Prevalence of infection among unvaccinated children for tuberculosis surveillance

Transcription

1 Indian J Med Res 72, July 1980, pp 7-12 Prevalence of infection among unvaccinated children for tuberculosis surveillance A.K. Chakraborty, K.T. Ganapathy and G.D. Gothi National Tuberculosis Institute, Bangalore Received September 25, 1978; revised article received November 22, 1979 A survey was carried out among 12,535 children in the age group 0-9 years of 90 villages to study the possible variation in the prevalence of tuberculous infection among the unvaccinated children in a village depending upon the varying prevalence of BCG scars in the same population. In each village, all the children in the age group of 0-9 years were registered and examined for the presence or absence of the BCG scar. Each child was tuberculin tested with 1 TU RT 23 with tween 80 and the reaction read between 72 and 96 h. Two proportions were calculated in each village viz., the proportion with BCG scars and that of infected children among those without scar and the villages were distributed by these two proportions. There was no evidence of any association between the two proportions. In none of the villages any association was seen between the two. In view of this finding, it is felt that the simple method of periodic tuberculin testing of the population in younger age groups could be developed into a method of tuberculosis surveillance even in areas where direct mass BCG vaccination is given. This would appear to be the cheapest, practicable and technically appropriate method of studying the overall tuberculosis situation. Surveillance 1 by repeated tuberculin testing of population is generally regarded as a useful method to study the trend of tuberculous infection in a community 2. However, where mass BCG vaccination is carried out, the presence of a large proportion of vaccinated population in the community may influence incidence and prevalence rates of infection in real terms as well as apparently by affecting the estimations. In this situation, the prevalence rates obtained on tuberculin testing the unvaccinated alone, may not be representative of the entire community, since infection rates among the vaccinated are not measurable. Such rates may not be comparable with those from areas where BCG vaccination is not practised. The alternative could be to compare the infection rates only 7 among unvaccinated population of the vaccinated areas from time to time, without construing them as infection rates for the entire community 2, provided exculsion of a sizeable proportion of BCG vaccinated population does not affect the prevalence rates of infection among the unvaccinated in the same community. It could be a convenient method for surveillance of the tuberculosis situation in areas where direct mass BCG vaccination without prior screening of population with tuberculin test is carried out and large proportions of those eligible are found unvaccinated at any point of time owing to operational reasons. Material from the study conducted by the National Tuberculosis Institute,

2 8 Tuberculosis surveillance by prevalence in unvaccinated Bangalore (NTI) in in a rural population of Bangalore district has been analysed to find out the changes in the prevalence of tuberculous infection in unvaccinated children aged 0-9 yr, depending upon varying prevalence of BCG scars in the same population. Material and Methods The study was conducted in 90 villages of Dodballapur sub-division of Bangalore district in The study area was contiguous to the area where a five year epidemiological study was conducted 3 by the National Tuberculosis Institute, Bangalore. The socioeconomic condition of population in the study area was not different from the area in which the above mentioned previous surveys were conducted. The District Tuberculosis Programme (DTP) was implemented in the area in Mass BCG vaccination of the population in 0-19 yr in age, without prior tuberculin testing, is being practised in the area since then. Registration : Children 0-9 yr in age were registered on individual cards by means of house-to-house visits in the study villages. The total number of persons aged 10 yr and more in each household was recorded on a household list. Younger population, as selected for this study, is better suited for a study of infection rates, since high proportions of persons are already infected in higher age groups 3, 4 reflecting accumulated tuberculosis experience and exposure to other mycobacteria. Further, more difficulties are experienced in reading/interpreting tuberculin test results in older population. Tuberculin testing : All the registered children aged 0-9 yr were brought to the Tuberculin Testing Centre set up in each village. Both the shoulders of these children were examined for the presence of BCG scar. Children without BCG scars were given intradermal tuberculin test with 1 TU RT 23 with 0.05 per cent tween 80 by trained turberculin testers. The longitudinal diameter of a tuberculin test induration was measured in mm by a trained reader and recorded by a secretary, between 72 and 96 h of the test. Population studied : The total population of the 90 villages was 42,343. Of these, 12,535 (29.6 per cent) were aged 0-9 yr (Table I), 6269 (50 per cent) of whom did not have BCG scars and were eligible for the tuberculin test. Tuberculin test were conducted among 6045 of these eligible children, constituting the total material for study of prevalence of infection. Distribution of the test read children by villages with different prevalence rates of BCG scars in them is shown in Table II. Results Level of demarcation between the infected and non-infected : On the basis of distribution of tuberculin induration in children aged 0-4 and 5-9 yr, which showed a bimodal pattern (Fig.), the level Table. I. Registered population, eligible for tuberculin test and coverage by age Eligible Age Total for Test group registerd tuberculin read yr test (96.4) (96.5) Figures in parenthesis give the percentage value

