Alaska Airlines has updated its policy on emotional support animals following an increase in incidents involving them on planes and in airports, including some customers and employees suffering bites.

Starting May 1, customers flying with psychiatric service animals must provide animal health and behavioral documents and a signed document from a medical professional at least 48 hours prior to departure.

The airline also will stop allowing amphibians, goats and animals with hooves, tusks or horns.

Alaska Airlines spokesman Tim Thompson said there will be an exception for trained miniature horses. Miniature horses are recognized as service animals under the Americans with Disabilities Act and are used by a small number of people with disabilities, said airline spokeswoman Bobbie Egan in an email.

The changes are in response to increasing problems with emotional support animals on planes, Thompson said. There have been incidents where animals have bitten customers and employees, Thompson said.

"Most animals cause no problems," said Ray Prentice, Alaska Airlines director of customer advocacy. "However, over the last few years, we have observed a steady increase in incidents from animals who haven't been adequately trained to behave in a busy airport setting or on a plane, which has prompted us to strengthen our policy."

Alaska Airlines in recent years has handled about 150 support animals every day.

The new policy does not affect certified service animals, which are typically dogs helping owners with physical disabilities. The new restrictions only apply to animals assisting emotional, psychiatric, cognitive or psychological disabilities.