For the first time, snipers have deployed in Tehran to combat a plague of "mutant" rats that are increasingly resistant to poison and have grown so big that cats are scared of them.

Ten teams of sharpshooters armed with rifles equipped with infra-red sights have bagged more than 2,000 of the brutish rodents in recent weeks, city officials told state media.

That's a drop in the ocean: Iran's rat population easily outnumbers the sprawling capital's 12 million inhabitants. The city council is now boosting the number of sniper squads to 40, officials said.

"It's become a 24/7 war," a grim-faced Mohammad Hadi Heydarzadeh, the head of Tehran municipality's environmental agency, declared on state television last month. "We use chemical poisons to kill the rats during the day and the snipers at night."

Tehran city authorities exterminate nearly one million rats a year and annually declare new, multimillion dollar campaigns to control the pest problem. But the rats are proving to be natural born survivors.

"They seem to have had a genetic mutation," Ismail Kahram, an environmental adviser to the city council, said. "They are bigger now and look different. These are changes that normally take millions of years of evolution," he told the website Qudsonline.ir last month. He said cats are now smaller than some of Tehran's rats, which can weigh up to 5kg.

The problem gets worse with the onset of warmer weather when the snows on the nearby Alborz mountains begin to melt, raising water levels and flushing rats out of their subterranean lairs.

Many flourish in the open water channels that criss-cross the capital, such as the roadside streams flanking Ali Asr, the Middle East's longest street. It is a bustling tree-lined thoroughfare, packed with fast-food outlets and restaurants, running from upmarket north Tehran to the city's poorer southern suburbs.

In this latter part of the capital, the rat population is thought to be six times higher than the number of people living there, state-run media reported in 2010.

The tons of poisons used each year are meant to make the rats feel thirsty, so they return underground in search of moisture, reducing a possible health hazard from the corpses.

The bodies of the rats blasted by snipers are either incinerated or taken to the Kahrizak district in southern Tehran where they are covered in lime and then buried.

The rat problem has been such a major issue for several years that a newspaper once ran a cartoon in which a rat tells a man: "Our numbers are more than yours, so you leave Tehran."

mtheodoulou@thenational.ae