Academic journal article Bulletin of the World Health Organization

Annual Risk of Tuberculous Infection in the Northern Zone of India

Academic journal article Bulletin of the World Health Organization

Annual Risk of Tuberculous Infection in the Northern Zone of India

Article excerpt

Voir page 578 le resume en francais. En la pagina 579 figura un resumen en espanol.

Introduction

The annual risk of infection is the preferred indicator of the epidemiological status of tuberculosis, especially in developing countries, because disease surveys are expensive and time consuming. The annual risk of infection is defined as the probability of acquiring new tuberculous infection of reinfection over a period of one year. It expresses the overall impact of various factors influencing the transmission of tubercle bacilli, specifically the load of infectious cases in cite community and the efficiency of case finding and treatment programmes. The study of tuberculosis status in a large country such as India always has been a challenge. Although the importance of tuberculosis to public health has long been recognized, only in 1955-58 did a nationwide disease survey first reveal the magnitude of the problem (1). Most of the subsequent epidemiological studies to estimate the prevalence of disease or annual risk of infection have been confined to limited geographical areas of south India, close to two national tuberculosis institutes. Thus, information on the prevailing epidemiological status of tuberculosis has been largely unknown for most parts of the country.

The National Tuberculosis Program (NTP) has been implemented in India since 1962; however, its performance in terms of case finding and treatment has been below expectation (2,3). The Government of India therefore resolved to intensify efforts to control tuberculosis and to adopt the Direct Observation Treatment Strategy (DOTS) for the programme, which was renamed as the Revised National Tuberculosis Control Program (RNTCP). The RNTCP started to be introduced across the country in a phased manner in 1998, and it has shown promise, especially in terms of the cure rate for cases with sputum smears positive for tubercle bacilli (4). While efforts to control tuberculosis were being revitalized, it was decided to study the prevalent epidemiological situation of tuberculosis by conducting a nationwide tuberculin survey to estimate the annual risk of infection in different parts of the country. We stratified the country into four zones, each with about one quarter of the country's population (Fig. 1). The survey was designed to obtain estimates of the average annual risk of tuberculous infection in each of the four zones. This paper presents the results from the north zone.

[FIGURE 1 OMITTED]

Methods

Study population and sampling

The study population comprised children aged 1-9 years without a scar from bacille Calmette-Guerin (BCG) vaccination. Older children were not included, as they were more likely to harbour infection with environmental mycobacteria that could interfere with interpretation of study results (5, 6). Children with a scar from BCG vaccination were excluded because BCG-induced tuberculin sensitivity could interfere with interpretation of the study results (6).

The states of Himachal Pradesh, Jammu and Kashmir, Punjab, Haryana, Uttar Pradesh, and Delhi made up the north zone. The sample size was estimated on the basis of an assumed minimum prevalence of infection of 8%, which was itself based on results obtained during earlier surveys in different parts of India (7-17). Other parameters considered for estimation of sample size are given in Table A (web version only, available from: http://www.who.int/bulletin/). The sample size needed was estimated as 12 000 children without BCG scar, to be investigated in 600 clusters. For the purpose of the survey, a village was considered as a rural cluster and an urban census enumeration block as an urban cluster. The clusters in the zone were distributed among two strata--rural (n = 453) and urban (n = 147)--in proportion to the rural and urban population.

A two-stage sampling procedure was used to select clusters within a stratum. …

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