The newest members of the no-fly list on Delta Airlines? Puppies and kittens acting as emotional support and service animals.

Furballs under the age of four months will no longer be allowed to fly as support and service companions, while all support animals will be banned on flights longer than eight hours, the airline announced on Tuesday. The ban does not affect pets traveling in enclosed carriers.

The new restrictions take effect on Dec. 18, just as millions of people are set to fly home for the holidays. Passengers who already purchased tickets and requested to fly with a support or service animal will have a grace period until Feb. 1, Delta said in a blog post. After that, they will be asked to adjust their reservations.

The change in policy comes after Delta reported an 84 percent increase in incidents involving service animals during a two-year period ending in 2017, including an instance last year when a combat veteran’s 70-pound emotional support dog attacked another passenger on a flight. Delta has also reported problems with animals urinating, defecating and biting while in-flight.

John Laugher, Delta’s senior vice president of corporate safety, security and compliance said the updates are necessary for safety purposes and to “protect the rights of customers with documented needs — such as veterans with disabilities — to travel with trained service and support animals.”

Delta’s restricted emotional support policy comes as airlines have had to crack down over the past year after passengers have tried to board with a menagerie of emotional support animals, including snakes, squirrels and a peacock. Most airlines allow passengers to fly with an emotional support animal if they present a doctor’s note stating their need; however, many websites also claim to quickly offer emotional support animal letters for a fee.

Many of the U.S. airlines now have policies clearly stating which types of animals are restricted. Among them are hedgehogs, goats, snakes and animals with tusks — which could be interpreted to mean that all emotional support walruses are best left at home, no matter how young they are.