|DISTRIBUTION OF TUBERCULOUS INFECTION AND DISEASE
IN CLUSTERS OF RURAL HOUSEHOLDS
|SS Nair, G Ramanatha Rao & P Chandrasekhar: Indian
J TB 1971, 18, 3-9.
Data from 62 randomly selected villages in a district
of south India, which formed part of a prevalence survey carried
out by the National Tuberculosis Institute, Bangalore, during 1960-61,
has been made use of. The survey covered 29,813 persons in 5,266
households. There were 70 cases with bacilli demonstrable either
in smear or culture and 300 suspect cases. Using the village map
(prepared by survey staff), case clusters were formed
first, with each case household as nucleus and adjacent households
within a maximum distance of about 20 meters on either side of the
case households. Households closest to the nucleus household on
either side have been called as 1st neighbourhood and those coming
next in proximity on either side as a 2nd neighbourhood and so on.
The case household and its four neighbourhood together was called
a cluster. If another case household was found within 4th neighbourhood
of the first case the cluster was extended by including the 4th
neighbourhood of the new case also. Such clusters were called composite
case clusters and clusters with only one case household as simple
case clusters. Similarly, suspect case clusters were formed and
differentiated as simple suspect clusters or composite suspect clusters.
Further, to serve as a control group, non-case clusters were constituted
from a systematic sample of 10% households that were not included
in case or suspect case clusters.
Out of 60 case clusters formed, only 7 have multiple
cases showing that there was no evidence of high concentration of
disease in case clusters. While the percentage of child contacts
(0-14 years) infected was considerably higher in case clusters (25.8%),
there was not much difference between suspect case clusters (14.9%)
and non-case clusters (9.8%). Similarly, there was not much difference
between simple and composite clusters. Infection among child contacts
was higher in case households as compared to their neighbourhoods.
To get some idea of the zone of influence of a case or suspect case,
prevalence of infection was studied for 10 neighbourhoods, in simple
clusters to avoid the influence of multiple cases. It appeared that
the zone of influence of a case may extend at least upto the 10th
neighbourhood. It was also noted that there was very little difference
between zones of influence of suspect cases and non-cases. Case
clusters in which the nucleus case had shown activity of lung lesion
(evident on X-ray reading) or had cough showed significantly higher
infection among child contacts. Clusters around cases positive on
both smear and culture did not show higher infection than those
around cases positive on culture only. (This may be due to sputum
examination of single specimen only).
Out of the total infected persons in the community,
only 2% were in case households and 7% in suspect case households,
over 90% being in non-case households. The zone of influence of
a case extending at least upto the 10th neighbourhood and the overlapping
of such zones of influence of cases, present and past, seems to
be the most probable explanation for the wide scatter of infection
in the community. Prevalence of infection among child contacts was
definitely higher in case clusters. But, the significance of this
could be understood only from a study of the incidence of disease
during subsequent years in different types of clusters. It is significant
that only 10% of the total infected persons in the community were
found in case clusters. The case yield in general population, cluster
contacts, household contacts and symptomatics attending general
health institutions have been also compared. The case yield in the
last group (10%) is much higher than the case yield from both types
of contacts (0.7% and 0.6%) which where only slightly higher than
the case yield from the general population (0.4%).
|KEYWORDS: RURAL, HOUSEHOLDS, CLUSTERS, CASE,
SUSPECT CASE, CONTACT, PREVALENCE, INFECTION, DISEASE, SURVEY.