Gerry DeRoche, chief executive of the National Education for Assistance Dog Services, said fraudulent service or support animals could displace legitimate ones because most airlines limit the number allowed in a cabin.
Jeffrey N. Younggren, a clinical professor at the department of psychiatry and behavioral sciences at the University of New Mexico, said studies about the benefits of emotional support animals were "spotty and inconsistent."
"Before we start loading up airplanes with emotional support animals, we need the research," he said.
Official-looking paperwork is available online to make pets look legitimate: Owners answer questions about their need for a support animal, and a doctor issues an assessment without ever evaluating the client, Mr. Younggren said.
"The whole thing is a mess," he said, adding that such websites have become a "growth industry" over the last five years.
David Favre, a law professor at Michigan State University and editor in chief of its Animal Legal and Historical Center, said fraudulent cases eroded trust about service animals.
"There are many thoughtless, ignorant or arrogant people out there who only think of themselves," he said. "Abuse is everywhere."
Even for trained animals, maneuvering through crowds or traveling in confined places like planes can be stressful, but they are conditioned not to act out. Untrained animals in those circumstances are prone to misbehave by growling, biting or having accidents.
Chris Diefenthaler, operations administrator at Assistance Dogs International, said one of the worst outcomes could be when a pet posing as a service dog attacks a legitimate one, leaving it so traumatized or injured it has to be retired or put down.
"There are no standards for evaluating the need for an emotional support animal, whereas there are concrete rules to determine if someone is eligible for a service animal," Cassie Boness, a graduate student in clinical psychology in the department of psychological sciences at the University of Missouri, said in a post on the university's website.
"But emotional support animals can be certified through an online process, and they can be someone's pet," she continued. "The growing use of emotional support animals tends to discredit the use of service animals, which is where much of the tension comes from since people do not understand the difference."
Also, people can shop online for vests, patches or harnesses that identify their pets as service animals, leading to peculiar situations.
For instance, Ms. Giovinazzo, who flies frequently, said airline workers sometimes ask for identification for Watson. A detailed one issued by his guide school will draw scrutiny, while one that reads "TSA approved" that she bought from Amazon "looks more official," she said.
Cathy Zemaitis, director of development for National Education for Assistance Dog Services, shared a photo taken at Los Angeles International Airport of a dog wearing a vest labeled "service animal," a muzzle and a diaper.
"A true service dog would never be muzzled nor would they be in a diaper," she said.