The incidence of infection with mycobacterium tuberculosis is an index of the risk of infection to which a community is exposed. An accurate estimation of incidence rate is of considerable importance in understanding the epidemiology of tuberculosis in organising control measures. A new method of estimating incidence of infection is discussed. The material from 3 studies of National TB Institute has been utilized. Study I: is a part of a survey of a random sample of 134 villages. No previous tuberculin testing or BCG vaccination had been carried out in the area, but each person was examined for BCG scar in order to exclude persons vaccinated probably from other areas. After a complete census, a Mantoux test with 1 TU of PPD RT 23 with Tween 80 given on two occasions (Round I and II). Those with reaction of 13mm or less at Round I were offered a test with 20 TU with Tween 80 within a week of 1 TU test. The interval between the rounds was about 18 months. From the analysis of the data from the first 50 villages for which complete information for both rounds was available, it was seen that there was a general increase in the size of reactions elicited in the second round. Study 2: tuberculin testing was carried out with 1 TU and 20 TU among selected ‘control’ groups which provided the data regarding the “enhancing of tuberculin allergy” seen in repeat tuberculin tests. Study 3: in the course of the longitudinal “survey reader assessments” were carried out periodically to judge the standards of the tuberculin test readers. Inter & intra-reader comparisons were made. The findings have been used to estimate the magnitude of reader variation. The data was also used to study variations in the technique of testing and reading.

It was estimated that on an average inter & intra-reader variations between the rounds were unlikely to exceed 6mm or more in more than 5% of the observations. The reading errors have an equal chance of being positive or negative except at extreme ends of the distribution where zero readings of Round I can only show an increase, and the very large reactions had a greater chance of showing only a decrease at a subsequent round. The study mainly concerns with the problems of estimating the incidence of tuberculous infection in a community. Calculations based on age-specific prevalence rates or on rates of tuberculin conversion or both subject to gross error, leading to unreliable epidemiological conclusions. For estimating the newly infected, a new approach has been suggested based on the drawing of a curve for the distribution of differences in reaction size from one round of tuberculin testing to another. It is assumed that if new infection causes a distinct rise in the degree of tuberculin sensitivity which is greater than the combined rise due to enhancement and reader variation, the distribution of differences between the rounds should indicate the newly infected. It is shown that the newly infected probably constitute a homogeneous group with an increase in mean reaction size of about 24mm and standard deviation of 4mm. Accordingly, 98% of the newly infected show an increase in reaction size of 16mm or more.

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.

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

Some of the parameters relating to duration of infected status and incidence of disease have been measured by analysing the data collected from the five year study. Between 1961-68, 119 villages in Bangalore district with total average population of about 62,000 were surveyed at intervals of 1, 3 and 5 years from the first survey. All persons were tuberculin tested with 1 TU RT 23 and those aged 5 years or more were X-rayed. Sputum of those persons showing any X-ray abnormality were collected and examined for AFB. Persons with X-ray abnormality but bacteriologically negative or with normal X-ray in all the preceding surveys, and who became culture positive with X-ray abnormality in the current survey were termed as "New cases". New cases who had shown 10 mm or more reaction to 1 TU RT 23 at I Survey were considered infected previously. New cases, tuberculin negative at I survey but who showed an increase of 16 mm or more between two consecutive surveys were considered infected midway between the two surveys.

Of the 42 new cases diagnosed from among the newly infected during 5 years, 81% came from those infected within one year. Incidence rate of cases among those who were infected within one year was about 5 times more than those infected earlier than one year. Incidence of cases steadily decreased with the increase in the duration of infection. Further, it was found that one fourth of all newly diagnosed cases came from the newly infected persons. However, the size of the pool of previously infected persons in a community being much larger, at least 72% of the new cases came from the reservoir of previously infected persons. The incidence of disease among the newly infected was almost the same in the three age groups i.e., 5-14, 15-34 and 35 years or more. But, the ratio of the incidence rates for the newly infected and the previously infected decreased from 13 for the age group 5-14 to 3 for the age group 35 years and above. In other words, the incidence of disease among the newly infected in the age group 5-14 was thirteen times more than for the previously infected in the same age-group whereas in the age-group 35 years and above, the incidence among newly infected was only thrice that among the previously infected.
Out of the 160 new cases diagnosed during the three repeat surveys, 21 per cent cases came from among those who were infected on the average for one year or less. This is almost in conformity with the hypothesis that one-fourth of all new active cases come from new infections less than a year old.

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.

Out of 56,146 persons without BCG scar examined at the first survey in 119 villages of Bangalore district (1961-63), 22,468 were subsequently examined 3 times over a period of five years by tuberculin test, X-ray and sputum at intervals of 1½ years to 2 years. No organized anti-tuberculosis services were provided in the study area. On the basis of tuberculin status and chest X-ray interpretations, the population was classified into 6 sub groups for the study of risk of sputum positive disease viz., Normal X-ray (N), Inactive Tuberculosis (AB) & Probably Active Tuberculosis (CD) and each of these into tuberculin positives and negatives.

