An American Airlines flight attendant was bitten by an emotional support dog on a flight from Dallas to North Carolina Monday, prompting union calls to further tighten rules on in-flight animals.

The incident occurred on American flight 3506 from Dallas-Fort Worth to Greensboro, North Carolina, an American Eagleflight operated by Envoy Air, a subsidiary of American. The plane was an Embraer 175, a regional jet.

The unnamed male flight attendant required five stitches on his left hand when he returned to Dallas, American spokesman Ross Feinstein said. The type of dog was not disclosed.

"What happened on yesterday’s American Airlines flight is completely unacceptable and inexcusable,'' the Association of Flight Attendants-CWA said in a statement late Tuesday.

The union, which represents Envoy flight attendants, said it has long supported the use of trained service animals to help passengers but said the industry needs standards for the less-defined category of emotional support animals. Critics have complained passengers are able to get instant certification for an emotional support animal, also called a comfort animal, online, and that many aren't properly trained.

"We need the (U.S.) Department of Transportation to take action now, so events like the one that happened yesterday do not continue to occur on our planes,'' the statement said. "This is fundamentally about maintaining safety, health and security for passengers and crew, while ensuring accessibility for those who need it.”

The DOT sought comments from travelers in 2018 and was flooded with replies, but so far no action has been taken.

Airlines have been tightening their policies in the wake of an increasing number of incidents involving in-flight animals and an overall increase in the number of pets brought on board. Passengers have tried to bring on a range of emotional support animals, including a peacock and a squirrel. American most recently revised its policies in March, limiting emotional support animals to cats and dogs and requiring a veterinary health form with vaccination details, its second revision in as many years.

In 2018, a child was bitten by an emotional support dog during boarding on a Southwest Airlines flight in Phoenix. In 2017, a 5-year-old girl was allegedly mauled by a pit bull in the gate area of an Alaska Airlines flight. The family sued the airline earlier this year. Also in 2017, an Alabama man was attacked by a veteran’s emotional support animal on a Delta Air Lines flight from Atlanta to San Diego. He sued the airline and the veteran earlier this year.