About forty years ago in Poland, an adventurous strain of the bacterium Staphylococcus aureus made a seemingly unprecedented move, a new study shows: it crossed over from humans to chickens and settled in to stay. The disease has since spread worldwide to become the leading cause of lameness in broiler chickens.

J. Ross Fitzgerald and graduate student Bethan V. Lowder of the University of Edinburgh, along with eight colleagues, discovered the big jump and reconstructed the pathogen’s diversification and pandemic spread. To do so, they compared DNA sequences from fifty-seven S. aureus samples isolated during the past half century from poultry living on four continents.

Remarkably, the team found, most poultry-infecting strains belong to a single genetic group and are closely related to a few human strains that circulated exclusively in Poland in the 1990s. That suggests a single, recent, human-to-poultry host switch.

The poultry strains subsequently lost genes involved in human pathogenesis and acquired ones that confer virtual imperviousness to attack by chicken immune cells, the team found. Thus, the avian strains seem to have adapted to their new host. That’s a first from the short list of pathogens that animals can pick up from humans (usually we hear about animal pathogens adapting to humans).

In the human-to-poultry case, the conditions seem uniquely optimal for spreading infection: a few multinational companies distributing huge numbers of live chickens worldwide. But microbes are resourceful; most likely, epidemiologists haven’t looked hard enough for other cases.

This research was published in the journal PNAS.