3 Chakraborty et al. 9 of demarcation between the two modes could be anywhere between 10 and 12 mm. The level of demarcation between the positive tuberculin reactors (infected) and negative reactors (non-infected) was chosen somewhat arbitrarily at 10 mm. However, it made little difference in infection rates if the other alternative of demarcating the modes at 12 mm was adopted, since the number of persons between the two modal distributions was too small. Similar experience has been reported elsewhere 5,6. Fig. Distribution of indurations to tuberculin by age in 90 villages. Prevalence of infection : Among the 3805 children without BCG scar, aged 0-4 yr, 98 (2.6 per cent) were infected (Table III). Of 2240 children aged 5-9 years, 200 Table II. Distribution of unvaccinated children in villages with varying proportions of children with BCG scars Proportion of 0-4 year 5-9 year children in a No. of No. of No. of No. of village with villages children villages children BCG scar (%) test-read test-read

4 10 Tuberculosis surveillance by prevalence in unvaccinated Table III. Prevalence of tuberculous infection by age Age group, No. test Reactors Percentage yr read ( 10 mm) reactors (8.9 per cent) were infected. Prevalence of infection was 4.9 per cent among the unvaccinated children in the 0-9 yr group. Prevalence of infection in the unvaccinated children depending on the proposition of BCG vaccinated in a village. For each village, the proportion of children with BCG scars and the proportion of infected children among those without BCG scars were calculated and depending on these proportions, each village was plotted on a scatter diagram and analysed by linear regression technique, which failed to reveal a significant association between the factors. The distributions of the villages by these two variables are presented for 0-4 and 5-9 yr (Table IV). No significant correlation was observed (correlation coefficient r= for Table IV and for Table IV). Lack of significant correlation was observed for the total 0-9 yr old children as well (r= ). These indicate that prevalence rates of infection observed among the unvaccinated children in 0-4, 5-9 yr and for the 0-9 yr group, as a whole, in a vaccinated village had no significant association with the proportion of BCG vaccinated children in the same village. Discussion Absence of obligatory notification of tuberculosis cases in developing countries does not permit the study of trend of the tuberculosis situation from routine records. Repeated epidemiological surveys, using tuberculin test, X-ray and sputum examinations may be carried out in these countries to judge the trend. This method of direct measurement of disease prevalence is expensive 7 and time consuming. Moreover, since prevalence of cases is low (to the extent of about 3 to 4 per thousand) for any change in that prevalence rate between two surveys to be statistically significant, either a very large population is required for repeated surveys 8 or the gap between any two surveys has, to be a long one. These constraints make it less applicable. The method of periodic tuberculin testing of the population in younger age groups for collecting information on prevalence of tuberculous infection on the other hand, appears to be simple and applicable in population where BCG vaccination is not given. Cost-wise, an infection prevalence survey may be half as expensive as a disease prevalence survey 9. Sample size of population required to be studied to obtain adequate data on the trend will be much smaller in case of study of infection rates than for disease rates. Information on annual risk of infection derived from infection prevalence surveys is also considered as a more appropriate index for the study of the most recent trend in tuberculosis situation than prevalence or incidence rates of the disease 2. However, in countries where mass vaccination is in vogue, as in India, the presence of a large proportion of population with BCG vaccination in the study area may create problems by influencing rates of incidence and prevalence of infection among the unvaccinated. Exclusion of the vaccinated makes the remaining a selected group;

