The poor peri-urban areas of developing countries, wherein living conditions are unsatisfactory with overcrowding, poor hygiene and inadequate sanitation are usually most affected by tuberculosis [12, 13]. Such living conditions, lack of access to health-care and/or poor health-seeking behaviour may lead to a vicious cycle of M. tuberculosis transmission [13, 14]. Furthermore, national notification data often do not reveal the overwhelming burden of tuberculosis in these impoverished populations [15]. We used systematic sampling to select the households from two neighbourhoods for inclusion in our study. As an efficient alternative to a complete census, this is considered a preferred method and has been used for tuberculosis knowledge and attitude assessment in a similar impoverished setting in India [16]
In this study, the prevalence (per 100,000) of pulmonary tuberculosis among those aged 15 years or more, who actually participated in the study was 329 (95% CI: 195–519) and prevalence adjusted for non-sampling was 438 (95% CI: 281–651). These figures appear among the highest in world [8]. It is substantially higher than an earlier reported national figure of 171/100,000 for Pakistan [8]. However, by taking into account the variability of our point estimate of prevalence of 329, the lower limit of the point estimate is not far away from WHO estimates, given the fact that chosen suburbs have higher tuberculosis prevalence than the average figure for Pakistan. Nonetheless, the observed difference could be due to the fact that in the present study we conducted active case detection among those with a cough of 2 or more weeks of duration. By contrast, the WHO figure is an estimate based on certain assumptions regarding Pakistan as a whole, which may not necessarily hold in the overcrowded and impoverished peri-urban settings of this study. Currently, federal or provincial health services do not conduct such active surveillance for pulmonary tuberculosis cases in the study area or elsewhere in the country. Given the confidence intervals for pulmonary tuberculosis prevalence (per 100, 000) of 329 (95% CI: 195–519) and adjusted prevalence of 438 (95% CI: 281–651), our findings are not far off the figure 557/100,000 previously reported in northern area of Pakistan [17]. Alvi et al [17] used a rapid village survey method, which entailed going to every house [18], almost comparable with the sampling design used in this study. Prevalence of pulmonary tuberculosis among those who provided at least one sputum sample was 16.2%, which is twice as high as reported in marginalized communities in Mexico [19]. The factors which possibly could explain this difference may include differing government spending priorities for public health, rates of case detection, and the subsequent treatment of cases which may have reduced the rates of secondary transmission in their settings.
Government health services in Pakistan base their pulmonary tuberculosis case-detection on self-reporting followed by smear testing of suspected cases. Had we followed a similar approach in this study we would not have detected 60% of the present cases at this stage. WHO's proposed target of detecting at least 75% of tuberculosis cases and treatment of 85% of detected tuberculosis cases therefore could not have been met. The greater number of pulmonary tuberculosis cases detected through active case finding as in this study does not exclude the possibility that some of these cases may have been detected at a later stage; however, in the meantime considerable potential for spread of M. tuberculosis would exist as a result of such delayed detection. Even with the more active surveillance implemented in this study, the resulting pulmonary tuberculosis prevalence estimates may have been somewhat underestimated since 25% of the subjects eligible for sampling did not provide sputum samples, and only subjects aged 15 years or more were included in the study. Moreover, some of the subjects may have been incubating M. tuberculosis infection or perhaps were tuberculosis – positive with non-productive cough.
None of the demographic [20–22], and socio-economic factors [23], was associated with pulmonary tuberculosis status of the subjects in this study. However, there were only 18 cases; the study had very low power to detect any such potential association of interest. Aside from cough of 2 or more weeks duration, which is incorporated within our algorithm, among the other symptoms considered, none was significantly associated with pulmonary tuberculosis status, which is in contrast to the finding of a study in Mexico [19], wherein blood in sputum was significant predictor of pulmonary tuberculosis. This clinical variable was not statistically significant in our study perhaps due to the low power and may merit further evaluation. Taking into account the results of the univariate analysis, we may therefore conclude that only cough of 2 or more weeks of duration seems to be a reliable predictor of pulmonary tuberculosis status in this population.
Of 15 strains tested for their antibiotic sensitivity, only 3 were resistant to streptomycin, indicating relatively low prevalence of Mycobacterial drug resistance in this setting. Also, none of the pulmonary tuberculosis-positive case had history of previous drug treatment.
Study limitations
In addition to our reliance on sampling, which inevitably will be less comprehensive than a complete census, we could not achieve the desired sample size of households from Rehri Goth, while households from Baba Island were slightly over-represented in our sample. To assess the impact of this potential sampling bias we reexamined socioeconomic status, health-care seeking behaviour and prevalence of pulmonary tuberculosis by stratified analysis and found them to be virtually homogeneous (data not shown); we therefore have no reasons to believe that this sampling bias had any significant influence on our results. The study was designed purposely to identify contagious cases. Diagnosing and treating sputum-smears cases are crucial to a tuberculosis control program. We might have missed some cases by the screening questions (some persons not reporting cough may be sputum positive) or due to lack of sputum samples from subjects positive on screening questions resulting in under-estimation of prevalence of pulmonary tuberculosis. Despite these limitations, our estimate of prevalence is one of the highest in world and other settings in the same region. However, aside from cough, we could not identify other risk factors for pulmonary tuberculosis in this setting; therefore, the determinants of tuberculosis in this community remain unclear. Such additional insight would be invaluable for designing appropriate interventions as well as for generalizability considerations. In the present study all cases came from independent households, therefore, our previously reported set of risk factors for M. tuberculosis infection among the contacts of sputum-smear positive tuberculosis in a marginalized population does not seem relevant in this population [24]. Furthermore, a recent study in a marginalized population reported that in high-incidence area like ours, M. tuberculosis transmission occurs mainly outside the household [25]. It is important therefore that future studies continue to focus on elucidating the risk factors for community-acquired M. tuberculosis infection in this and similar settings.