RISK OF PULMONARY TUBERCULOSIS ASSOCIATED WITH EXOGENOUS REINFECTION AND ENDOGENOUS REACTIVATION IN

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. Krishnamurthy1

and K. Chaudhuri2

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

that survey interval as against 57 cases actually diagnosed.

It was also estimated that 1.9% of the total

population would be having recent infection, 1.3%

would be previously infected with recent infection

and 32.7% with previous infection but no recent

infection leaving 64.1% who are not infected at all

(uninfected). Among the new cases diagnosed, 28%

would have progressive primary disease, 41% cases

arise due to exogenous reinfection and 31% due to

endogenous reactivation. In other words, the 1.9%

population with recent infection contributes 28% of

the total new cases, the 1.3% reinfected population

contributes 41% and the 32.7% infected population

without further reinfection contributes the

remaining 31% of the total new cases.

Introduction

It is estimated that about one third of the Indian

population are infected with M.tuberculosis (National

Tuberculosis Institute, 1974). It is also reported that

about 75% of the total new pulmonary tuberculosis cases

are diagnosed from the previously infected and 25%

from the uninfected persons (Krishnamurthy et al.,

1976). The new cases arising from the previously

infected population may have developed the disease

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

cases arise from three different groups of population

viz.,

*Paper presented at the First Joint Conference on Tuberculosis and Chest Diseases held at Madras in

December, 1989.

1. Statistical Assistant, National Tuberculosis Institute, 8, Bellary Road, Bangalore 560 003.

2. Director, National Tuberculosis Institute, Bangalore.