Emotional-support animals are on the rise on planes and at airports. From 2016 to 2017, the number of emotional-support animals traveling onboard commercial flights increased by 74%.

Airlines and passengers have complained that the vague rules covering emotional-support animals have allowed people to cheat the system and pass off untrained family pets as support animals, to the detriment of actual service animals.

Airlines for America, a group that represents several major US airlines, wants the US government to tighten its definition of service animals to exclude emotional-support animals.

Emotional-support animals are becoming an increasingly common sight at airports and on passenger planes in the United States.

According to Airlines For America — a trade group that represents major US airlines including American, United, JetBlue, Southwest, and Alaska — the number of emotional-support animals, or ESAs, traveling aboard commercial flights jumped 74%, from 481,000 in 2016 to 751,000 in 2017.

But for airlines and many of its customers, ESAs are becoming increasingly problematic. This is especially true in instances where unscrupulous passengers use fraudulent documentation to game the system, thereby allowing their pets to fly for free.

Airlines say that many of these untrained pets posing as support animals are a threat to passengers, crew, and even properly trained service dogs.

Reported incidents have ranged from mauled passengers to defecating pigs. One United Airlines passenger even tried to bring an emotional-support peacock on a flight.

Now, Airlines For America wants the US Department of Transportation to tighten the definition of service animals to "include only dogs that are individually trained to do work or perform tasks for the benefit of an individual with a disability," the organization said in a statement to Business Insider.

While this definition would include psychiatric service dogs, ESAs would no longer be covered. As a result, should this definition be adopted, airlines would in theory no longer have to accept emotional-support animals.

In addition, the DOT has proposed updated rules of its own that will narrow the definition of service animal down to just dogs, cats, and miniature horses. At the same time, this would allow airlines to turn away other service animals such as snakes, other reptiles, ferrets, rodents, and spiders.

Further, the DOT has given airlines the task of weeding out the impostors from those who really need the assistance of a service animal. According to a summary attached to the DOT's "Interim statement of enforcement priorities," airlines will be allowed to question passengers whose disability is not clearly visible about the nature of their disability and how the service animal is of assistance.

The US government is also looking to codify regulations limiting the number of support animals to one, requiring passengers with service or support animals to first stop at the check-in desk, and strengthening requirements on medical documentation.

So far this year, American, Delta, JetBlue, Alaska, and United have all already enacted stricter policies for both service and emotional-support animals, incorporating many of the new rules proposed by the DOT.

These policies have been met with positive responses from the Association of Flight Attendants-CWA union as well as the American Veterinary Medical Association.

Mental-health and disability-rights activists have pushed back on these new regulations, calling them discriminatory.

In a letter submitted to the DOT, the National Disability Rights Network expressed concern that new rules giving airlines discretion would single out people with mental health conditions.

The DOT closed the issue for additional comments on June 7, 2018 and is currently still working through the possible regulatory changes.