The annual incidence of sputum positive disease observed was 1.45 per thousand among 18,207 eligible persons aged 5 years and more. The incidence of the disease in tuberculin positive group was 7 times as compared to that among tuberculin negatives. The incidence rate of bacteriological disease was 0.79 per thousand among X-ray normals (N) of the first survey; it was 3.73 per thousand among persons with inactive tuberculous lesion and non- tuberculous shadows (AB) and 26.04 per thousand among the group of persons with active or probably tuberculous shadows (CD). Of the total incidence cases, 76% were contributed by the tuberculin positives. The group of active or probably active shadows (CD) contributed 26.6% of the total new cases. The population without any radiological abnormality (N) contributed 48.2% of the new cases.

A study was carried out mainly to find out the prevalence and incidence of sputum negative active pulmonary tuberculosis (suspect cases) among 35,876 persons aged 5 years and above in rural areas of Bangalore district during 1968-72. Two surveys (I & II) at an interval of 3 months, succeeded by a follow up examination of the X-ray abnormals of the earlier surveys, were conducted in the same villages. Examinations at each survey consisted of tuberculin test, X-ray and sputum examinations. X-rays were interpreted individually at the time of each survey by single picture interpretation method and subsequently by Joint Parallel Reading (JPR) method to arrive to a diagnosis. In the JPR method X-ray readings and their comparison was done by a panel of three X-ray readers with full knowledge of age, sex, result of sputum examination and tuberculin test of each person with chest abnormality at any of the three surveys.

On a single picture interpretation the overall prevalence rate of suspect disease was found to be 5.4 per thousand at I survey and 4.59 per thousand at II survey. There was no significant difference in the overall age and sex specific prevalence rates of suspect disease between I & II surveys. Incidence of suspect disease at the end of 3 months was 2.24 per thousand. By JPR method the prevalence rates of suspect disease was 3.2 per thousand at I survey and 3.6 per thousand at II survey. The prevalence rates by single picture method were overestimated to the extent of 38% at I survey and 19% at II survey when compared with those found by JPR method. At I survey prevalence rates on JPR method was significantly lower than by single picture method. This was not so at II survey. Similarly, incidence rate of 0.2 per thousand of suspect disease on JPR was about 1/10th of that found by single picture method.

The incidence of bacteriologically positive cases in 6 months from among suspect cases on JPR was found to be 28%. Majority (76%) of non-tuberculous or inactive tuberculous shadows continued to remain as such after 6 months and about a quarter (23%) became normal. Incidence of bacteriologically positive cases from this group was minimal. Of 19,640 persons with normal X-rays 134 (0.7%) developed new shadows in 3 months; 103 (0.5%) cleared after 2-12 weeks (fleeting shadows). Mis-interpretation of the latter as active tuberculous may falsely boost the estimates of suspect disease to the extent of about 5%.

In a five year epidemiological survey conducted by National Tuberculosis Institute (NTI) from 1961 to 1968, the population was mainly classified into three epidemiological groups (i) with no radiological abnormalities seen in the lungs (Group N) (ii) having X-ray shadows of non-tuberculous etiology or tuberculosis etiology but judged as inactive (Group M) and (iii) with shadows of tuberculosis etiology judged possibly or definitely active but negative on culture (Group S). The objective of this paper is to compare the characteristics of cases coming from the above three groups (N, M and S) in respect to bacillary disease status (a) at the time of diagnosis and (b) after a lapse of time (Fate). Out of the total 172 new cases diagnosed during three follow ups, 70 were diagnosed between I & II surveys, 40 between II and III and 62 between III-IV surveys. In the two 18 months follow up periods, 45 of the total new cases had come from Group N, 31 cases from Group M and 34 cases from Group S, corresponding figures for 24 months follow up (III & IV surveys) were 26, 26 and 10 respectively.

In the 18 months follow up it was observed that proportion of new cases positive on culture in the three groups were not significantly different. Comparison of fate of cases coming from three groups were similar in terms of cure, death and culture positivity. The findings point out clearly that not only development of disease but also the fate of cases is independent of pre diagnosis status of the new cases.

From all the 3 groups, disease developed more rapidly in some cases than in others. This reveals that tuberculosis cases are not an uniform entity from the point of view of development of the disease and cure.

The incidence rates of sputum positive pulmonary tuberculosis (cases) from the five year follow ups of a rural population done by National Tuberculosis Institute were reported on the basis of studies at intervals of one and a half to two years. Information on fate of cases was also likewise reported. These parameters appear to be imprecise since incidence and fate of cases at shorter intervals were not taken into account. Thus, the information on incidence of pulmonary tuberculosis in India is meager as compared to that on prevalence of disease. Therefore, a study mainly to find out the incidence and fate of cases at shorter intervals of 3-6 months was undertaken in 87 randomly selected villages of Nelamangala sub-division, Bangalore district which was one of the 3 sub-divisions where repeated epidemiological surveys had been conducted between 1961-68. The sample of villages in the present investigation was other than that included in the earlier report. Organized Case-finding, anti-tuberculosis treatment and BCG vaccination neither existed nor could be provided in the area till the completion of the study. The present study was conducted between 1968-1972.

