A dog is seen on the lap of its owner in a plane in Chiba, Japan on January 27, 2017.

United Airlines will no longer accept emotional-support kittens and puppies under four months of age and will ban emotional-support animals outright on long flights as it tries to crack down on soiled cabins and biting incidents that have harmed passengers and crew.

The Chicago-based airline joins rival Delta Air Lines in tightening its rules. Delta took the lead in banning young puppies and kittens last month.

Airlines have been tightening restrictions after a surge in emotional-support animals on flights. Passengers in recent years have brought animals other than dogs and cats — including a turkey, a pig and a duck — on board as emotional-support animals. United last year denied boarding to a passenger traveling with a peacock as an emotional-support animal.

Support and service animals fly free of charge and without a carrier under the 1986 Air Carrier Access Act. But passengers and crew members have complained about animal allergies, dirtied cabins and aggression from the animals.