The objective was to establish the prevalence rates for tuberculosis infection, radiologically active pulmonary tuberculosis and bacteriologically confirmed diseases for different age and sex groups. Tumkur District in Mysore State consisting of 2,392 villages, 10 towns of was selected for the study. The district headquarter town Tumkur was excluded from the survey. Random sample of 62 villages and 4 town blocks having a population of 34,746 persons constituted the study population. All the individuals available in the registered population were given a Mantoux test with 1 TU RT 23 with Tween 80. Longitudinal diameter of induration was read 3-4 days after the test. At the time of tuberculin test, all persons aged 10 years and above were offered a single 70mm photofluorogram. For each picture read as abnormal, a spot specimen of sputum of the individual concerned was collected at the time of reading the tuberculin test. Age and sex distribution of infection and disease were studied.

Various parameters concerning the prevalence of infection and disease in the community were reported. Prevalence rate of infection in all ages and both sexes of the population was found to be 38.3%, radiologically active tuberculosis 1.86% and 0.41% sputum positive disease. The infection and disease increased with age; of the total diseased, half were in age group 40 years and more and about 2/3 among males.

Studies on the distribution of tuberculous infection and disease in households have mostly been restricted to the examination of contacts of known cases. Clinical experience has lead to a strong belief that tuberculosis is a family disease and contact examination is a “must” for case-finding programmes. A representative picture of the distribution of infection and disease in households can be obtained only from a tuberculosis prevalence survey.

This paper reports an investigation, based on a prevalence survey in a rural community in south India. The survey techniques and study population have been described in an earlier report. Briefly, the defacto population was given a tuberculin test with 1 TU of PPD RT 23 with Tween 80 and those aged 10 years and above were examined by 70mm photofluorography. All the X-ray pictures were read by two independent readers. Those with any abnormal shadows by either of the two readers were eligible for examination of a single spot specimen of sputum by direct smear and culture. The defacto population numbered 29,813 and tuberculin test results were available for 27,115. After excluding BCG scars, the study population of 24,474 was distributed over 5,266 households which were further classified as “bacillary case household” with atleast one bacteriologically confirmed case, “X-ray case household” with atleast one radiologically active case but with no bacillary cases and ‘non-case household’ with neither a bacillary nor an X-ray case. Total bacillary cases were 77 and were distributed in 75 household. 74 households had one case each and one household had 3 bacillary cases.

The findings of the study have thrown considerable doubt on the usefulness of contact examination in tuberculosis control; (1) over 80% of the total number of infected persons, in any age group, occurred in households without cases, (2) cases of tuberculosis occurred mostly singly in households, and the chance of finding an additional case by contact examination in the same household is extremely small, (3) a common belief has been that prevalence of infection in children in 0-4 age group is a good index of disease in households, but in this study about 32% of households with cases of tuberculosis had no children in this age group, (4) in houses with bacteriologically confirmed case only 12% of the children in 0-4 age group showed evidence of infection, a possible explanation of such a low intensity of infection could be that there is resistance to infection. It is well known that some children even after repeated BCG vaccination do not become tuberculin positive. It is felt that a large number of children do inhale tubercle bacilli, but a primary complex does not develop or even if it develops, the children remain tuberculin negative. A hypothesis has been made that in addition to resistance to infection, there is something known as “resistance to disease”. Otherwise, it is difficult to explain why under conditions of heavy exposure in infection, only some individuals develop evidence of infection and very few develop disease thereafter.

Generally there is no acceptable definition of the term “case of pulmonary tuberculosis”, although such a definition is of fundamental importance both in clinical medicine where results of various chemotherapeutic regimens are compared, as well as for the comparison of different epidemiological data. The main purpose of this paper is to focus attention on the difficulties of defining a case on the basis of bacteriological examination, X-ray examination and tuberculin test. Data from two successive prevalence surveys in a random sample of 134 villages in Bangalore district with a population 70,000 have been utilized to illustrate some of the difficulties in defining a “case” of pulmonary tuberculosis for reporting the prevalence or incidence of the diseases. The entire population was tuberculin tested with 1 TU RT 23 with Tween 80 at both rounds and those 5 years of age and older were examined by 70mm photofluorogram. The sputum specimens (spot and overnight) were collected from those with any abnormality on X-ray as recorded by either of the two independent readers. Both the specimens were examined by fluorescent microscopy and Ziehl-Neelsen technique and by culture.

