Southwest Airlines announces new rules for emotional support animals The new rules will take effect September 17.

Southwest Airlines announced Tuesday that it will soon enforce new restrictions on customers traveling with emotional support animals.

Beginning September 17, only one dog or cat, either in a carrier or on a leash, will be allowed per customer on Southwest flights.

"Our updates are based on a careful review of the Department of Transportation’s (DOT) recent enforcement guidance and feedback we received from our Customers, Employees, and several advocacy groups and animal-related organizations," the Dallas-based airline said on its community discussion forum, southwestaircommunity.com.

Southwest’s policy says emotional support animals provide "support for an individual with a mental health-related disability and is not trained to perform a specific task(s) or work." They are distinct from trained service animals, which Southwest defines as animals "individually trained to perform a task(s) or work for a person with a physical and/or mental disability."

Customers who wish to travel with their emotional support animals on Southwest, once the changes are in effect, must present the airline with a "current letter" from their doctor or medical health professional on the day of their departure, the announcement said.

All emotional support and service animals are still required to be trained to behave in public, according to Southwest. Any animal that displays disruptive behavior can be denied boarding.

The change will make Southwest the only airline that limits emotional support animals to just dogs and cats, but it is not the first major carrier to adjust its emotional support animal rules.

Delta, United, American and JetBlue separately announced new restrictions earlier this year following a string of emotional support animal incidents, including a dog reportedly attacking a Delta passenger last year, and a woman who was denied boarding in January because her emotional support peacock failed to meet United’s guidelines.

Airlines are now not the only companies tightening their emotional support animal rules. Royal Caribbean also announced Tuesday that emotional support animals "may not sail onboard Royal Caribbean International ships," effective immediately.