|PROBLEMS CONNECTED WITH ESTIMATION OF THE INCIDENCE
OF TUBERCULOSIS INFECTION
|Raj Narain, SS Nair, P Chandrasekhar & G Ramanatha
Rao: Indian J TB 1965, 13, 5-23.
The incidence of infection with mycobacterium tuberculosis
is an index of the risk of infection to which a community is exposed.
An accurate estimation of incidence rate is of considerable importance
in understanding the epidemiology of tuberculosis in organising
control measures. A new method of estimating incidence of infection
is discussed. The material from 3 studies of National TB Institute
has been utilized. Study I: is a part of a survey of a random
sample of 134 villages. No previous tuberculin testing or BCG vaccination
had been carried out in the area, but each person was examined for
BCG scar in order to exclude persons vaccinated probably from other
areas. After a complete census, a Mantoux test with 1 TU of PPD
RT 23 with Tween 80 given on two occasions (Round I and II). Those
with reaction of 13mm or less at Round I were offered a test with
20 TU with Tween 80 within a week of 1 TU test. The interval between
the rounds was about 18 months. From the analysis of the data from
the first 50 villages for which complete information for both rounds
was available, it was seen that there was a general increase in
the size of reactions elicited in the second round. Study 2:
tuberculin testing was carried out with 1 TU and 20 TU among
selected control groups which provided the data regarding
the enhancing of tuberculin allergy seen in repeat tuberculin
tests. Study 3: in the course of the longitudinal survey
reader assessments were carried out periodically to judge
the standards of the tuberculin test readers. Inter & intra-reader
comparisons were made. The findings have been used to estimate the
magnitude of reader variation. The data was also used to study variations
in the technique of testing and reading.
It was estimated that on an average inter &
intra-reader variations between the rounds were unlikely to exceed
6mm or more in more than 5% of the observations. The reading errors
have an equal chance of being positive or negative except at extreme
ends of the distribution where zero readings of Round I can only
show an increase, and the very large reactions had a greater chance
of showing only a decrease at a subsequent round. The study mainly
concerns with the problems of estimating the incidence of tuberculous
infection in a community. Calculations based on age-specific prevalence
rates or on rates of tuberculin conversion or both subject to gross
error, leading to unreliable epidemiological conclusions. For estimating
the newly infected, a new approach has been suggested based on the
drawing of a curve for the distribution of differences in reaction
size from one round of tuberculin testing to another. It is assumed
that if new infection causes a distinct rise in the degree of tuberculin
sensitivity which is greater than the combined rise due to enhancement
and reader variation, the distribution of differences between the
rounds should indicate the newly infected. It is shown that the
newly infected probably constitute a homogeneous group with an increase
in mean reaction size of about 24mm and standard deviation of 4mm.
Accordingly, 98% of the newly infected show an increase in reaction
size of 16mm or more.
|KEY WORDS: RISK OF INFECTION, TUBERCULIN ALLERGY,
ENHANCEMENT, INCIDENCE, INFECTION.
| CHANGES IN THE PREVALENCE RATES OF INFECTION IN
YOUNGER AGE GROUPS IN A RURAL POPULATION OF BANGALORE DISTRICT OVER
A PERIOD OF 5 YEARS
|AG Kurthkoti & Hardan Singh: NTI Newsletter
1985, 21, 28-40.
The utility of repeated estimates of prevalence
rates of infection in children as a tool for surveillance in tuberculosis
is now well recognized. Two prevalence surveys at an interval of
5 years were conducted by National Tuberculosis Institute, Bangalore,
with the main objective of studying changes in prevalence rate of
infection among children in the age group of 0-9 years. A total
population of 42,343 residing in 90 randomly selected villages of
Doddaballapur taluk, Bangalore, were registered; of them, 12,535
were children in the age group of 0-9 years. Children were further
classified into two sub groups 0-4 and 5-9 years, with or without
BCG scars. The unvaccinated children in these two age groups formed
the study population.
The population in the study area during the 2nd
repeat survey was similar to that of first survey with regard to
age, sex distribution, except that a growth rate of 1.1% per year
was registered. The BCG scar rate, among children in the age group
0-4, 5-9 years, was 8% & 39% respectively at survey I. All the
unvaccinated children below 10 years were given tuberculin test
with 1 TU PPD RT 23 and reactions were read 72 to 96 hours after
tuberculin testing. In the first survey, level of demarcation to
classify the infected children was 10 mm and above, while in II
survey it was 12 mm and above. It was observed that the prevalence
rate of infection from I survey to II survey was not altered (2.58%
& 2.46%) in the 0-4 years of age, while there was an increase
in the rate from 8.93% to 12.3% in 5-9 years of age in the II survey.
