A month after a flight attendant was bitten by an emotional support animal, the U.S. Department of Transportation told the airline industry Thursday that carriers can't bar certain dog breeds because airlines deem them dangerous, handing a victory to pit bull fans.

But the federal agency gave airlines the green light to require passengers to produce records on vaccinations and training to determine if a specific animal poses a threat on a plane. And bans on certain species -- snakes, for instance -- will be allowed to stand.

The effort by the Department of Transportation to clarify its policy on animals in planes is the latest chapter in the long-running saga over emotional support animals. An increasing number of airplane passengers have been bringing animals, some quite exotic, contending that they were needed for emotional support during flights; airlines suspected the passengers were merely trying to save money.

To control the proliferation, Southwest, JetBlue and United, among others, last year began tightening restrictions on airborne animals, particularly the unusual species. Delta Air Lines' ban last year of all "pit bull-type dogs" as service animals or emotional support animals proved especially controversial.

An airline trade group and a flight attendants union voiced support for the federal agency's efforts to clarify its rules regarding animals. The Department of Transportation is expected to begin enforcing the guidelines later this year after they are published as part of a formal "notice of proposed rulemaking."

Sara Nelson, president of the Assn. of Flight Attendants-CWA, called the new guidelines "an important step to address what has become a mess of animals loose in the aircraft cabin."

Airlines for America, a trade group for the country's largest carriers, said that many fliers have been fraudulently passing off their pets as emotional support animals to avoid having to pay animal transport fees.

"With over a million passengers bringing [emotional support animals] on flights last year, airlines and airports saw a sharp increase in incidents such as biting and mauling by untrained animals," the group said. "The DOT's guidance is an important step toward addressing this growing problem and ensuring a safer and healthier travel experience for all."