Airlines no longer will be required to accommodate travelers who want to fly with emotional support animals such as pigs, rabbits and turkeys under a final rule announced Wednesday by the U.S. Department of Transportation.

The new rule now defines a service animal to be a dog that is “individually trained to do work or perform tasks for the benefit of a person with a disability” and limits the number of service animals a person can travel with to two. It also requires airlines to treat psychiatric service animals as they would other service animals.

Emotional support animals aren't considered service animals under the new rule, which drew more than 15,000 public comments before it was finalized. Although the rule does not bar them from traveling in passenger cabins, airlines will not be required to accommodate them.

“The final rule announced today addresses concerns raised by individuals with disabilities, airlines, flight attendants, airports, other aviation transportation stakeholders, and other members of the public, regarding service animals on aircraft,” DOT officials said in a statement.

The rule is set to go into effect 30 days after it is published in the Federal Register.

Over the years, airlines have had to accommodate a growing variety of animals as the definition of what is considered a service animal expanded to include animals that travelers said they needed for emotional and psychological support when flying. That led to growth in the number and types of animals traveling in passenger cabins.

Industry trade group Airlines for America estimated the number of emotional support animals traveling aboard commercial flights increased from 481,000 in 2016 to 751,000 in 2017.

The growth led to conflict between passengers, particularly when animals misbehaved.

Last summer, an American Airlines flight attendant received five stitches after she was bitten by an emotional support dog on a flight out of Dallas/Fort Worth. DOT officials said they also have received reports of biting incidents in which children were severely scarred. Some incidents led to lawsuits.

Service animals are those that have been trained to perform a certain function, and under the Americans With Disabilities Act, reasonable accommodations must be made for a person using one. However, no training is required for emotional support animals, which has led some to question their legitimacy.

Before the rule change, federal law didn't address the issue of emotional support animals, so airlines had little recourse but to accommodate them.

The department's action drew praise from airline groups and employee unions.

“Airlines are committed to promoting accessibility for passengers with disabilities and ensuring their safe travel,” said Nicholas E. Calio, CEO of Airlines for America. “The Department of Transportation's final rule will protect the traveling public and airline crew members from untrained animals in the cabin, as well as improve air travel accessibility for passengers with disabilities that travel with trained service dogs.”

Sara Nelson, international president of the Association of Flight Attendants-CWA, called the new rule a “victory for the flying public and crew.”

“It sets clear definitions and guidance to ensure people with disabilities and our veterans have necessary service animal assistance while maintaining the safety, health and security of all passengers and crew onboard our planes,” Nelson said. “This final rule will ensure that untrained pets will never roam free in the aircraft cabin again.”

The Transportation Department first tried to address the issue in 2016 through its advisory committee on accessible air transportation, but the group was unable to resolve which species should be allowed. The issue has been contentious in part because some groups have argued that animals other than dogs should be consider for inclusion.

Transportation Department officials previously said they limited the definition of service animal to dogs because the vast majority of service animals are canines.

The new rules include provisions that may make some parts of the journey easier for those who travel with service animals.

Passengers will no longer be required to physically check in at the airport, and can check in online.

The rule prohibits airlines from refusing to transport a service animal solely based on its breed, but does give carriers the ability to refuse to transport animals that behave aggressively or are a “direct threat” to the health or safety of other people.

The rule also gives airlines the ability to require paperwork attesting to an animal’s health, behavior and training. If the animal will be traveling on a long flight, the person it is traveling with must verify “it will not relieve itself or is able to do so in a sanitary manner.” Paperwork must be provided within 48 hours in advance of the flight if the passenger's reservation was made before that time.

Service animals also must fit within their handler’s foot space on the aircraft and be harnessed, leashed or tethered in the airport and on the plane.