Based on individual trends within cities, Washington was by far the leader in rat increases, followed by San Francisco, Toronto, New York City and Amsterdam. Washington's rising rat reporting trend was three times greater than Boston's and 50% more than New York's, the study said. Washington city officials did not respond to requests for comment.

Large rat populations can have a damaging effect on cities. Researchers found that living with rats " impacts people and their psyche."

Global warming is helping rats thrive in major cities around the world, with Washington DC seeing the fastest growth in rats