FINDING rats in the town dump is hardly cause for comment in most of the world. Rattus norvergicus (the Norway rat) has spread to all but a few bits of the planet, giving rise to the urban myth that city dwellers are never more than six feet away from a rodent. However, the western Canadian province of Alberta has prided itself on being one of those rat-free bits for more than half a century. So when an infestation was discovered in early August outside Medicine Hat, a city of 72,000 people, it was headline news.

Pest-control officers installed high-definition cameras to track the rats, set up poisoned traps to catch them and released two bull snakes to kill those too wary to be trapped. The snakes, which look like rattlesnakes but are non-venomous constrictors, had been caught after citizens complained. They are normally released in a wilderness area when found in town, but in this case they were deployed to the dump.

Pictures of dead rats (those not disposed of by the snakes) seemed to signal early success. But as the corpses continued to pile up—there were 103 by August 27th—and rats were sighted in residential areas, the city opened a new front in the war: Operation Haystack. This involved stacking bales of hay stuffed with poison at 15 locations. Alison Redford, the provincial premier, promised the extermination effort would be “unrelenting.”

Alberta, where farming is important, has taken rat-control seriously since the 1950s. It maintains a rat-control zone 600km (375 miles) long by three km wide along the southern section of its border with Saskatchewan, where the greatest threat is thought to exist. A rat patrol pays special attention to farms and towns in that zone. The province publishes recipes for the most effective poisons: a mixture of rolled oats and icing sugar laced with Warfarin does the trick. And it has made it illegal to own a pet rat, although hospitals and universities are exempt.

According to Lianne McTavish and Jingjing Zheng, researchers at the University of Alberta, the provincial government’s poster campaign in the 1950s, modelled on wartime propaganda, was also intended to unite Albertans behind an image of a “clean province that was both distinct from its prairie neighbours and for the most part populated with vigilant, hardworking citizens eager to remove unwanted intruders”. The message sunk in. When rats were reported in a Calgary alley in 2004, the pest-control officers were nonplussed to find a posse of neighbourhood residents armed with brooms and wooden batons helping to club the rodents to death.

So few Albertans have seen a live rat that they have trouble identifying them, even though the government has helpful rat quizzes on its website. “We get hundreds of calls a year and less than 10% involve a rat,” says Phil Merrill, the provincial rat and pest specialist. So the recent increase in sightings in Calgary and Edmonton, where authorities received more calls in one day in August than they normally receive in a year, does not necessarily mean the province is being overrun. Past calls have resulted in the apprehension of juvenile muskrats, squirrels and northern pocket gophers. All are pests, but none is considered as dangerous in Alberta as the rat.