It was a presidential tweet that again aggravated America's racial tensions.

Key points: A pest control company says DC is the fourth "rattiest" area in America

A pest control company says DC is the fourth "rattiest" area in America The company rated Baltimore ninth on its list

The company rated Baltimore ninth on its list Gentrification and mild winters have been blamed for a spike in complaints in DC

Donald Trump's bad-mouthing of Baltimore, which he called a "disgusting, rat and rodent infested mess", was part of a sprawling attack on prominent African-American politician Elijah Cummings, who represents a majority-black district in the city.

The outburst incensed the local newspaper, which retorted it was "better to have some vermin living in your neighbourhood than to be one".

But the bigger, ongoing political debate in the US capital about whether the Commander-in-Chief is a racist, has obscured the reality of the United States's reported rodent population.

Washington has ramped up its efforts to bring the city's rodent problem under control. ( ABC News: James Glenday )

Washington DC, including the areas immediately around the White House and Capitol Hill, is thought to have more rats in the ranks than most of the country.

According to pest control company Orkin, DC is the fourth "rattiest" area in America, beaten only by the more-populated Chicago, Los Angeles and New York.

Baltimore came in ninth.

White House has long had a rat problem

The White House was built 227 years ago, and has suffered rodent infestations ever since. ( Reuters: Kevin Lamarque )

Vermin has inhabited the city for centuries, and America's executive mansion has had a long, well-documented battle with vermin.

According to a 1990 report in the Houston Post, a rat swam past first lady Barbara Bush in the White House pool. It was then drowned by the President George HW Bush.

Jimmy Carter was reportedly upset by the stench when rodents died and decayed in the walls.

Loading

Back in 1889, Caroline Harrison, wife of president Benjamin Harrison, noted the scale of the building's rodent problem in her diary.

"The rats have nearly taken the building, so it has become necessary to get a man with ferrets to drive them out," she wrote.

"They have become so numerous and bold that they get up on the table in the Upper Hall."

Donald Trump claimed "no human would want to live" in Baltimore. ( AP: Pablo Martinez Monsivais )

The Trump administration has reportedly brought in pest control to capture rats, mice, cockroaches and stink bugs.

Mice have even managed to infiltrate the Situation Room.

This could explain why the President may have complained to his golf partners that the White House is a "real dump."

In December, discussion about the scale of rat infestation picked up when a Fox News White House correspondent tweeted about a "big brown rat" that moved past his foot.

Loading

Since then, several other reporters have noted the baiting operations taking place on the mansion's grounds.

Washington's 'rat man' knows he's got a big problem

Gerard Brown, who describes himself as the city's "head rat man", knows he has a big problem.

His team from the local health department took the ABC on a patrol through an alley, not far from the United States Congress.

DC's "head rat man" Gerard Brown (second from right) says "sanitation is pest control". ( ABC News: James Glenday )

There were bins with bite marks, food scraps scattered about and plenty of holes.

"In the last two years, we saw an increase of 55 per cent in rat complaints," Mr Brown said.

As we spoke, a large rodent scurried in front of us from a garden to the rear of a restaurant.

The team has been doubling down on extermination efforts. It has tried bait, poison, dry-ice and wire mesh.

Currently, it is trialling a birth-control substance, aimed at stopping the creatures from breeding.

The rapid gentrification of central Washington is thought to have helped trigger an explosion in the rat population.

Many more wealthy families and young professionals have moved downtown to what were once predominantly African-American neighbourhoods.

Washington's "head rat man" Gerard Brown says "sanitation is pest control". ( ABC News: James Glenday )

The area has seen much more construction and the new residents are propping up a series of new cafes and boutique eateries.

Despite their association with poverty, rats generally thrive where there is food, water and shelter.

"We are currently providing that in abundance. There's been a 25 per cent increase in new restaurants in the past two years," Mr Brown said.

"We have also been having more mild winters, which is good for rats. It means they can keep breeding. We predicted this would be a problem several years ago."

Washington's rat team has a tip for Australia

In most places, rats currently seem to be thriving.

Bobby Corrigan, a noted urban rodentologist who has advised officials across America and around the world, including in Melbourne, said most cities he visited in recent years were grappling with an increase in the animals.

"Rats are, for the most part, a global issue," he said, before adding he was soon off to Auckland on a rat-related mission.

In ideal conditions, the animals may have sex up to 20 times a day and give birth to large litters after three-and-a-half weeks, meaning a small infestation can rapidly become a big one.

Experts say the best way to reduce rat numbers is to get rid of rubbish. ( Supplied: Bobby Corrigan )

The creatures can damage homes, chew through cables and spread Leptospirosis.

Seven dogs in Sydney have died of the rare bacterial infection this year.

It can also be fatal in humans.

Despite all the different tools the Washington team uses to kill rats, they have one simple tip for Sydney and indeed any other city trying to control the population of the creatures.

"Clean up your trash, particularly food scraps," Mr Brown said.

"Garbage is rat food. If they can't get enough, rats get stressed, they turn on each other, they don't breed as much, they starve."

"Sanitation is pest control. It's the best thing we can do."