Hakan Kjellberg of the commercial pest control firm Anticimex said Stockholm's rats also benefit from a scattershot approach to the infestation, The Local reported.

"There has been a lack of comprehensiveness about Stockholm 's attempt to control rats," says Kjellberg. "Yet, rat control is one situation that really demands a coherent and continuous plan. After all, the rat population is amazingly adaptable and certainly doesn't respect neighborhood borders."

Since rats tend to stay out of sight, no one is sure if Stockholm has more rats than other European cities. Some people who have seen them claim they are bigger.

But The Local said the rats tend to be more visible in areas frequented by tourists, including downtown subway stations and parks bordering the island city's harbor.