5 Chakraborty et al. 11 Table IV. Distribution of villages according to prevalence of BCG scars and prevalence of infection in unvaccinated children. 0-4 yr Prevalence of Proportion of children with BCG scar (%) Infection Total (%) Total yr < Total Total infection rates in them may not necessarily be truly representative of the entire population. Although, it could be suggested that, in order to overcome the problem in vaccinated areas, small areas in different parts of the country could be left unvaccinated for tuberculosis surveillance, it would be unethical to do so. The alternative seems to be to compare the infection rates among the unvaccinated population of the vaccinated areas at different points of time, without projecting the infection rates to the entire community, provided exclusion of various proportions of BCG vaccinated persons does not influence study of infection rates among those not vaccinated. It is seen from our study that exclusion of various proportions of children with BCG scars did not have any correlation with the prevalence of infection among the unvaccinated in the same villages. Absence of prevaccination screening with tuberculin test in the area might have ensured that the proportion of children left unvaccinated in these areas was not the result of any process of systematic exclusion. Hence, the method of tuberculosis surveillance by repeated tuberculin testing of unvaccinated population in areas with BCG vaccination programme, may yield acceptable data. The size of the unvaccinated population however, needs to

6 12 Tuberculosis surveillance by prevalence in unvaccinated be adequate enough to yield statistically valid results on change in situation. In India, due to operational reasons, the proportion of unvaccinated children in the vaccinated areas is high and sufficient number of such children are, likely to be available in most places, almost always. It is of interest to observe that prevalence rates of tuberculous infection among children in 0-4 and 5-9 yr groups in the 90 villages of the present study were not different from those in 119 in other villages studied between in the neighbouring rural areas of the same district 3, where the BCG vaccinated population was only about 2.5 to 3.7 per cent compared with nearly 50 per cent in the present study. Similar findings were also obtained in a repeat survey in Tumkur district 5, where exclusion of persons with BCG scars in various proportions had no correlation with prevalence rates among the unvaccinated. Acknowledgment The authors are grateful to Dr N.K. Menon, Director, National Tuberculosis Institute, Bangalore, for valuable suggestions and advice. They wish to thank Shri K.R. Rangaswamy and M.S. Krishna Murthy, team leaders and members of EPS Field Teams responsible for field work; Smt N. Parimala, Shri R. Channabasavaiah and Shri J.A. Steaphen of Statistical Section; Shri Nagaraj for the drawing and Smt Kamala Rathnaswamy for secretarial assistance. References 1. Hitze, K.L. Surveillance of human tuberculosis. WHO Document (1972) Waaler, H., Galtung, O. and Mordal, K. The risk of tuberculosis infection in Norway. Selected papers, (The Royal Netherlands Tuberculosis Association. The Hague Holland) 16 (1975) National Tuberculosis Institute. Tuberculosis in a rural population of South India : a five year epidemiological study. Bull WHO 51 (1974) Chakraborty A.K., Ganapathy, K.T., Nair S.S. and Kul Bhushan. Prevalence of non-specific sensitivity to tuberculin in a south Indian rural population. Indian J Med Res 64 (1976) Gothi, G.D., Chakraborty, A.K., Nair, S.S., Ganapathy, K.T. and Banerjee, G.C. Prevalence of tuberculosis in a South Indian district. Indian J. Tuberc 26 (1979) Narmada, R., Raj Narain, Raju, V.B., Naganna, K., and Sundaram, R.S. Incidence of. tuberculosis among infected and uninfected children. Indian J Med Res 65 (1977) Gothi, G.D., Radha Narayan, Nair, S.S. Chakraborty, A.K., Srikantaramu, N. Esti-mation of prevalence of bacillary tuberculosis on the basis of chest X-ray and/or sympto matic screening. Indian.J Med Res 64 (1976) Chakraborty, A.K. Some problems of epidemiological assessment of a DTP. NTI News: letter 10 (1973) Naganna, K. and Ramesh, A.S. Cost of pulmonary tuberculosis prevalence surveys. Indian J Med Res 61 (1973) Reprint requests: Dr AK. Chakraborty, Senior Medical Officer, National Tuberculosis Institute No. 8, Bellary Road, Bangalore