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022
TUBERCULOSIS IN RURAL SOUTH INDIA: A STUDY OF POSSIBLE TRENDS AND THE POTENTIAL IMPACT OF ANTI-TUBERCULOSIS PROGRAMMES.
HT Waaler, GD Gothi, GVJ Baily and SS Nair: Bull WHO 1974, 51, 263-71.

This paper estimates the natural trend of tuberculosis in rural south India and the potential epidemiological impact of a few selected programmes on this trend, by using the values of important variables and parameters derived from a longitudinal epidemiological study conducted in 1961-68 in Bangalore district by the National Tuberculosis Institute (NTI), Bangalore. The values are fed into an epidemetric model and the final outputs of computerization derived are incidence of disease (in both absolute and relative terms) and cumulative future prevalence of disease.

(1) An annual average input of new generations of 3.16% has been derived for a population of 1 million by using a simplified fertility rate formula. A constant reduction 0f 1% per year has been assumed until fertility rate has reached 50% of its starting value. The assumption is that any reduction in fertility due to current family planning programmes will have a considerable impact on the size of the population and on the epidemiological situation. Further demographic assumptions are, excess mortality applied to groups of active cases and fatality among untreated cases. (2) The population is subdivided into the following epidemiological groups: (i) non-infected, (ii) infected for – (a)< 5 years, (b)= 5 years, (iii) protected by BCG, (iv) active cases - (a) non-infectious, (b) infectious and (v) previous cases. Initially groups (iii) and (v) are given zero values. The future risk of infection is adjusted to the force of infection, which is assumed to be reduced to 1/7th when a case is successfully treated. Morbidity rates include transfers from infected group to active cases group during 5 year periods. (3) A spontaneous healing rate of 50% and a cure rate of 80% after chemotherapy are assumed. Protective effect of BCG is given three values: 30%, 50% and 80%, with uniform annual reduction of 1% (4) Case detection and treatment (CF/T) is given two values: 66% and 20%. Coverage for BCG limited to 0-20 years is assumed to be 66% or 30%.

The computer simulation output for natural trend shows that the absolute number of new cases increases considerably while the incidence rate do not warrant firm conclusions about any long term trend. All programmes considered have considerable potential impact. The CF/T programmes will reduce the incidence after 25 years by only 12% compared to reduction of 17% by the BCG programme. In general, the effect of CF/T will be more immediate and of BCG will be seen much later. To avoid the drawbacks of incidence as an indicator of tuberculosis situation, the cumulated future prevalence is taken as the tuberculosis problem. To adjust for the present significance of future cases as part of the problem certain discount rate have been applied. The CF/T programme and the BCG programme with 50% protection lead to 69% problem reduction, if not discounted. With increasing discount rates, CF/T has an advantage over BCG. The actual problem reduction will be higher than that estimated if improvements in the standard of living are expected during the coming years.

In conclusion, data on the dynamics of tuberculosis situation in rural south India, obtained by NTI, Bangalore when fed into a mathematical model, many predictions about the future tuberculosis situation were made under a wide range of hypothetical assumptions.

KEY WORDS: TREND, MODEL, BCG PROGRAMME, RURAL POPULATION, IMPACT, CONTROL PROGRAMME.

044
INCIDENCE OF TUBERCULOSIS CASES IN CONTACTS - A SIMPLE MODEL
AK Chakraborty, Hardan Singh & P Jagota: Indian J Prev & Soc Med 1980, 11, 108-11.

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.

KEY WORDS: INCIDENCE, CASE, CONTACTS, MODEL
 

 
  ASSESSMENT & EVALUATION  
 
 
169
A SIMPLE MODEL FOR PLANNING AND ASSESSMENT OF PROGRAMMES FOR TUBERCULOSIS CONTROL
SS Nair: Indian J Public Health 1977, 21, 111-31.

BCG vaccination (prevention) and Case-finding followed by treatment (cure) are two universally accepted methods for controlling tuberculosis. BCG trials in selected populations have provided some information on the protective value of BCG, generally over short periods of time (below 20 years) and mainly among younger populations. Efficacy of different drug regimens for treatment of tuberculosis are well established and a number of studies on the effectiveness of different types of treatment programmes on the patient population are available. However, the manner in which BCG and treatment affect the four epidemiological indices of prevalence & incidence of infection and prevalence & incidence of disease in the community, over a period of time, has not been reported in detail.

This paper describes a simple set of models which can be used to predict the trend from these indices under different types of TB programmes (including no programme) which can be depicted as a combination of 5 programme parameters. How these models can be used for planning and assessment of programmes have been demonstrated by some examples. The trend in the incidence of disease obtained from this model is similar to that reported by Waaler et al in 1974. Unlike earlier models, the present model starts with cases and the calculations involved are simple enough to be handled by calculators and computer facilities are not necessary.

KEY WORDS: MODEL, PLANNING, ASSESSMENT, CONTROL PROGRAMME.
 
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