By Alice Popovici

NEW YORK (Reuters) - A New York pilot program to root out rats by attacking them in the colonies they set up in parks, subways and sewers has proven so successful that the city is expanding it to all five boroughs, a city official said on Thursday.

The city will increase funding for the program in its proposed new budget to $2.9 million from $400,000 and staff will grow from nine to 50, including a sanitation staff, exterminators and a rodent biologist.

“Depending on the initial level of severity, we’ve been able to have very significant effects,” Daniel Kass, deputy commissioner for environmental health, said of the department’s efforts to eradicate rats. “The biggest challenge is always managing the sources of food for rats.”

Beginning in October, the pilot project identified and studied “rat reservoirs” in such neighborhoods as the East Village and East Harlem in Manhattan, and Bronx’s Grand Concourse. Exterminators set out bait for the rats, closed up burrows and worked with the neighboring community on best practices to avoid attracting them in the future.

The city has no official estimate on rat numbers, but a Columbia University doctoral student last year estimated the population at about 2 million. That is far fewer than traditional estimates of 8 million, or one rat for every human in the nation's most populous city.

The study by statistician Jonathan Auerbach was based on public complaints about rat sightings, which the city tracks and publishes online.

“There’s no really valid way to try to count the amount of rats,” Kass said. One way to determine if the population is decreasing is to look at rat activity in the neighborhoods that participated in the pilot project, he said.

Based on staff reports, Kass said the program led to an 80 to 90 percent reduction in rat sightings in the seven neighborhoods involved in the initiative.

"They’ll always be there,” Kass said. “The question is how small a population you can create.”

(Editing by Frank McGurty and Eric Walsh)