Analysis of data has shown that the term “a case of pulmonary tuberculosis” does not represent a single uniform entity, but embraces cases of several types, differing considerably in their tuberculin sensitivity, results of X-ray and sputum examination, in the reliability of their diagnosis and mortality experience. The status of cases found at initial and subsequent surveys showed changes with time, and such changes show considerable differences for the various types of cases. It was felt that a single straight-forward definition of a case was not possible to suit all situations. One has to use more than one definition. Although theoretically, finding a single bacillus in sputum should be adequate proof of pulmonary tuberculosis, it was shown that finding of a few bacilli (3 or less) was very often due to artifacts and should not be the basis for a diagnosis. It has also been found that positive radiological findings, in the absence of bacteriological confirmation, indicate only a high risk of the disease and not necessarily pulmonary tuberculosis. Direct microscopy appears to be a consistent index of disease but in community surveys has the limitation of missing a substantial proportion of cases and of adding some false ones.

In view of the difficulty of providing a single definition of a case of tuberculosis, four indices have been suggested. (1) Cases definitely positive by direct smear; (2) Cases definitely positive by culture; (3) All cases positive by culture (including less than twenty colonies); (4) Sputum positive cases which are radiologically active. Each of these could be used for different situations. However, it was concluded that, there seems to be no option but to use more than one definition for assessing the prevalence and incidence of disease.

The prevalence and incidence rates of tuberculous infection and disease in the community are known in the age group 10 years and above from several surveys carried out so far. The present paper provides various parameters of tuberculosis in particular in the pediatric age group. A random sample of 119 villages in 3 taluks of Bangalore district were surveyed 4 times from May 1961 to July 1968 at intervals of 18 months, 3 years and 5 years of the initial survey. Tuberculin test was done for the entire available population with 1 TU PPD RT 23 with Tween 80, and 70mm X-ray for all available persons aged 5 years and above. Two samples of sputum were obtained from the X-ray abnormals, and examined by smear and culture.

It was found that prevalence of infection increased with age from 2.1% at 0-4 year age group to 16.5% at 10-14 year age group, compared to 47% at 15 years and above age group. Prevalence of disease in 5-14 year age group was considerably lower than in age group 15 years or more. Tuberculosis morbidity increased with the size of tuberculin reaction and it was high among children with reaction 20mm or more. Incidence of infection increased with age from 0.9% per year in age group 0-4 years to 2.8% per year among that of 15 years and above. Incidence of disease also showed the same phenomenon-, rising from 0.5% in age group 5-9 to 4% per year in the age group 15 years and above. There were 10 sputum positive cases in 5-14 years of age in first survey, of them, 8 became negative and one died. While from among 152 cases in 15 years and above age group, 48 became negative, 72 died and 32 remained positive. The fate of cases of pulmonary tuberculosis in 5-14 years age was not as serious as in 15 years and above age group. The survey had no means of examining miliary and meningeal tuberculosis.

Children as well as adults with larger reaction of 20mm or more to tuberculin test had higher mortality. This could be considered due to tuberculous infection after taking into account death due to non- tuberculous reasons in both the infected and uninfected groups. Use of chemoprophylaxis might be considered for those who give history of contact with open cases and have tuberculin reaction size 20mm or more.

