Medicine Hat officials have found rats in a landfill and are setting out poison in an attempt to help Alberta keep its rat free status.

"Upon further investigation of a single rat found in a farm yard south of Highway 41A, several rats were spotted in the Medicine Hat landfill," city officials said in a press release.

"These rats were found as a result of this monitoring, which shows that our monitoring program is working. Additional bait stations have been put out to poison rats, and staff are checking sites daily."

Landfill superintendent Paul Nicholson says 39 rats have been killed so far.

Officials have monitored a "three-mile buffer zone" around the landfill for rat activity since 2012, when an infestation of rats was discovered.

Nicholson says crews have been vigilant about checking for rats since 2012 and have seen very little activity until this spring.

"We will be vigilant as long as we have to be vigilant. If that's one year, two years, five years — we will continue the course," said Nicholson.

He says he's confident they eradicated the rats from 2012 and it's likely the latest ones have arrived in the fall or winter by getting into a truck.