NEW YORK • San Francisco's airport deploys a pig to calm frazzled travellers. Universities across the United States have dogs and one has a donkey on campus to soothe students during finals.

Llamas comfort hospital patients, pooches provide succour at disaster sites and horses treat sex addiction.

The trend, which has accelerated since its stirrings a few decades ago, is underpinned by a widespread belief that interaction with animals can reduce distress.

Certainly, the groups offering pets think this, as do some mental health professionals.

But the popular embrace of pets as furry therapists is kindling growing discomfort among some researchers who said it has raced far ahead of scientific evidence.

Earlier this year in the Journal Of Applied Developmental Science, an introduction to a series of articles on "animal-assisted intervention" said research into its efficacy "remains in its infancy".

A recent literature review by Ms Molly Crossman, a Yale University doctoral candidate, cited a "murky body of evidence" that sometimes has shown positive short-term effects, often found no effect and occasionally identified higher rates of distress.

Overall, animals seem to be helpful in a "small-to-medium" way, she wrote, but it is unclear whether the critters deserve the credit or something else is at play.

Mr James Serpell, a director at the University of Pennsylvania School of Veterinary Medicine, said: "It's a field that has been sort of carried forward by the convictions of practitioners... That kind of thing has almost driven the field and the research is playing catch-up. In other words, people are recognising that anecdote isn't enough."

Using animals in mental health settings is nothing new.

In the 17th century, a Quaker-run retreat in England encouraged mentally ill patients to interact with animals on its grounds. Neurologist Sigmund Freud often included one of his dogs in psychoanalysis sessions.

Yet the subject did not become a research target until American child psychologist Boris Levinson began writing in the 1960s about the positive effect his dog had on patients.

But the evidence to date is problematic, according to Ms Crossman's review and others before it.

Most studies had small sample sizes, she wrote, and an "alarming number" did not control for other possible reasons for a changed stress level, such as interaction with the animal's human handler.

Studies also tend to generalise across animals, she noted.

If participants are soothed by a golden retriever, it does not mean another dog - or another species - will evoke the same response.

Even so, media headlines are often about the happiness bounce.

To many animal lovers and pet owners, the back-and-forth might sound horribly wonky. There is something intuitive about the good feelings animals give people. Why over-analyse it?

Mr Alan Beck does not disagree.

Mr Beck, who directs the Centre for the Human-Animal Bond at Purdue University, Indiana, cites one common theory for why animals might be therapeutic.

He said: "Throughout history, animals gave us some comfort. So if it works for you and me in a relatively normal environment, maybe it has a special role for someone who has a depression and stress disorder."

Focusing too much on scientific support sometimes feels like a form of "physics envy", he added, "where you try to quantify everything without appreciating it".

But there are good reasons for rigorous research on animals and mental health.

In 2012, the US Department of Veterans Affairs said it would not cover costs of service dogs for veterans with post-traumatic stress disorder, citing "a lack of evidence" to support efficacy. It is in the midst of a multi-year study on the topic, which could lead to government funding for these pooches.

Another reason, the scientists said, is for the animals' sake.

Ms Crossman pointed to a 2014 incident at Washington University as an example of animal therapy gone wrong. A bear cub brought to campus during finals week nipped some students, causing a rabies scare that almost ended with the animal being euthanised.

WASHINGTON POST