000.
The "Tuberculosis explosion" in Asia can be already seen in Northern Thailand.
WHO analyzed the factors that may have contributed to the emergence of such a global crisis of tuberculosis as seen above, which are shown in Table 2.
Table 2 Factors causing the “Global Crisis”(WHO)
- Neglect of TB by the government
- Poorly managed and incorrectly conceptualized control program causing failures
- Demographic factors
- HIV epidemics
2. TB Control concepts viewed from an Epidemiology Model
a) A Model of Natural History of Tuberculosis
Figure 1 describes the mechanism of tuberculosis in a human population. Suppose now we have 100 persons who have been infected with tubercle bacilli.
They are observed for lifelong for the clinical development of TB. It is estimated that roughly 10% of them may develop smear-positive tuberculosis at varying times.
Then, the patients are observed under no intervention, and we have seen that they may die or be cured, hypothetically on the average after two years.
During these two years they are active as sources of infection. We assume, very hypothetcatlly that one source of infection can infect five persons during a year.
Then because we have 10 cases each active for two years, we can expect 10 × 2 × 5 = 100 new infections from this group of patients.
Having started from one hundred of the infected, we have gone around the cycle, and we are now left with another 100 infections, which in turn is the start of the new cycle.
In the population where the values of the indicators, or the parameters for clinical breakdown, disease duration and infectivity are set as above, the tuberculosis situation will be constant forever, every time one hundred infected persons producing 10 cases, resulting in another 100 new infections.
However, if something happens to the factors related to clinical breakdown so that the risk of breakdown is reduced to 8%, what will happen to the epidemics?
