Story highlights Big rats are reportedly running rampant in 26 districts in Tehran

The government uses poison, and teams of sharpshooters are killing the rodents with air guns

The biggest headache is Norwegian rats, which can grow to 16 inches

The rats die hard in Iran.

Iran has the biggest beef with Norwegian rats, Mohammad Hadi Heydarzadeh, a top environmental official, told CNN.

He said the critters have "entered Iran from abroad by cargo ships."

Though it's unclear if the rats come with paperwork, a search of ratbehavior.org shows that that particular kind of rat can grow 16 inches long. And they are very aggressive, "fighting, chasing, biting and boxing."

While the Iranians have used poison to kill the rats during the daytime, at night the teams of sharpshooters use infrared lenses to hunt them down.

"We have identified the rats' places of congregation by using (computer) software," Heydarzadeh explained.

By the end of 2013, the Iranians hope to have 40 teams of shooters working, he added.

So far, 2,205 rats have been killed. Their bodies are carted off and incinerated or buried in special dumps, the environmental official explained.

University researchers are working with the government to assess whether their newest plan of attack is working.

But Heydarzadeh feels good.

"The number of these rodents in Tehran," he said, "is on the decline."