3.9. Outcome of longitudinal survey - natural history of TB

Longitudinal surveys are time and resource consuming without yielding the immediate results. It requires courage to assume this type of work. The foresight made the NTI to take up the challenging and repetitive work of the longitudinal survey. Not that repeat surveys had not been done earlier elsewhere. Dr Frimodt Moller had done it in south India84 and the NDTC had done it in north India85. But the planning and approaches of NTI were different in that it included in-depth repeat surveys which were conducted at fixed intervals. Thus the natural history of the disease, as prevalent in a rural community, could be studied methodically.

In November 1977, Dr Gothi delivered the Wander-TB Association of India Oration: Natural History of TB. In the Oration, he had incorporated many of these new aspects culled out carefully from the longitudinal survey. He found that about 5-8% of the total infected persons may develop primary or post primary disease. He also drew on the valuable contributions made by a host of experts both from India and abroad. These insights added a new dimension to the thinking and sharpened prognosticating abilities as well. He stated that the epidemic of TB spans into centuries. The anti-TB measures specially drugs in particular, have not only changed the outlook for individual patient but by reducing infectivity period have speeded up the decline of TB in the community as seen in Japan and Eskimos in Canada86. Twenty five years later, in 1992, Chakraborty and others87 used the data to deliberate the 23 year trend and suggested an average decline of 3.2% per annum, a trend probably representing the natural dynamics. These were eye openers because it is not easy to escape from the text book definitions or narratives of famous clinicians rich in experience and to gain afresh unbiased picture. These insights impelled the NTI to plan newer studies: Incidence of pulmonary TB and change in bacteriological status of cases at longer intervals of 12 years (1974); and at shorter intervals of three months (1978). The latter study revealed that cases converted or reverted even at shorter intervals and this appeared to be going on continuously in the community. Incidence of cases over a period of three months was 0.99 per thousand which was not very different from the annual rate of 1.03 per thousand. However, incidence of cases, cure and death from among the existing as well as fresh cases kept on balancing each other so that the prevalence rate of cases studied at shorter intervals did not show variations88.

In 1979, Gothi and others presented the findings of a repeat survey conducted in Tumkur in the same set of villages and towns and town blocks of the initial survey 12 years later89. The first survey was conducted in 1960-61 when there was no organised anti-TB programme in the district. Since 1964 the area was under the cover of NTP. A DTP was organised in the entire district in 1964. One of the objectives of this repeat survey was to determine if there was any change in the prevalence rates of infection and disease following the implementation of the DTP. The findings were: over a period of 12 years no appreciable change in the overall prevalence rates of tuberculous infection was observed in the same area. The fate of sputum positive cases diagnosed at first survey was : about two-third dead, one-fourth negative and 8.1% were positive for M.tb. It appears that surveys conducted at intervals of 10-15 years may reveal little change in the prevalence rates, as TB is in endemic phase or follows a slow downward trend. One could conclude that 12 years period is too short an interval for natural dynamics of disease and the DTP has not shown any dent on the problem89.

In 1982, Chakraborty and others published a report on the population of the longitudinal survey resurveyed 16 years later (1961-77). The population sample was restricted to 22 villages of Bangalore district. Even this repeat survey showed that the prevalence of cases did not differ significantly from survey to survey. The variation from first survey to fifth survey was 3.96 to 4.92 per thousand. However, there was a shift in the mean age and better survival rate of cases diagnosed at later surveys90.