The days of passengers bringing their pets on airplanes as emotional-support animals could be ending.

The U.S. Department of Transportation on Wednesday proposed that only specially trained dogs qualify as service animals, which must be allowed in the cabin at no charge.

Airlines could ban emotional-support animals including untrained dogs, cats and more exotic companions such as pigs, pheasants, rabbits and snakes.

Airlines say the number of support animals has grown dramatically in recent years. They lobbied the Transportation Department to crack down on what they consider a scam — passengers who call their pets emotional-support animals to avoid pet fees that generally run more than $100 each way.

“This is a wonderful step in the right direction for people like myself who are dependent on and reliant on legitimate service animals," said Albert Rizzi, founder of My Blind Spot, an advocacy group for people with disabilities. He said some people “want to have the benefits of having a disability without actually losing the use of their limbs or senses just so they can take their pet with them.”

The main trade group for large U.S. airlines praised the proposal. Nicholas Calio, president of Airlines for America, said, “The proposed rule will go a long way in ensuring a safer and healthier experience for everyone.”

Flight attendants had pushed to rein in support animals, and they too were pleased.

"The days of Noah’s Ark in the air are hopefully coming to an end,” said Sara Nelson, president of the Association of Flight Attendants. She said some of her union's members were hurt by untrained pets.

Veterans groups also sided with the airlines, arguing that a boom in untrained dogs and other animals threatens their ability to fly with properly trained service dogs. Last year, more than 80 veterans and disability groups endorsed banning untrained emotional-support animals in airline cabins.

On the other side are people who say that an emotional-support animal helps them with anxiety or other issues that would prevent them from traveling or make it more stressful. They aren't a very organized group, but there are lots of them.

Southwest Airlines handles more than 190,000 emotional support animals per year. American Airlines carried 155,790 emotional support animals in 2017, up 48% from 2016, while the number of checked pets dropped 17%. United Airlines carried 76,000 comfort animals in 2017.

Transportation Department officials said in a briefing with reporters that they proposed the changes to improve safety on flights. Some passengers have been bitten by support animals, and airlines complain that they relieve themselves on planes and in airports.

The public will have 60 days to comment on the proposed changes. Officials highlighted a few areas where they are most eager to get comments, including whether miniatures horses should continue to qualify as service animals.

The Transportation Department proposes a narrow definition in which a service animal could only be a dog that is trained to help a person with a physical or other disability. Passengers with a service dog would have to fill out a federal form on which they swear that the dog is trained to help them. A dog trained to help with psychiatric needs would qualify as a service animal.

Current rules do not require any training for emotional-support animals. However, airlines can demand that the animal's owner show them a medical professional’s note saying they need the animal for support.

The proposed rules would prohibit airlines from banning particular types of dog breeds if the animal qualifies as a service dog, although they could refuse to board an individual dog they deem a threat. Delta Air Lines, which bans pit bulls, said it is studying the proposal.

The president of the Humane Society of the United States said airlines like Delta had maligned pit bulls. Kitty Block said the Transportation Department’s proposal to prohibit breed-specific bans “sends a clear message to airlines that their discriminatory practices are not only unsound, but against the law.”

The new rules would also bar the current practice by many airlines of requiring animal owners to fill out paperwork 48 hours in advance. A department official said that practice can harm disabled people by preventing them from bringing their service dog on last-minute trips.

The proposal also says people with service animals must check in earlier than the general public.

Airlines could require that service animals be on a leash or harness and fit in its handler’s foot space. They could limit passengers to two service animals each, although it is unclear how often that happens under the current rules.

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AP Writer Cathy Bussewitz contributed to this report.