Data from 62 randomly selected villages in a district of south India, which formed part of a prevalence survey carried out by the National Tuberculosis Institute, Bangalore, during 1960-61, has been made use of. The survey covered 29,813 persons in 5,266 households. There were 70 cases with bacilli demonstrable either in smear or culture and 300 suspect cases. Using the village map (prepared by survey staff), ‘case clusters’ were formed first, with each case household as nucleus and adjacent households within a maximum distance of about 20 meters on either side of the case households. Households closest to the nucleus household on either side have been called as 1st neighbourhood and those coming next in proximity on either side as a 2nd neighbourhood and so on. The case household and its four neighbourhood together was called a cluster. If another case household was found within 4th neighbourhood of the first case the cluster was extended by including the 4th neighbourhood of the new case also. Such clusters were called composite case clusters and clusters with only one case household as simple case clusters. Similarly, suspect case clusters were formed and differentiated as simple suspect clusters or composite suspect clusters. Further, to serve as a control group, non-case clusters were constituted from a systematic sample of 10% households that were not included in case or suspect case clusters.

Out of 60 case clusters formed, only 7 have multiple cases showing that there was no evidence of high concentration of disease in case clusters. While the percentage of child contacts (0-14 years) infected was considerably higher in case clusters (25.8%), there was not much difference between suspect case clusters (14.9%) and non-case clusters (9.8%). Similarly, there was not much difference between simple and composite clusters. Infection among child contacts was higher in case households as compared to their neighbourhoods. To get some idea of the zone of influence of a case or suspect case, prevalence of infection was studied for 10 neighbourhoods, in simple clusters to avoid the influence of multiple cases. It appeared that the zone of influence of a case may extend at least upto the 10th neighbourhood. It was also noted that there was very little difference between zones of influence of suspect cases and non-cases. Case clusters in which the nucleus case had shown activity of lung lesion (evident on X-ray reading) or had cough showed significantly higher infection among child contacts. Clusters around cases positive on both smear and culture did not show higher infection than those around cases positive on culture only. (This may be due to sputum examination of single specimen only).

Out of the total infected persons in the community, only 2% were in case households and 7% in suspect case households, over 90% being in non-case households. The zone of influence of a case extending at least upto the 10th neighbourhood and the overlapping of such zones of influence of cases, present and past, seems to be the most probable explanation for the wide scatter of infection in the community. Prevalence of infection among child contacts was definitely higher in case clusters. But, the significance of this could be understood only from a study of the incidence of disease during subsequent years in different types of clusters. It is significant that only 10% of the total infected persons in the community were found in case clusters. The case yield in general population, cluster contacts, household contacts and symptomatics attending general health institutions have been also compared. The case yield in the last group (10%) is much higher than the case yield from both types of contacts (0.7% and 0.6%) which where only slightly higher than the case yield from the general population (0.4%).

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 study was undertaken in a sample of 103 villages of 3 sub-divisions of Bangalore district as a part of the 5 year study of epidemiology of tuberculosis between 1961-68. The follow ups were done at 1.5, 3 & 5 years after the first survey. The entire population was offered tuberculin test with 1 TU RT 23, a second test with 20 TU RT 23 to those persons who were having reactions of 0-13 mm to 1 TU. All aged 5 years or more were offered 70mm photofluorograms at each survey. Two specimens of sputum were collected from persons having abnormal X-ray shadows for examination of tubercle bacilli. Procedures were uniform at each survey. The population was divided into three groups on the basis of their tuberculin reactions: (a) reactors to 1 TU (infected with M.tuberculosis), b) non-reactors to 1 TU but reactors to 20 TU (infected with atypical mycobacteria), c) non-reactors to both 1 TU & 20 TU (not infected with either M.tuberculosis or other mycobacteria). Incidence of disease and crude mortality were studied separately among these groups.

The five year incidence of culture positive disease was the highest among 1 TU reactors and the least among reactors to 20 TU. In the younger age group (5-14 years) the five year incidence of culture positive disease among reactors to 20 TU was significantly lower compared with that among 20 TU non-reactors. The reduction of incidence of culture positive cases in the former group over that in the latter was 75% for culture positive cases and 61% for combined culture positive and negative disease. As regards crude mortality, the overall rate was significantly lower among 20 TU reactors compared with non-reactors. Even if the significance of the finding on crude mortality is debatable, it could be concluded that non-specific infection provides some protection against development of tuberculosis, at least in younger age groups.