A potential breeding site in the city's northeast has put Alberta's rat-free claim at risk.
A northeast home is now surrounded by traps as Bylaw Services deal with what they believe is anything from two to 20 rats in the area.
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The city and the province regularly deal with individual rats that hitch a ride into the province, but this situation is causing concern because it looks like the rats were breeding.
Dr. Douglas Morck, a rat expert at the University of Calgary, says they do so at an incredibly fast rate.
"They are hugely destructive, so they burrow, they gnaw, they can gnaw a whole house down," he said. "They are incredibly destructive, they eat our grain [and] they carry diseases."
Calgary bylaw officers say they expect to have all the rats killed off in the area within a couple of weeks. While the fine for bringing a rat into the province is $5,000, no charges are expected in this case as it looks like they were brought in unknowingly from another province.
The city has dealt with three other cases of rats brought in on boats and trailers from Saskatchewan this year.