V.K.CHADHA
National Tuberculosis Institute, Bangalore - 560 003

Introduction

The study of epidemiology provides the scientific basis for tuberculosis (TB) control. It aims at identifying factors which increases the likelihood of acquiring infection and progression to disease or death besides measurement of disease frequency and distribution which are vital for assessment of disease trends in the community and planning of resources. Information on epidemiological trend of a disease in the community gives an insight into its behavioural pattern over a period of time and enables us not only to assess the impact of intervention programmes but also to foresee the likely scenario in future.

Tuberculosis continues to be a major public health problem. It is transmitted through air-borne route, when a patient suffering from infectious TB (sputum positive pulmonary TB) coughs or sneezes. Majority of the transmission of tuberculosis infection takes place indoors. An untreated case of sputum smear positive pulmonary TB case infects about 10-15 persons per year. Poor ventilation, over crowding and certain climatic conditions facilitate the transmission of infection.

The risk of developing TB, given that infection had occurred, is determined by the integrity of the cellular immune system. An average of 10% of those infected break down into disease over their life time, half of this occurring within the first 2 years of infection. On the other hand, there is 5-7% risk of break down of infection into disease every year among persons co-infected with Human immunodeficiency Virus (HIV) and tubercle bacilli (Narain et al 1992). Presence of other conditions like under-nutrition, silicosis, diabetes mellitus and corticosteroid therapy are also associated with higher disease incidence.

EPIDEMIOLOGICAL INDICATORS OF TUBERCULOSIS

With the advent of chemotherapy leading to reduction in TB deaths, mortality rates do not provide the correct picture of disease situation. Annual notification rates in developed countries, where most of the population has access to health care services and reporting by health care providers is mandatory and comprehensive, represent incidence of TB. On the other hand, reliability of morbidity data from developing countries is affected by lack of adequate access to health care, availability of diagnostic procedures and completeness of notification or reporting systems. Since repeated disease surveys are difficult and impracticable, Annual Risk of Infection (ARI) is currently the preferred epidemiological indicator of TB situation and its trend in developing countries. The age distribution of incidence cases also helps to assess the transmission patterns.

TB PROBLEM IN INDIA

Though TB has been known to be a major public health disease in our country for a long time, its magnitude was not known until National Sample Survey (NSS) by ICMR in 1955-58 revealed the country-wide burden and similarity of disease prevalence in urban and rural areas (ICMR 1955-58). Subsequent disease surveys in various parts of the country (Table 1) during last 40 years indicate that TB remains a major problem even after introduction of chemotherapy and implementation of National Tuberculosis Programme (NTP). India alone contributes one fourth of the global incidence of TB. Even though majority of the diseases occur among middle aged and elderly males, more women die of TB than due to all of maternal causes combined. There are an estimated 13-14 million TB cases at any point of time, of which one fourth are infectious in nature. It is the largest killer of adults and there is one death because of TB every minute in this country. Various tuberculin surveys conducted in the past have revealed the Annual Risk of tuberculosis Infection (ARI) of 1 to 2.5% in different parts of the country (Figure 1). This is 50 - 100 times of that in developed countries. Overall, 40% of our country's population are infected with tubercle bacilli.

The longitudinal studies in Delhi, Bangalore and Tumkur did not indicate any change in disease prevalence over the periods of study. Only a marginal decline in disease prevalence and incidence were observed over a period of 15 years in BCG trial area of Chingleput, where initial prevalence of disease was much higher (Goyal et al 1978; Chakraborty et al 1982; Gothi et al 1979; Tuberculosis Prevention Trial 1980). However, a small shift in disease prevalence to older age groups, specially among males has been observed in these longitudinal studies. Continued high rates of disease prevalence incidence and ARI (1-2.5%) (Figure 1) suggest that TB remains a major public health problem (Chakma et al 1996; Chakraborty et al 1995; Ray and Abel 1995)

EPIDEMIOLOGICAL TRENDS OF TB

There is ample evidence that TB problem in developed countries had been declining since the turn of this century. In most developed countries 12-13% annual decline in risk of infection was observed after introduction of case-finding and treatment programmes in contrast to 4-5% decline per annum prior to chemotherapy (Cauthan et al 1988). Thus, TB problem declined rapidly after the advent of chemotherapy in developed countries, especially among children and young adults. It remained high in most of the developing countries even after the advent of chemotherapy. This was mainly because of low case-finding efficiency and poor treatment programmes. The declining trend TB in industrialized countries and some of the middle income countries since the beginning of the century witnessed a reversal after mid-eighties, as a result of the advent of HIV epidemic and other socio-demographic factors. There was further worsening of the TB problem in developing countries as well. The impact of HIV pandemic expected to be more serious in developing countries where most of the young adults are already infected with tubercle bacilli. Presently, 90% of HIV infected individuals are estimated to be residing in Asia and Africa. The increase in cases of multi-drug residential tuberculosis (MDR-TB) is an added variable responsible for the recent rising trend of TB and India is the most likely breeding ground for MDR-TB. While TB has come back to haunt the developed world, it never went away and continues to rage in the developing world. TB situation is expected to worsen in the immediate future, as a result of demographic factors (population growth and changes in age structure of the population) and increase in HIV seroprevalence rates. Three fourths of this increase is expected to be due to demograpic factors and the rest from a balance between decline in incidence due to intervention measures and increase in incidence due to HIV epidemic. As the drug resistance spreads, TB threatens to become an incurable disease for future generations and TB deaths will rise further, also because of higher mortality of HIV associated TB.

Reducing the duration of infectiousness by early case detection and prompt and effective treatment remains the most viable tool for controlling the disease of TB. In view of the poor efficiency of current case finding and treatment programmes, urgent efforts are needed to intensify the TB control programme. The implementation of Revised Strategy of National TB Control Programme (RNTCP) is the major step in this direction.