In this April 1, 2017, file photo, a service dog named Orlando rests on the foot of its trainer, John Reddan, of Warwick, N.Y., while sitting inside a plane at Newark Liberty International Airport.

Delta Air Lines has revised its animal policy again, this time limiting emotional-support animals to one per customer and banning "pit bull type dogs" as either comfort or service animals.

The changes go into effect July 10. The airline said the updates resulted from safety concerns, after several workers were bitten. Delta said it carries about 700 service or comfort animals daily, or nearly 250,000 per year.

The revisions are the latest examples of the major airlines grappling with how to serve passengers who want to bring animals with them in the cabin. Delta had previously updated its animal policy in March, after a 70-pound dog flying as a comfort animal bit a passenger in the next seat on a June 2017 flight from Atlanta to San Diego.

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"The safety and security of Delta people and our customers is always our top priority," said Gil West, Delta's chief operating officer. "We will always review and enhance our policies and procedures to ensure that Delta remains a leader in safety."

Each of the major airlines has updated animal policies in recent months, after the Transportation Department was unable to reach a consensus on emotional-support animals between travelers, advocacy groups for the disabled and airlines.

The department is collecting comment about potential changes in animal regulations, after failing to reach a compromise between passengers, advocacy groups for the disabled and airlines. Nearly 3,000 comments have already poured in, with a deadline of July 15.

Airlines basically have at least four ways for passengers to bring animals on flights. But uncertainty about standards for the different categories has led to conflicts between passengers and airlines.

Most airlines allow passengers to ship animals in crates with luggage. But occasionally pets die under those circumstances, and high-profile incidents raised concerns. The department counted 506,994 animals transported in cargo last year, including 24 that died, 15 that were injured and one that was lost.

Airlines generally allow passengers to bring smaller pets in containers in the cabin, so long as they fit beneath the seat. But that option costs $75 to $125, depending on the airline.

The dispute over regulations and airline policies is about what animals can travel for free with passengers in the cabin without cages.

The Americans with Disabilities Act recognized dogs and miniature horses as trained service animals. The Air Carrier Access Act then said service animals could fly in the cabin with passengers, while also opening the door to broader range of emotional-support animals, which assist passengers with mental-health issues.

Airlines typically require travelers to have a note from a medical provider describing the need for an emotional-support animal and documentation of the animal's health. But some crew members and passengers suspect that travelers bring pets as comfort animals to avoid fees.

In recent years, the variety of emotional-support animals exploded to include monkeys, pigs and ducks as emotional-support animals, although airlines didn't have to accept reptiles, ferrets or rodents. These comfort animals, which didn't require specified training, sometimes upset other passengers.

As the variety of animals on flights multiplied, so did complaints. The department received 2,443 complaints from travelers with service animals on U.S. and foreign airlines in 2016 and another 2,499 last year.