Sixty Norway rats have been killed so far in a garbage dump outside Medicine Hat, a city of 61,000 in Alberta's southeastern corner, and officials are taking steps to eradicate the rest of the colony.

"We've got them isolated in a specific area within the dump and we have the dump also contained," said Vaughn Christensen, the provincial official who runs the province's rat control program. "And then ... for a number of miles surrounding the dump we have an active baiting program."

Alberta has billed itself as being rat free for more than seven decades after moving in 1950 to wipe out the rodents which eat agricultural crops and spread disease. The province maintains a 29-kilometer (18-mile) buffer zone along its eastern border with Saskatchewan where bait traps are monitored in order to watch for any rat populations headed west.

The Medicine Hat colony was spotted after rats began turning up in traps in the spring and through public reports.

While some infestations are occasionally spotted and dealt with, Christensen said it is rare to find a colony the size of the one in Medicine Hat's landfill.

"It's hard to put a number to the size," he said. "But if we recover 60 rats above ground then it's logical to assume, because they (live) primarily underground, there's more there. ... But I think we're talking hundreds, not thousands, and not the hundreds of thousands you might find in other parts of the world."

(Reporting by Scott Haggett; Editing by Richard Chang)

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