They cost the US $19bn each year, a sixth of the $120bn annual estimate for all invasive species. In 2017, New York’s mayor pledged $32m to tackle rats; in Mumbai, most vehicle fires are caused by rats. And while the urban myth that you’re never more than 6ft from a rat may not hold up, you’re probably not too far from one right now.

Except if you are in Alberta. Home to the cities of Calgary and Edmonton, and with a population of about 4.3m, Alberta is famous for oil, national parks and ice hockey. But it also has a lesser-known claim to fame: it’s the only part of the world with significant urban and rural populations that does not have a breeding rat problem.

So how did an area the size of Texas achieve a feat unparalleled anywhere in the world? Was it luck or the result of strategic genius? And what’s Alberta gained from keeping rats out?

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‘Kill At Sight!’

“We really have a geographical advantage,” says Merrill. “We started before we had rats – the rats came to our eastern border in about 1950 – and we said we don’t want rats, so we checked all the farms along the border where they were and poisoned them out. And we just don’t allow any more rats to come in.”

Geography certainly played a role – for a large area, there are few potential entry zones. Rats can’t survive the cold of the north or in the Rocky Mountains to the west. The southern border with Montana is mountainous and too sparsely populated for rats to spread.

That leaves defence of the eastern border. Rats arrived on the east coast of North America in the late 18th Century and slowly spread west, reaching neighbouring Saskatchewan in the 1920s.

“We’re not any smarter than Saskatchewan,” says Merrill. “The rats got to them about 30 years before us. The [provincial] governments at the time weren’t very developed – Saskatchewan wasn’t ready for them. By the time they hit our border, we had a department of health and a department of agriculture, and we had a system ready that we could actually do something.”

And they did. Rats were declared a pest in 1950, making rat control mandatory. Poison was used to kill rats that had made it into Alberta and to treat buildings that might shelter them in a strip 300km long and 20-50km wide along the eastern border. A rat control zone was established (it remains in place today) and pest control officers (PCOs) appointed to police it.