The increase in the infection rate could be attributed to the rising
trend of infection, over reading by tuberculin-readers', skills
of both tuberculin tester and reader, boosting of tuberculin reaction
or scarless BCG vaccination. In conclusion, the study of changes
in the prevalence rate of infection in the younger age group is
simple, cheap, less time consuming. The data can be used for calculating
annual risk of infection as well trend of transmission of infection.
|KEY WORDS: TREND, RISK OF INFECTION, PREVALENCE,
SURVEILLANCE, RURAL COMMUNITY.
|RISK OF PULMONARY TUBERCULOSIS ASSOCIATED WITH EXOGENOUS
REINFECTION AND ENDOGENOUS REACTIVATION IN A SOUTH INDIAN RURAL POPULATION
- A MATHEMATICAL ESTIMATE
|VV Krishna Murthy & K Chaudhuri: Indian J TB
1990, 37, 63-67.
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 associated with exogenous reinfection and endogenous
reactivation has not been computed in Indian conditions, the data
collected during a longitudinal study by National TB Institute,
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 reinfection 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% previously
infected population contributes the remaining 31% of the total new
|KEY WORDS: RISK OF INFECTION, INCIDENCE OF INFECTION,
EXOGENOUS REINFECTION, ENDOGENOUS REACTIVATION, RURAL COMMUNITY, MATHEMATICAL
|CASE FOR A REPEAT EPIDEMIOLOGICAL SURVEY IN INDIA
|AK Chakraborty: Indian J TB 1992, 39, 209-12.
The question of carrying out a repeat epidemiological
survey in India has been engaging the attention of many for quite
some time. The first nationwide tuberculosis prevalence survey was
conducted in India during 1955-58. It served as an eye opener and
produced data which were profitably used by the planners to decide
about the form and state of national control programme. Doing a
repeat survey will be useful only if it would be capable of yielding
epidemiological information on the future course of action. At the
time of formulation of the District Tuberculosis Programme (DTP),
it was perhaps presumed that programme would work with optimum efficiency
as in the operational studies and as such the real performance was
not envisaged. Secondly, due to low prevalence rates of tuberculosis
as shown in all the surveys could reflect a small rate of change
or no change at all, thus these longitudinal surveys with inadequate
samples, did not have enough discriminatory power to observe a statistically
valid change with time.
It is now globally realised that instead of looking
at mortality rates or small changes in the prevalence rates of cases,
it is the Annual Risk of Infection (ARI) which holds the key to
epidemiological trend in a community. However, through a model recently
constructed at the National Tuberculosis Institute, it is possible
to extrapolate the findings of well planned small surveys in certain
areas. It gives an idea what to expect over a period of 50 years
- a slow decline. Therefore, when the present efficiency
of Case-finding programme is about 33%, treatment efficiency also
of the same order or even worse and with persistent rise in the
population, it is futile to talk of epidemiological assessment through
repeat surveys. Instead, we should concentrate on raising the efficiency
of the DTP as near to the level which could be called the critical
level of efficiency. Till then nation wide surveillance through
the calculation of ARI is the only choice.
|KEY WORDS: REPEAT SURVEY, ASSESSMENT, DECLINE,
RISK OF INFECTION.
|WANING OF BCG SCAR AND ITS IMPLICATIONS
|R Channabasavaiah, V Murali Mohan, HV Suryanarayana,
MS Krishna Murthy, & AN Shashidhara: Indian J TB 1993, 40,
It has been postulated that BCG scar disappears
in a good number of children and some of the vaccinated children
will get included in the non- vaccinated group and cause difficulty
in interpreting the results of tuberculin test. It was decided to
analyse information on BCG scar status in the younger population
of a rural community in 3 taluks of Bangalore district with an objective
to find out whether disappearance of BCG scar is dependent on the
age of the child, size of post-vaccination induration at initial
survey and tuberculin sensitivity status of children in whom BCG
scar has disappeared, in comparison with children in whom the BCG
scar has not disappeared. In all, 1095 children aged 0 to 14 years
were found with BCG scar in 119 randomly selected villages during
an epidemiological survey done in 1961 at the time of intake. Following
two groups of children were studied for disappearance of the scar.
Of them, a) 796 children who had BCG scar at the first survey, and
whose BCG scar status was available at 4th survey, b) 299 who showed
no BCG scar at first survey but were found with BCG scar at 2nd
survey and whose BCG scar status was available at 4th survey.
Of the BCG scars recorded at intake, 26.4% and
32.5% disappeared subsequently during three and a half and five
year periods respectively. The waning of BCG scars was independent
of age of the child and tuberculin sensitivity status at intake.
Tuberculin sensitivity status in children in whom scar had disappeared
was the same as that found in children in whom scar had persisted
at intake and after five years. The misclassification of children,
in whom scars have disappeared, as unvaccinated leads to a difficulty
in interpreting the results of tuberculin test done for the purpose
of computation of the Annual Risk of Infection. Further,
the extent of misclassification increases in proportion with the
increase in BCG coverage of the population. This finding justifies
the practice of identifying the demarcation level on the basis of
the distribution of tuberculin induration sizes for classifying
the infected persons in a population in each survey.
|KEY WORDS: BCG SCAR, WANING, RURAL POPULATION,
RISK OF INFECTION.