If you’ve lost a pet, chances are you’ve heard of the Rainbow Bridge.

This bridge is a mythical overpass said to connect heaven and Earth — and, more to the point, a spot where grieving pet owners reunite for good with their departed furry friends. It is a poem, origins disputed, that launched the pet bereavement movement, inspired countless pet loss blogs and fueled a lucrative marketplace for rainbow bridge-themed dog urns and lava bead bracelets.

It is, in free verse form, “Chicken Soup for the Soul” for an exploding $69 billion pet care industry.

The poem has been passed between animal lovers since the 1980s, but its origins are fuzzy. Although it is usually attributed to an unknown author, at least three men claim to have written it — one, Paul C. Dahm, even holds a copyright for a variation. All three wrote similar books in the 1990s on pet loss after claiming to have penned “The Rainbow Bridge.”

“I wrote what I then called ‘Pet Heaven’ and forgot about it until 10 years later, when I lost my own dog and heard about ‘Rainbow Bridge,’ ” said one claimant, Wallace Sife, who recalls writing his version for a California friend’s dog club newsletter. “It was a modified version of what I had written.”

Sife is the founder of the Association for Pet Loss and Bereavement, a volunteer counseling chat room and online resource. His 1993 book, “The Loss of a Pet,” is now in its in fourth edition and is considered a go-to source in the pet grief field.

Whatever its origins, the poem and books that followed validated a demographic in need of support and helped create a new social construct — that pet deaths are devastating for people. Before, Sife said, this was a disenfranchised sorrow, one neither fully acknowledged nor socially acceptable. As that changed, the multi-stanza schmaltz of “The Rainbow Bridge” only spread.

The poem — which assures that you and your deceased pet will eventually “cling together in joyous reunion, never to be parted again” — is now the condolence standard for veterinary practices. Colin Dwyer, a New York City-based veterinarian, laughed when asked about it. But he conceded that he and his staff regularly refer to this mythical crossing and sometimes even offer a copy of the poem to heartbroken pet owners.

It offers “an easy way to have something to say,” Dwyer said. “I don’t think it’s any better or worse than saying, ‘Oh, she’s in doggy heaven.’”

Even professionals have taken solace in the promise of the rainbow bridge. Veterinarian Julie Ann Luiz Adrian, an associate professor at the University of Hawaii, said it comforted her after her cat, Friday, died when Adrian was still a student.

“It talks about the kisses from them, their nose twitching,” said Adrian, referring to a line that appears in some versions. “Something in it resonates with people.”

And their sorrow is real, said Adrian, who would know. She published a study last year that concluded people follow the same trajectory of grief no matter who or what — human or animal — they are mourning. Almost 30 percent of pet owners reported prolonged grief lasting six months or longer, Adrian found, while 12 percent suffered severe grief that resulted in major life disruption. More than 5 percent suffered post-traumatic stress.

The level of grief was determined by how the owner perceived the animal, Adrian said.

“Was the animal truly part of the family?” she said. “For some of us, and especially for the newer generation who are replacing pets with kids, our pets are our children.”

So is “The Rainbow Bridge” a sort of grief anthem for millennials? In some ways, probably so. The trend of humanizing what are now often called “companion animals” has helped grow the pet industry a steady $2 billion to $3 billion a year since 2010, according to the American Pet Products Association. But spending jumped by $6 billion, to nearly $67 billion, between 2015 and 2016 — the same year the millennial generation overtook baby boomers as the primary pet-owning demographic.

Three in five millennials own pets, according to APPA’s most recent survey, compared to about 50 percent of the general population. APPA research also found millennials are almost twice as likely to purchase urns or memorial items for their pets than baby boomers or Gen Xers.

APPA does not track pet bereavement as an independent category, spokeswoman Tierra Bonaldi said. But anecdotally, the offerings for grieving owners are only getting more baroque.

“Pet owners can now select everything from personalized headstones and urns to nose-imprinted pendants,” Bonaldi said. “They can even turn their pets’ ashes into a diamond with memorial cremation jewelry.”

Corporate policy is catching up, with firms including Kimpton Hotels and Restaurants, Mars, Trupanion and Petco now offering pet bereavement leave.

Yet amid the advances in pet loss research, a widening dead pet remembrance market and changing company benefits, “The Rainbow Bridge” endures. In the waiting room at Roundtop Animal Hospital in Upstate New York on a recent Saturday, Cianna Fox, 25, said one need not be a millennial “pet parent” to relate to the poem.

“Honestly, it’s like, when you are in that situation and have to put a pet down, it’s nice to think there may be a happy ending,” she said. “And the poem makes you think of a deeper meaning to their death.”

Brooklyn photographer Winnie Au, 31, said she had never heard of the bridge until the death in 2013 of her corgi, Tartine. Friends gave her condolence cards — yes, plural — and several referred to the otherworldly dog park “just this side of heaven.”

“It seems really goofy, but when you are that upset, it’s a comforting thought,” said Au, now the owner of a basset hound, Clementine.

Sife said losing a pet forces owners to face their own mortality, no matter how they choose to grieve. Still, the rainbow bridge — in all its variations — “is what people want to hear,” he insists. “It’s what we call in the trade ‘warm fuzzies.’ ”

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