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 196168 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) noninfected, (ii) infected for – (a)< 5 years,
(b)= 5 years, (iii) protected by BCG, (iv) active cases  (a) noninfectious,
(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 020 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.
