Ind. J. Tub., 1990, 37, 63

RISK OF PULMONARY TUBERCULOSIS ASSOCIATED WITH EXOGENOUS
REINFECTION AND ENDOGENOUS REACTIVATION IN A SOUTH INDIAN
RURAL POPULATION-A MATHEMATICAL ESTIMATE*

V.V. Krishnamurthy
1
and K. Chaudhuri
2

Summary : It has been reported that a substantial
proportion of the new cases arise from the previously
infected population. Hence, it appears that exogenous
reinfection and/or endogenous reactivation play a major
role in the development of post-primary disease. Though
the risk of disease due to recent infection is known, the risk
of disease associated with exogenous reinfection and
endogenous reactivation has not been computed in Indian
conditions. The data collected during a longitudinal study
by NTI, Bangalore was analysed to estimate the above
mentioned risk rates. The risk of disease associated with
exogenous reinfection was 6.55% per year compared with
0.21% yearly due to endogenous reactivation. To test the
validity of the computed risk rates these were applied to
the interval between the 3rd and 4th surveys. It was then
estimated that 64 new cases should have been diagnosed in

either due to exogenous superinfection/reinfection
(EXRI) or endogenous reactivation (ENRA). On the
other hand, new cases from the uninfected population
arise following a comparatively recent primary infection.
As a substantial proportion of the new cases are
diagnosed from the previously infected persons, EXRI
and/or ENRA should be playing a major role in the
development of post-primary disease. Though the risk of
disease among the previously infected persons is five
times that among the uninfected, the risk of disease
associated with EXRI and ENRA is not known in Indian
conditions. In this analysis, an attempt has been made to
mathematically estimate the above mentioned risk rates
for the Indian population aged five years and above.

Materials and Methods
The data collected from four epidemiological
surveys during a longitudinal study of tuberculosis in a
rural population of South India is analysed (National
Tuberculosis Institute, 1974). As the interval between
the I-II and II-III surveys was identical (1½ years), the
data collected during the first three surveys are analysed.
For the details of the survey procedures, the above cited
study may be referred. Persons having normal chest
x-ray or those with abnormal chest x-ray in the
preceding survey(s) but bacteriologically negative who
became culture positive with persisting X-ray
abnormality in the subsequent survey were termed new
cases. Infection occurring within 1½ years is considered
as new infection or reinfection, as the case may be. New


where A, B and C are the risks of disease associated
with new infection, reinfection and endogenous
reactivation respectively and the abbreviations NI, RI
and NRI are as explained above. The first term in the
above equation viz., NI x A is equivalent to the total
new cases arising from the group of persons with
recent infection. If cases diagnosed from the newly
infected population could be identified and removed,
then the above equation reduces itself to :

RI × B + NRI × C = New cases arising from the
previously infected population.

When the size of RI and NRI groups becomes known,
there will be two unknowns i.e. B and C in the
equation. By formulating two equations, the values of
B and C could be worked out.

Findings

Size of the population with "recent reinfection (RI) "
and "no recent reinfection (NRI) "

To estimate the size of RI and NRI, it is assumed
that the risk of infection is uniform for the population
as a whole, comprising both infected and uninfected
persons. Similar assumption has been made by other
workers also (Sutherland, 1973). The incidence


Primary
Tuberculosis
New cases due to
endogenous
reactivation
New cases due
to exogenous
reinfection
RISK OF PULMONARY TUBERCULOSIS 65

rate of infection (column 4) between I and II and II and
III surveys is available from the already reported five
year study, (National Tuberculosis Institute, 1974). By
applying these incidence rates to the infected
population, the respective sizes of RI and NRI can be
estimated.

It was estimated that among 10,565 persons (aged
5 years and above) infected at the I survey and
examined at the II survey, 415 persons had become
reinfected between I-II surveys and I0,150 had no
reinfection (Table 1). The corresponding figures for
II-III surveys were 229 and 9,165 persons respectively
(Table not presented).

Among the 70 new cases diagnosed at the II
survey, 48 cases were diagnosed from the group of
infected persons at I survey, 19 cases from the
uninfected (the tuberculin sensitivity status at I survey
was not available for three cases). The corresponding

types of infection Type of infection

Risk of disease
(%)
(Annual)
Recent infection
3.19*
Exogenous reinfection
(Reinfection)
6.55
Endogenous reactivation
0.21

* Krishnamurthy et al

With the information on the size of previously
infected population, with or without recent reinfection,
it is possible to construct the profile of the population
in respect of tuberculous infection (Table 3). The table
is self explanatory.

Among the infected persons (11, 161), 5% were
initially uninfected but got recently infected between
surveys I and II (596), 4% were reinfected (415) and
91% (10,150) had no reinfection.
Table 3. Population aged 5 years and above (examined at I and II surveys) at II survey by infection status

Status of tuberculous infection Number % to total population
remain uninfected 19896 64.1
*Uninfected }
recently infected 596** 1.9**
Previously infected with recent reinfection 415 1.3
Previously infected with no recent reinfection 10150 32.7
Total population 31057 100.0

* Total uninfected = 20492; ** National Tuberculosis Institute, 1974 Among the 67 new cases diagnosed at the II
survey, for whom tuberculin sensitivity was known at I
survey, 28% had progressive primary tuberculosis,
41% had disease due to exogenous reinfection and the
remaining 31% due to endogenous reactivation (Table
4).

Contribution by different groups of population to the
total new cases at the II survey

Population with recent infection constituting
1.9% of the total population have contributed 28%
of the total new cases (progressive primary disease);
infected population with reinfection constituting
1.3% of the population have contributed 41% of the
new cases (disease due to exogenous reinfection),
and tie remaining 31% of the total new cases

reinfected together contribute 69% on the total new
cases arising in the community. Only 31% of the
new cases arise from the previously infected persons
with no recent reinfection, presumably due to
endogenous reactivation.

Acknowledgement

The authors are grateful to Dr A.K. Chakraborty,
Deputy Director, CGHS, Bangalore for his comments
and suggestions at the draft stage of the paper,
Mrs.Anuradha for statistical assistance, Miss T.J.
Alamelu for secretarial help and Mr B.R. Narayana
Prasad graphics.
References
Krishnamurthy, VV, Nair, SS, Gothi, GD, a
Chakraborty, AK. Incidence of Tuberculosis
among newly infected population and in relation to
the duration of infected status; Indian Journal of
Tuberculosis 1976, Vol. 33/1, 3.
National Tuberculosis Institute, Bangalore.
Tuberculosis in a rural population of South India :
a five year epidemiological study, Bull World
Health Orgn, 1974, 51, 473.
Sutherland, I. Endogenous reactivation and exogenous
reinfection, IUAT 1972,47,123.