Delta Air Lines’ pet policy has some new, stricter rules. The airline announced the new rules for service and support animals, by requiring advanced documentation requirements for animals boarding their planes.

“This comes as a result of a lack of regulation that has led to serious safety risks involving untrained animals in flight,” Delta said in a statement. “The new requirements support Delta’s top priority of ensuring safety for its customers, employees, and trained service and support animals, while supporting the rights of customers with legitimate needs, such as disabled veterans, to travel with trained animals.”

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Delta‘s new rules will require customers traveling with a service or support animal show a letter prepared and signed by a doctor or licensed mental health professional. Those with psychiatric service animals and emotional support animals will also need to provide a signed document confirming that their animal can behave — to prevent untrained, sometimes aggressive household pets from traveling without a kennel in the cabin. The traveler will also need to show proof of health or vaccinations 48 hours in advance of the flight.

“These measures are intended to help ensure that those customers traveling with a trained service or support animal will no longer be at risk of untrained pets attacking their working animal, as has previously been reported,” Delta said.

The measures were established with the help of the airlines’ Advisory Board on Disability, a 15-member board of disability advocates.

The airline said they carry around 700 service and support animals each day, or 250,000 each year. Customers have attempted to bring some interesting support animals on their planes, including comfort turkeys, possums, snakes, spiders, and more. Delta will no longer accept exotic or unusual service or support animals.

“Ignoring the true intent of existing rules governing the transport of service and support animals can be a disservice to customers who have real and documented needs,” Delta explained. “Delta has seen an 84 percent increase in reported animal incidents since 2016, including urination/defecation, biting, and even a widely reported attack by a 70-pound dog.”

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Delta employees also reported increased acts of aggression (barking, growling, lunging and biting) from service and support animals, which is not typical of service animals who have been properly trained.

Delta is creating a Service Animal Support Desk for customers traveling with service and support animals. You can find more information about the new Service & Support Animal rules here.