U.S. airlines want to make it harder for people to bring "passive emotional support" animals on their aircrafts, the Wall Street Journal reports.

Between the lines: American Airlines said in a letter to the Department of Transportation that regulations for flying with animals "are unnecessarily broad and easily abused. People have been bitten, licked, jumped on and growled at; aisles have been blocked, and animals have urinated and defecated on our planes," per the WSJ.

Airlines typically charge passengers $125 or more to fly with a pet, the WSJ reports, while service animals and animals meant for emotional support fly for free.

• Yes, but: Some emotional support animals are deemed so after passengers get certification "from overly permissive online agencies or dress them up in official-looking vests to obtain free passage."

What's happening:

• Trade groups and member airlines are requesting that the Department of Transportation "bring their guidelines in line with the Americans with Disabilities Act," per the WSJ, to more strictly define what service animals are.

Between the lines: The number of emotional support animals jumped from 481,000 to 751,000 from 2016 to 2017, trade group Airlines for America reports.

• In January this year, United denied flight access to an "emotional-support peacock" that a woman wanted to bring aboard.
• In 2014, a female passenger brought an emotional-support pig on her U.S. Airways flight.
• Daniel, an emotional-support duck, waddled onto a flight in 2016 with his owner who has PTSD.
• Spirit Airlines told a student she couldn't bring her emotional-support dwarf hamster on the flight; she ended up flushing it down the toilet.
• American Airlines banned specific emotional support animals, per the Chicago Tribune: "[A]mphibians, goats, hedgehogs, insects, nonhousehold birds and animals with tusks, horns or hooves."

What they're saying:

• Some disability groups complain that if airlines don't recognize support and service animals — including animals other than just dogs — than people "may have to find another way to travel."
• But the American Psychological Association said in a statement to the WSJ: "Emotional support animals are clearly different from service animals and as such should not be afforded the same status as service animals."