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Journal of Public Health Advance Access published online on June 19, 2009
Journal of Public Health, doi:10.1093/pubmed/fdp056
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© The Author 2009, Published by Oxford University Press on behalf of Faculty of Public Health. All rights reserved

High rate of transmission among contacts in large London outbreak of isoniazid mono-resistant tuberculosis


F. Neely, Specialist Registrar1
H. Maguire, Consultant Medical Epidemiologist1,2
F. Le Brun, Statistician3
A. Davies, Specialist Registrar in Microbiology, Senior Lecturer in Microbiology4
D. Gelb, Statistician3
S. Yates, Tuberculosis Specialist Nurse5

1 Health Protection Agency (HPA), London region, 7th Floor, Holborn Gate, 330 High Holborn, London WC1V 7PP, UK
2 St Georges Hospital Medical School, London SW17 8QT, UK
3 Health Protection Agency (HPA), Centre for Infections, 61 Colindale Avenue, London NW9 5EQ, UK
4 Royal Free Hospital, Pond Street, London NW3 2QG, UK
5 North Middlesex University Hospital Trust, Sterling Way, London N18 1QX, UK


Address correspondence to H. Maguire, E-mail: helen.maguire{at}hpa.org.uk


  Abstract

Background For a continuing London outbreak of isoniazid mono-resistant tuberculosis (TB), we aimed to determine transmission rates and risk factors for contacts of early cases, in order to inform future guidance on contact tracing.

Methods Paper-based proformas were completed by TB nurses, and then analysed using EpiInfo/SAS statistical software.

Results Forty community contacts (11%) became cases, 45 (13%) were recommended chemoprophylaxis and 270 (76%) were discharged clear of infection. The highest transmission rate was among contacts exposed to two or more cases (29% became cases) and close contacts of sputum smear-positive cases (22%). Other risk factors were being male and exposure to drug-using cases or cases with prison links. The number needed to be screened (NNS) to detect one case was lowest [5 (95% CI: 4–8)] for contacts of sputum smear-positive pulmonary cases, although the NNS was still only 20 (95% CI:8–72) for casual contacts of smear-positive cases.

Conclusions Transmission of disease to contacts was high (11%) compared with other documented outbreaks (0.7–2%). The results support recommended guidelines for contact tracing but also provide grounds to recommend, for outbreak cases, screening of casual contacts of smear-positive cases and contacts exposed to more than one case, drug users or prisoners.

Keywords: contact tracing, drug resistance, outbreak, transmission, tuberculosis


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