"The rat who inhabited the sewers and basements of Victorian London was bigger, meaner and generally better adapted to urban life than his predecessor [the black rat]."
In the 18th Century, Britain was invaded by gray rats, a much larger species than black rats. The creatures, the size of small cats, would "gnaw the hands and feet of little children". London was full of gray rats which later inhabited the sewer system and found their way into people's houses.
Professional rat catchers, hired to kill the beasts, used trained ferrets and hounds to kill them. Another way to catch the rats was to lure them with "toasted cheese or bacon or fried liver or tallow or oatmeal" laced with arsenic (http://www.geriwalton.com/index.php/2013/12/child-rat-catchers-of-victorian-era/).
Jack Black, the rat catcher for Queen Victoria, would trap rather than kill the rats. He would sell the rats to gamblers who put on rat fighting exhibitions, a popular sport at the time. Rat fighting was a misnomer: it was really rat chasing. The rats were released to awaiting dogs to determine which hound could kill the most rodents. Some dogs could kill a rat every 2.7 seconds (http://listverse.com/2015/01/04/10-bizarre-victorian-era-jobs/).
While the pay for catching rats was decent compared to that of other lower class jobs, the hazards were great: rat catchers were bitten frequently. Rats were notorious carriers of disease including the Black Plague. Despite the hazards, rat catchers persevered.
Rat catching was not just reserved for the Victorian era. I googled rat catchers and discovered that it is an ongoing profession, only they are now called "pest control technicians. Rat infestations still exist in year 2016. The rodents multiply at a rapid rate: two rats can become 1,000 in the short space of a year http://www.dailymail.co.uk/news/article-513491/The-Rat-Catcher-Meet-man-mission-rid-cat-size-beasts-outnumber-us.html.