Canada's shipping ports have had rat infestations for hundreds of years, even up the Great Lakes. DNA studies show that once a local population became established long ago, it defended itself repeatedly from incursions, and carries on. Alberta has no sea ports, so rats hitch rides there on trains, trucks, and in packaging. The scale is much, much smaller, so Alberta is somewhat able to eradicate them.

The exterminator told me you either have rats or mice. That's 'cuz the rats are good at eating the mice.