Abstract
BackgroundScreening household contacts of active tuberculosis (TB) patients is recommended for TB control. Due to resource constraints this rarely occurs in lower income countries. Demographic and clinical features of index cases may influence the likelihood of onwards TB transmission. It has also been proposed that accumulation of intracellular lipid bodies within M. tuberculosis cells may also enhance bacterial transmissibility. This study explored whether clinical and bacteriological observations recorded at baseline in TB cases in Malawi could help identify those with the highest risk of onwards transmission, to prioritise contact tracing.MethodsIn this case-contact study, data on clinical presentation, sputum bacterial load and the percentage of lipid body positive acid-fast bacilli (%LB + AFB) on sputum smears were recorded in adults with sputum smear and culture positive pulmonary TB before initiation of therapy. The Tuberculin Skin Test (TST) was used to detect infection with M. tuberculosis amongst household contacts under the age of 15 years. TST positivity of the child contacts was related to characteristics of the index case.ResultsThirty four index cases brought 56 contacts (median: 1, range: 1–4 contacts each). 37 (66%) of contacts had a positive TST. Cavities or a high percentage of lung affected on index patient CXRs were associated with TST positivity. Multivariate analysis of non-radiological factors showed that male sex, HIV-negative status and raised peripheral blood white blood count (WBC) in index patients were also independent risk factors of TST positivity. Lower %LB + AFB counts were associated with TST positivity on univariate analysis only.ConclusionTST positivity is common amongst household contacts of sputum smear positive adult TB patients in Malawi. Contact tracing in this high risk population could be guided by prioritising index cases with CXR cavities and extensive radiological disease or, in the absence of CXRs, those who are HIV-negative with a raised WBC.
Highlights
Screening household contacts of active tuberculosis (TB) patients is recommended for TB control
This study aims to explore the extent to which clinical, radiological and microscopy-based bacteriological factors could help identify highly sputum smear positive (SSP) index cases with the highest risk of onwards transmission, so that contact tracing efforts can be targeted in low resource settings
The high TST positivity seen in child contacts of SSP adult TB patients highlights the importance of contact tracing
Summary
Screening household contacts of active tuberculosis (TB) patients is recommended for TB control. This study explored whether clinical and bacteriological observations recorded at baseline in TB cases in Malawi could help identify those with the highest risk of onwards transmission, to prioritise contact tracing. Active case finding in household contacts has proved to be effective in Malawi [6], the city of Blantyre alone notifies one thousand new SSP TB patients per year (unpublished Malawi TB Control Programme data). Implementation of a structured approach to prioritise contact screening would be Hector et al BMC Infectious Diseases (2017) 17:259 helpful, especially in settings where case detection rates remain low [8, 9]
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