The aim of modern Public Health Programmes, is a reduction in the total amount of disease in the community. The unit for treatment and cure is not an individual but a sick community. With this new aim, it becomes essential to know the size and extent of tuberculosis in the community as it will be helpful not for purposes of planning only but essentially for the assessment of their effect on the problem. An attempt is made to review the important features of the available knowledge about infection, morbidity and mortality through various surveys. (i) Prevalence of Infection: Tuberculosis infection is widespread in both urban and rural areas of almost all parts of the country. Nearly 40% of the population are infected. To avoid the effect of non- specific allergy and get a more reliable demarcation, tuberculin reactions of 14mm and more were considered as positive by National Tuberculosis Institute. (ii) Prevalence of morbidity: The prevalence of radiologically active tuberculosis in the population is likely to be 1.5%, Prevalence of bacteriologically confirmed diseases is 0.4%. Based on single sample of sputum examination, the prevalence of infectious cases in the country is probably an under estimate. About two million are infectious at any one point of time. (iii) Mortality: Deaths from tuberculosis in the country is not definitely known. The impression of clinicians that death due to tuberculosis have fallen sharply may not be true. Half a million deaths will appear an underestimate. About 250 per 1,00,000 persons i.e., one million deaths due to tuberculosis per year seems to be a reasonable estimate. (iv) Bovine Tuberculosis: Only a few cases in man caused by the bovine tubercle bacillus have been reported although 2.75% to 25% of cattle have been found tuberculin reactors.

To put in a nut shell, the problem of tuberculosis in India is a gigantic one and our means of fighting it with the single tool of BCG, do not even touch the fringe of the problem.

The paper has illustrated the use of mathematical model (epidemetric model) for the prediction of the trend of tuberculosis in a given situation with or without the influence of specific tuberculosis control programme. The paper also advocates the use of models for evolving applicable control measures by reflecting their interference in the natural trend of tuberculosis in control areas. These models were constructed by applying methods which have been developed and utilised in other social sciences.
The precise estimates of the various parameters entering the model must be available if realistic long term results are to be achieved through model methodology. The need for exact data regarding prevalence and incidence of infection and disease, necessitates longitudinal surveys in large random population groups. It is, however, the present authors firm opinion that it would be fruitful for almost any health department, to compare their best available epidemiological knowledge in a system of relationships in order to quantify their concept of the situation. Such an exercise in mathematics would, in any case, serve to sharpen the epidemiologists thinking and would lead them to appreciate what data they need most urgently. The model may help in predicting the trend of tuberculosis in a given situation.

A rural population of 65,000 belonging to 119 randomly selected villages of Bangalore district was repeatedly examined four times during 1961 to 1968, by tuberculin test, X-ray and sputum examinations, to study the epidemiology of tuberculosis without any active anti-tuberculosis measures. The interval between the first and the fourth examination was 5 years. The coverage of various examinations at different surveys were very high.

The main findings of the study are: Prevalence rate of tuberculous infection in the population was about 30% (among females 25% and males 35%). The overall prevalence rates of infection were fairly constant at all the four surveys, but a steady decrease in the prevalence of infection was observed in the age group 0-24 years. Annual incidence rate of infection on the average was about 1%. During the study period, the incidence of infection showed a decline from 1.63% to 0.8% for all ages combined. Prevalence rate of disease ranged from 337 to 406 per 1,00,000 population during the study period, the highest being at the time of first survey and lowest at the time of third survey. For the younger age group of 5-34 years, the rates showed continuous decrease during the study period. Annual incidence rate of disease ranged from 79 to 132 per 1,00,000 population, highest being between first and second surveys and lowest between second and third surveys. The incidence rate in younger age groups below 35 years showed a decline during the study period. Those with tuberculin test induration of 20mm or more had highest annual incidence rate of disease. The annual incidence rate of bacteriologically confirmed disease in the three radiological groups of population was (i) 185 per 1,00,000 with normal X-rays, (ii) 958 per 1,00,000 with abnormal shadows judged as inactive tuberculous are non-tuberculous and (iii) 4,530 per 1,00,000 with abnormal shadows judged as active or probably active tuberculous but bacteriologically not confirmed. The third group constituted 1% of the total population and contributed 34% of the total incidence cases. In each of the above three radiological groups, the incidence of disease was highest among those with tuberculin test induration of 20mm or more to 1 TU RT 23 with Tween 80. Those with 20mm or more tuberculin test induration in the third radiological group constituted 0.45% of the total population but contributed 27% of the total incidence cases. Incidence rate for males was nearly double that of females. More than half of the new male cases were 35 years of age, whereas more than half the females were below the age of 35 years. Out of 126 cases followed up at three subsequent surveys over a period of 5 years, 49.2% died, 32.5% got cured and 18.3% continued to remain sputum positive. Both death and cure rates were highest during the first one and a half year period.

About 30% of newly detected cases come from population uninfected at an earlier survey. Both infection and disease showed a decline in the younger age group. There was no evidence of an increase in drug resistance among newly diagnosed cases. Incidence of cases showed a higher natural cure. These findings indicate that tuberculosis cases are not a uniform entity. There can be different gradations from the point of view of diagnosis and ability to benefit from treatment. The differences between male and female patients with regard to death and cure rates support this view

The trend of tuberculosis in a sample of 22 villages of Bangalore district observed over a period of about 16 years (1961-77) is reported. Distribution of tuberculin indurations did not show a clear cut demarcation between infected and non-infected. The method adopted to demarcate the cut off point has been described herewith: Distribution of tuberculin induration size of 0-14 years was attempted and extrapolated to higher age groups. Even in these younger age groups the antimodes were not clearly defined, so the antimode was arrived by fitting two normal curves as two likely modes.

