We write on behalf of the Outbreak Control Committee (OCC) investigating an outbreak of isoniazid monoresistant tuberculosis (TB) affecting over 260 cases (222 in London) to alert clinicians about recent transmission of a multidrug resistant (MDRTB) component with unusual characteristics.

A unique genetic fingerprint on Restriction Fragment Length Polymorphism (RFLP) typing¹ at the Health Protection Agency Mycobacterium Reference Unit (HPA MRU) has allowed tracking of the strain. Fifty percent of cases were born in the UK; they are from a wide ethnic and social background with foci in high risk groups including the homeless, injecting drug users, and prisoners.² Inhalation of crack cocaine is common and may have contributed to the spread. All outbreak cases are recommended to receive directly observed therapy (DOT) unless adherence is confirmed. Adherence to treatment has been poor in one third and several have acquired MDRTB. Some of these were active in the community while infectious, and we have seen primary MDRTB in two young people with no known epidemiological link apart from relative geographical proximity.

Of six outbreak MDR cases in London, three are distinct strains with rare mutations (D516Y, H526R, S531W) in the rpoB gene, demonstrable by commercial genetic probing. These were found in patients poorly concordant with treatment. A wild type genotype not detectable on commercial molecular testing routinely used in the UK³ has been found in one poorly compliant case followed by two (primary) new cases suggesting community acquisition.

The British Thoracic Society guidelines^4,5 recommend that all TB cases are microbiologically confirmed where possible. Adequate samples should be taken before treatment and isolates sent to the HPA MRU to enable detection of outbreak cases as well as sequencing of MDRTB strains. This is necessary so that appropriate treatment can be initiated as soon as possible and enhanced contact tracing carried out for these outbreak cases.