“We don’t know exactly why,” Mr. Demodice said. “I think it might be because there is an overpopulation underground because the usual habitat for this animal are the sewers, underground, not above ground.”

“Our work is to push them back down,” he said.

But why are they proliferating? Could it be everybody’s favorite scapegoat — the European Union and its faceless, unaccountable bureaucrats?

Yes, it could.

New regulations from Brussels, the European Union’s headquarters, have forced countries to change how they use rat poison, said Dr. Jean-Michel Michaux, a veterinarian and head of the Urban Animals Scientific and Technical Institute in Paris.

The old way of poisoning rodents involved a sort of deadly snack service in which park employees put lethal pellets directly into the burrows where the rats lived or sprinkled a poison powder along the underground byways used by the rats.

The poison would cling to the rat’s fur “and then when the rat cleaned itself, licked itself, it absorbed the powder and was automatically poisoned,” Dr. Michaux said.

Either method meant the rat was likely to come in direct contact with the deadly substance. Typically, the rat died two or three days later. The poison is an anticoagulant that eventually causes dehydration, internal bleeding and death.