Citing a substantial increase in the number of passengers bringing emotional support animals on its flights, as well as an increase in incidents, United Airlines announced Thursday it’s tightening the rules on bringing such animals on planes.

Starting March 1st, United will require passengers with emotional support animals to provide evidence the animal has “been trained to behave properly in a public setting and acknowledge responsibility for the animal’s behavior,” as well as proof of vaccination from a veterinarian. This is in addition to providing 48 hours’ notice to the airline and an enhanced letter for the passenger from a medical professional. There are no changes in United’s policy for service animals.

“Year-over-year, we have seen a 75 percent increase in customers bringing emotional support animals onboard and as a result have experienced a significant increase in onboard incidents involving these animals,” the airline said in a statement. “The Department of Transportation’s rules regarding emotional support animals are not working as they were intended to, prompting us to change our approach in order to ensure a safe and pleasant travel experience for all of our customers.”

United’s move follows Delta’s announcement in January that it would require passengers with support animals to submit an animal request form and confirmation of animal training form 48 hours before departure. Delta cited an 84 percent increase in passengers traveling with support animals from 2016 to 2017, including an incident of a passenger being bit by a dog on a flight. And on Saturday, a woman was denied boarding at Newark International Airport on a United flight when she tried to bring her emotional support peacock on the plane.

Without stricter guidance from federal regulators, more airlines are likely to take more steps to prevent incidents with animals on flights.