Carriers of the bacterium causing typhoid fever could be important in its spread.

Typhoid fever, caused by Salmonella enterica serovar Typhi, causes 200 000 deaths each year, principally in developing countries. An international collaboration – led by Mark Achtman of the Max Planck Institute for Infection Biology in Berlin, Germany, and includingGordon Dougan from the Wellcome Trust Sanger Institute and Jeremy Farrar from the Wellcome Trust's Major Overseas Programme in South-east Asia – has analysed strains of Typhi from around the world, shedding new light on its evolution and suggesting possible approaches to its control.

The researchers looked at the DNA sequence of 200 gene fragments in more than 100 strains of Typhi.Patterns of variation within these revealed its likely evolutionary history and global spread.

Interestingly, some strains of Typhi seem to have survived unchanged over thousands of years. (Unusually, the ancestral strain, from which all current strains are derived, still exists today; the genetic data suggest it arose after humans migrated out of Africa but before the Neolithic period.) On the other hand, widespread antibiotic use has led to rapid evolution in certain genes associated with antibiotic resistance.

The results suggest that there may be two ways in which Typhi is spread: through an acute form, causing disease, that can evolve rapidly due to the selective pressures from antibiotics; and through a slowly evolving form that is present in asymptomatic carriers. Such a role for carriers would have major implications for public health strategies.

External link

• Roumagnac P et al. Evolutionary history of Salmonella typhi. Science 2006;314(5803):1301–4.