This study conducted among 30,576 persons has shown that incidence of cases over a period of three months was 0.99 per thousand and was not much different from the annual rate of 1.03 per thousand reported on the basis of repeated surveys at longer intervals. That the three months rates were not a quarter of the annual rates meant that the procedure of calculating incidence rates on the basis of surveys done at varying intervals after adjusting for the interval had to be used with great caution. The study of fate of cases showed that cases converted or reverted even at shorter intervals and this appeared to be going on continually in the community. However, incidence of cases and cure and death from among the existing as well as the fresh cases kept on balancing each other so that the prevalence rates of cases studied at shorter or at longer intervals did not show variations.

Contact examination is not recommended as a routine procedure for Case-finding in the District Tuberculosis Programme. The rationale for not including contact examination as a routine Case-finding measure is: (1) prevalence rate of tuberculosis among the contacts is not much higher than in the general population (2) at the time of diagnosis of an index case, a second case may not be found in the same household. Though more prevalence cases cannot be diagnosed by contact examination, is it possible that by keeping the household contacts, as a group, under surveillance, future incidence of cases in the community can be substantially prevented? A model situation has been created by using hypothesis derived from various studies conducted in India, designed to answer the question. Variables used in the model are: 40% of the general population are infected at any point of time, there is only one prevalence case of TB at any given point of time in an average household of five, 40% of the non-infected population in a contact household are infected per year, incidence of disease among newly infected group is seven, times of the incidence among previously infected, incidence of disease in general population is 0.13% and from among previously infected persons 0.3% per year develop sputum disease.

At an incidence rate of 0.13% per year among general population aged >5 years, it is expected that 111 cases would arise in a year in the population of 1,00,000 under study. Thus, of the 111 cases occurring in the community, 101 arise from those who are not contacts.

The proportional contribution of new cases from the contact group to the total incidence cases in the entire community is so small, that even if all the contacts are kept under surveillance, BCG vaccinated or placed on chemoprophylaxis, still over 90% of incidence cases cannot be prevented from occurring. This is apart from the fact that keeping them under surveillance will be highly costly and is an operational problem of considerable magnitude.

The data from a longitudinal survey conducted in Bangalore district from 1961-1968 by National Tuberculosis Institute was analysed to find out the prevalence, incidence and fate of suspect cases. In brief, the survey was conducted in 119 randomly selected villages in three taluks of Bangalore district and repeated within the next five years. At each survey, eligible population was subjected to tuberculin, X-ray & sputum smear and culture examinations.

The overall prevalence rate of suspect cases among persons aged five years and more was 1.06% at I survey, 0.68%, 0.49% and 0.43% at II, III and IV survey respectively. In males, the prevalence rate was 1.19% at I survey & 0.62% at IV survey corresponding figures for females were 0.94% and 0.24% respectively. A decline of prevalence of suspect cases from 1.06% at I survey to 0.43% at IV survey was observed. The overall incidence of suspect cases was 0.16% between I & II surveys, 0.10% between II & III, and 0.06% between III & IV surveys. The overall as well as age specific annual incidence rates between III & IV surveys were significantly less than that between I & II surveys. At all the three intervals the incidence increased with the age. Incidence of suspect cases in males was more than that in females. Change in disease status over a period of time is termed as "fate". The disease status was classified as (i) cure (ii) continued to be suspect case (iii) converted into bacillary cases and (iv) dead. The percentage of cure (51.9%, 53.2% and 50.3%) and conversion into bacillary cases(7.2%,5.8% and 5.4%) were almost the same at all the three intervals. But the percentage of those who remained suspect cases reduced from 33.5% at the end of 18 months to 17.5% at the end of 60 months. On the other hand, the death rate increased from 7.4% at the end of 18 months to 26.8% at the end of 60 months. The decreasing trend of continuing to be suspect cases at the rate of 10% between two observations, appears to be corresponding to the increasing trend in the death rate as seen from the observations made at the three intervals.

During a longitudinal survey, carried out in 119 randomly selected villages of Bangalore district for studying the time trend of tuberculosis, the average infectivity of a case over a period of one and a half years was found to be six. In 1986 i.e., 25 years after the start of I survey, 61 persons belonging to one village called Nunnur who were found newly infected between I & II surveys, were interviewed. Further, a general study of the layout of the houses and public facilities in the village was made. However, in Nunnur, there was just a single bacteriological case (index case) identified at the I survey. This index case was resident of household numbered 80 in the main village. This case study investigates the background of the observed high infectivity. The incidence rate of infection in Nunnur was 9.5% in 1½ years which is higher than the overall average rate of 4% as well as rate for 30 other single case villages i.e., 3.5%. The investigation reveals that at least 21 persons., found newly infected at II survey, had varying levels of contact with the index case. The remaining 40 infected persons could not be linked, either directly or indirectly, to any other known bacteriological case including the index case in the village. All the persons identified as infected at II survey were distributed throughout the village, beyond the likely zone of infection of the index case.