Tuberculosis Epidemiology: Migrants Do Not Spread TB

At a recent meeting of the Society of General Microbiology in Bath, Surrey University scientists said recent reports that migrant workers and immigrants to the United Kingdom are responsible for spreading TB in Britain are incorrect.

"By DNA fingerprinting the different strains of the bacterium Mycobacterium tuberculosis which causes TB we can tell where an infection originally came from, and whether it is a new variety or a reactivated strain from years ago," explained professor Jeremy Dale of the School of Biomedical and Molecular Sciences at the University of Surrey-Guildford. "We found that the range of bacterial types in patients born in the UK was markedly different from that in patients born overseas." Hence, the findings show that the patterns of TB infection in UK-born patients suggest that many cases are caused by reactivation of past infection. Only a small portion of people infected by TB will develop the disease, while in the rest the bacteria stay dormant, with the possibility of being reactivated at a later date.

By contrast, the DNA patterns of TB bacteria found in overseas-born patients were characteristic of the country of origin of the patients, indicating that there has been little spread of TB from migrant communities into the UK-born population. "We tested samples from London obtained between 1995 and 1997. London has about half the UK's cases of TB. The DNA fingerprints of the bacteria showed marked differences in patients born overseas, and presumably these are imported strains," Dale said.

The researchers are studying evidence of how TB spreads over long distances, with the hope of helping health officials devise improved strategies for controlling the disease in the future. A group of closely related TB bacteria, called the Beijing family, was found to be dominant among the identifiable strains they examined. The Beijing family was first isolated in China and is now the major family of strains in many parts of Asia. This family has been responsible for many of the most serious outbreaks of drug-resistant TB.

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