United Airlines is tightening its policy on emotional support animals on its flights.

Starting Monday, support animals are no longer allowed on flights longer than eight hours.

United says it’s making the change because of a rise in on-board animal-related incidents.

In addition, the airline will no longer allow kittens or puppies less than four months as support animals, in-cabin pets or service animals, saying animals that young typically haven’t had all the necessary vaccinations.

Delta and Spirit also recently changed their policies, now requiring at least 48 hours notice to bring service animals on flights.