An invisible enemy: the battle to save black-footed ferrets from the plague

Yersinia pestis killed millions of people across Europe in the middle ages. Today it remains a deadly threat to one of the US's most endangered species

Every year, from August to November, Travis Livieri becomes nocturnal. The field biologist goes out in his truck in Conata Basin, South Dakota, armed with a spotlight in search of one of the most endangered mammals in North America: the black-footed ferret. When the light catches the reflective green shine of the ferret's eyes, he waits for the animal to disappear into a burrow and then lays a trap at the entrance.

Once the ferret is trapped, Livieri coaxes it into a long black tube and anaesthetises it before giving it a vaccine shot. Then he takes a blue marker and draws a line from the ferret's left ear to its right shoulder. About a month later, he returns again at night to the same location to give the ferret its booster, drawing a line from its right ear to its left shoulder. Ferrets marked with an X are safe from the plague.