The choice of demarcation level, therefore, is somewhat arbitrarily made on the basis of the distributions and these varied from survey to survey; between 10 mm at survey I and 16 mm at survey V. The actual and standardized infection rates showed more or less declining trend in 0-4 years, 5-9 years and 10-14 years age groups. The prevalence of cases was not significantly different from survey to survey (varying from 3.96 to 4.92 per thousand from first to fifth survey). However, there was a shift in the mean age of cases, and better survival rate of cases diagnosed at later surveys.

The utility of repeated estimates of prevalence rates of infection in children as a tool for surveillance in tuberculosis is now well recognized. Two prevalence surveys at an interval of 5 years were conducted by National Tuberculosis Institute, Bangalore, with the main objective of studying changes in prevalence rate of infection among children in the age group of 0-9 years. A total population of 42,343 residing in 90 randomly selected villages of Doddaballapur taluk, Bangalore, were registered; of them, 12,535 were children in the age group of 0-9 years. Children were further classified into two sub groups 0-4 and 5-9 years, with or without BCG scars. The unvaccinated children in these two age groups formed the study population.

The population in the study area during the 2nd repeat survey was similar to that of first survey with regard to age, sex distribution, except that a growth rate of 1.1% per year was registered. The BCG scar rate, among children in the age group 0-4, 5-9 years, was 8% & 39% respectively at survey I. All the unvaccinated children below 10 years were given tuberculin test with 1 TU PPD RT 23 and reactions were read 72 to 96 hours after tuberculin testing. In the first survey, level of demarcation to classify the infected children was 10 mm and above, while in II survey it was 12 mm and above. It was observed that the prevalence rate of infection from I survey to II survey was not altered (2.58% & 2.46%) in the 0-4 years of age, while there was an increase in the rate from 8.93% to 12.3% in 5-9 years of age in the II survey. The increase in the infection rate could be attributed to the rising trend of infection, over reading by tuberculin-readers', skills of both tuberculin tester and reader, boosting of tuberculin reaction or scarless BCG vaccination. In conclusion, the study of changes in the prevalence rate of infection in the younger age group is simple, cheap, less time consuming. The data can be used for calculating annual risk of infection as well trend of transmission of infection.

The National Tuberculosis Control Programme is in operation since 1962, and its quantitative achievement is being monitored indirectly through records and reports received from District Tuberculosis Centres. For direct evidence of impact of the programme, tuberculin surveys are useful in reflecting the recent epidemiological situations prevailing in the area. Tuberculosis being a disease of secular nature, a periodic follow up with five years (arbitrary) interval may be preferred over the continuous follow up, for finding the trend of tuberculosis situations in an area.

Keeping in view the importance of tuberculin surveys, National TB Institute (NTI) has evolved a surveillance system which can be adopted by any state in India. The state teams can be trained at NTI in registering population, tuberculin testing & reading, so as to carry out the surveillance in their respective areas. It is essential to create a central organisation for surveillance of tuberculosis using the tuberculin test. The centre would be responsible for technical & administrative support and monitoring. NTI could provide technical expertise in formulating the surveillance system, a training methodology and an in service training to the designated staff.

In a chronic disease like tuberculosis, the exact levels of prevalence or incidence of infection and disease are of lesser importance than its time trend. Surveys should be conducted repeatedly if possible, in order to study the latter. Longitudinal surveys, conducted by National Tuberculosis Institute (NTI) & New Delhi TB Centre, could provide information only on the incidence and prevalence of the disease & infection and not on the time trend due to inadequate sample size of the population selected for the surveys. To measure an annual decline of 1% after 12 years, NTI should have taken a population of 4,45,000 for Tumkur survey instead of 35,000 actually taken. An attempt to measure the trend with the help of epidemetric model also suffers from the inherent infirmity of the small population size. It gave little statistical support to the coefficient of variations of the observed rates, thus imparting little discriminatory power to the observed rates. The error of taking inadequate sample size of the population for these surveys, could be attributed to: (1) The statistical concept of epidemiological assessment through repeated measurement of TB problem had not yet concretised in the minds of the Epidemiologists and Programme Planners. (2) A very high rate of decline was expected after the implementation of the District TB Programme (DTP). (3) The purpose of longitudinal surveys was to get information only on the incidence of infection & disease and not to measure the change. (4) It was not envisaged in 1962 when DTP was being formulated, that there would be no change situation in the prevalence rate of tuberculosis after implementation of DTP from that found in National Sample Survey carried out during 1955-58. The hypothesis underlying static situation was formulated by the Indian epidemiologists later taking their clue from Grigg's momentous work.

Mean time it was established that the Annual Risk of Infection (ARI) holds the key for evaluating the epidemiological trend in a community. From the available data from Longitudinal Survey of NTI it has been found that almost identical rates of ARI were calculated as incidence rates of infection actually observed during the initial surveys. Over a period of 23 years, there has been an annual decline in the risk of infection for the area at the rate of 3.2%. Estimation of incidence of smear positive cases on the basis of the ARI could be made (1% ARI being equivalent of 50 cases per 100,000 population). The findings commensurate with observations made 23 years later, wherein incidence of cases was observed 23/100,000 population and ARI of 0.6% (a parametric relationship seen). The programme operation of average 33% efficiency for nearly three decades would give an annual declining trend of the following extent: 1.4% in case rate, 2.0% in smear positive case rate and 3.2% in ARI. Alternatively the above trend could also represent the natural dynamics.