Commentary: Sputum prevalence data suggest Mexican TB rates will explode on contact with HIV epidemic

Merrick Zwarenstein

Tuberculosis (TB) is not under control in most developing countries. The World Health Organization and the International Union against Tuberculosis and Lung Diseases are co-ordinating the increasing global effort against the disease.1 Among other elements, this programme proposes passive case finding (symptomatic patients will present spontaneously to care), a standardized approach to the diagnosis and clinical treatment of patients (under direct observation), and intense monitoring of patients to microbiologically proven cure. The belief is that complete implementation of these elements will obtain cure rates over 85%, after which the incidence of TB will fall due to the shrinking pool of infective patients. This plan has won widespread support from most developing countries and international funding agencies, with the only controversial issues being the need for direct observation of pill taking, and the role of second line therapies for multi-drug resistant TB.

The plan requires no population level monitoring of TB— this is replaced by the intense focus on using health services data to measure cure rates for newly diagnosed patients. This is a welcome improvement from a previous era in which the seriousness of the disease was hidden by unreliable population based annual risk of infection (ARI) studies. In these ARI studies conclusions about the prevalence of TB were drawn from models based on changes in prevalence of skin response to challenge with TB antigen. Immunological correlates of TB infection are not ideal parameters for assessing progress in dealing with the epidemic, because of the very confusing epidemiology of TB, with early childhood infection, and the unknown contribution that reactivation and reinfection make to the adult prevalence of the disease. During the 1980s in South Africa,2 the ARI fell, in spite of the stagnation of the economy and society under apartheid, and in the face of the developing HIV/AIDS epidemic. This implausible scenario should warn us of the importance of designing surveys which use reliable patient relevant outcome measures (such as prevalence of cough with positive bacteriology), rather than biologically and epidemiologically elegant, derived parameters.

It is no small achievement that Sanchez–Perez et al.3 have estimated the prevalence of TB in Chiapas, based on sputum positivity among adults with 15 days or more of cough. The parameter they have tried to estimate reflects symptomatic and infectious tuberculosis, and is therefore directly relevant both to health services planning, and to public health. Sanchez-Perez use gold standard techniques (three sputum samples, and culture) for diagnosis, and in this remote region, with its civil war these techniques do not work out as well as they might in Manhattan. As a result of geographical and political limitations to their otherwise meticulous random sampling strategy, some war torn communities may have been excluded from the study. Also, difficulties in transport from this remote locale to hospital laboratories seems to have resulted in many culture specimens becoming contaminated. It is likely that the authors are correct in arguing that these limitations mean that their prevalence estimate is lower than the true figure.

This admirable study gives us more reliable information on the state of the disease in Chiapas than would the much simpler ARI survey. Given the low prevalence of HIV in the Americas, the very high prevalence of TB should sound a loud wake-up call to public health practitioners and politicians. If they wish to avoid the situation in Africa, in which a tidal wave of TB is descending in the wake of rising HIV prevalence, health services for TB need to be immediately improved.

Notes

Medical Research Council, PO Box 19070, Tygerberg, 7505, South Africa.E-mail: merrick.zwarenstein{at}mrc.ac.za

References

1 See website at: http://www.who.int/gtb/dots/index.htm

2 Packard RM. Holding back the tide: TB control efforts in South Africa. In: A Century of Tuberculosis: South African Perspectives. Coovadia HM and Benatar SR (eds). Cape Town: Oxford University Press, 1991, pp.42–57.

3 Sánchez-Pérez HJ, Flores-Hernández JA, Jansá JM et al. Pulmonary tuberculosis and associated factors in areas of high levels of poverty in Chiapas, Mexico. Int J Epidemiol 2001;30:386–93.[Abstract/Free Full Text]





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