There are chew marks on the side of an old wooden granary. Dry oats are strewn everywhere. Holes have been dug under the shed. It's still early fall and yet the grass is matted down, as though it's been covered with snow.

These are the signs of an enemy Phil Merrill has spent decades battling.

"We've got rats," he says, stepping out of his truck, a grey Ford F-150 adorned with Alberta Rat Control on the side. "I can see the trails they've left in the grass. Dang!"

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Mr. Merrill, 64, is standing in a field in southwestern Saskatchewan, just two kilometres from the Alberta border. This is the frontline of a war he's been fighting for nearly 40 years as a pest control officer. The expressive Albertan has been the head of the provincial rat patrol for nearly a decade. Alberta has been rat-free – excepting the odd straggler – for the past 65 years; it's the only jurisdiction in the world to have fought rats and won, though colonies continue to thrive just outside the provincial boundary. Mr. Merrill is tasked with keeping them out.

Albertans have a take-no-prisoners attitude towards rats. For decades the province ran advertising telling residents to fear rats and kill them on sight. There's a heavily inspected rat control zone along the Saskatchewan border and pet rats are banned throughout the province. But Alberta's key weapon is the provincial rat patrol, a team of eight pest-control officers armed with shotguns, tasked with inspecting the control zone and spreading 10 tonnes of poison annually. No other jurisdiction has created a dedicated unit with the role of quickly snuffing out any rats who make it across a border.

"Alberta's got a real Alberta Advantage – we don't have rats," Mr. Merrill said between gulps of chocolate milk, his favourite drink, while driving the 500-kilometre control zone that falls under his purview. "There's a cost to rats. A farmer with a wooden bin would have to pay $1,500 for a new floor each time they chew through. The problem isn't that they eat that much wheat, but they contaminate a lot."

Despite attempts, no one has been able to estimate how much Alberta has saved from having no rats. If they got through, the rodents would destroy stores of grains on farms, undermining a pillar of the provincial economy. They also carry disease and would chew through household pipes and wires in Edmonton and Calgary. After more than a half-century without rats, the savings have been in the millions of dollars, according to Mr. Merrill.

Raised in southern Alberta, Mr. Merrill studied pest management at Simon Fraser University. He has worked as an inspector and pest specialist in Alberta for most of his life, and rats are clearly a passion. It's an odd job, but one that has yielded many stories. There was the rat that got loose on an Air Canada flight to Fort McMurray and somehow made it to the cockpit; a stubborn rat in a Lethbridge supermarket that was chased by employees carrying brooms; or the pet store that chose to fly three rats to the SPCA in Vancouver rather than euthanize them.

Alberta's war on rats started in 1950 when the rodents were first found in the border hamlet of Alsask, Sask. With the Rockies to the west, badlands to the south and vast forests to the north, Alberta had never had rats. But after two centuries of slow progress west from the port cities of the East Coast, rats had finally arrived at Alberta's doorstep. With a post-Second World War government that wielded significant resources, understood agriculture and feared the damage that rats could do to crops and buildings, the Alberta government created the rat patrol.

The Social Credit government of the day also established a rat control zone that stretches 29 kilometres into Alberta along the Saskatchewan border. The control zone starts at the U.S. border and continues to the northern forests near Cold Lake, Alta. There are about 3,170 farm sites within the zone and each is inspected annually. Special attention is lavished on older granaries with wooden floors.

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The rat patrol's main weapon is a bucket of aquamarine poison pellets. The bait is made of grain, barley and poison, all held together by wax. "Poison just works the best, by far," Mr. Merrill says as he pulls a bucket of the anti-coagulant pellets from the back of his truck. They'll kill most rats within a few days. The patrol has mostly phased out the use of snap traps.

Some in Alberta question whether the province is truly rat-free. Mr. Merrill explains that what the status means is that the province has no breeding rat population. Hundreds of rats do come into Alberta on commercial trucks from the U.S. and other provinces, but they're almost always alone and die alone.

The wooden granary in Alsask that is the source of Mr. Merrill's frustration is outside of his jurisdiction. The neighbouring Alberta town of Oyen has been a recent hotspot for rat activity and he thinks the old structure is where the rats are coming from.

A paper sign stapled onto the granary says that a pest control officer working for the local county last visited in May. Mr. Merrill shakes his head. "That just ain't enough. They need to be here every week. And they probably need to empty that granary."

The granary looks like a backyard shed and was probably built in the 1970s. The oats inside have been left alone for five years and there are probably hundreds of rats living under the granary. Surrounded by kilometres of empty, wind-swept prairie, the rats have found a warm place to live with a nearly unlimited supply of food.

"Damnit, I thought they'd all be dead," said Mr. Merrill, noting that officials in Saskatchewan have flagged the granary. "We've got to do something about this or they'll keep getting across that border."

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If the granary were only a few kilometres to the west in Alberta, Mr. Merrill would have the shed emptied and lifted by a crane. Armed with poison and shotguns, the rat patrol would have the rodent situation solved quickly. If that didn't work, they would set fire to the granary.

Saskatchewan doesn't have a rat patrol, but the province has stepped up its rat control effort in recent years. Two-thirds of its regional municipalities are now rat-free and the province could be entirely rat-free within a decade.

"That's where the rats stop and the patrol starts," says Romeo Prescott, a farmer in Alsask, pointing at a nearby cell tower on the Alberta border. "Growing up we knew that Alberta was militant about rats. We figured we had so many here because they'd send them back to Saskatchewan. It would be great if our province would be as aggressive as Alberta. My rat patrol on this farm is my cats."