Pigs can be therapy animals too. So can horses and rats and cats and llamas and …

Shrieks of laughter echoed off the walls of the hospital as Thunder the mini pig flopped onto his side and the children huddled around him, scratching his pink, hairy belly.

He and his wet-nosed partner, Bolt, drew patients in wheelchairs and bandages, and even a few doctors in white coats and scrubs to the Rose Garden Christmas tree area at Tampa General Hospital for a few minutes of cheer Friday.

The 10-month-old miniature pigs are some of the many therapy animals spreading smiles at Tampa Bay area hospitals this holiday season.

The day before, Spirit the Unicorn, a white miniature horse that wears a horn, his own set of tennis shoes and a diaper, went room to room in the halls of Tampa General.

It started with dogs, which have been used for therapy in a variety of ways for years. Labradors and golden retrievers undergo rigorous training to be service dogs to aid people with disabilities. In addition, dogs can serve as emotional therapy animals, helping victims through trauma, or therapy dogs at hospitals, nursing homes and at schools. But the interest and popularity of therapy animals has grown to include horses, pigs, rats, cats, guinea pigs and even llamas.

"Our pigs are therapy animals, not service animals, which means they can visit places like hospitals or schools for a friendly visit," explained Heather Barrow, who along with her 10-year-old daughter, Claire, travels with the pigs across the area for therapy trips.

"I was in the hospital for a month when my son was born premature," Barrow said. "Claire remembered the therapy dogs that came every week while we were there, which really brightened her day. So when she told us she wanted pigs, she said she also wanted them to brighten others' days, if they could."

There's no one agency that certifies animals for therapy, said Elisabeth Van Every, a spokeswoman for Pet Partners, which registers and screens nine types of animals that qualify as therapy pets. Over the past decade, Pet Partners' national database of registered therapy animals has more than doubled, she said.

"I think there's been more attention generally to it and some research, but most of it is just popular awareness," Van Every said. "Animals have a beneficial effect on human health, and that's the reason you're seeing the rise."

Van Every cited some medical research, from places like the Human Animal Bond Research Institute and by a Dr. Aubrey Fine, which shows that our furry friends make us happy.

"There's no overarching body that governs therapy animals or sets standards and criteria," she said.

But Pet Partners, she added, works within infection protocols required at hospitals and screens therapy animals to make sure they actually enjoy their work.

"We provide education for the handler, screen the human and animal team to make sure they're prepared," she said. "We would love to see some consistency."

At Courteous Canine, a dog-training facility in Lutz, the eight-week therapy dog training course has surged in popularity recently, said operations manager and trainer Marie Macher.

"We've definitely seen an increase in interest of wanting to get their dogs trained to be therapy dogs," she said. "A lot of the people we see come through the course are already working in those fields. They're speech pathologists or work in social services, and they have a dog at home that has the potential to help them with their career."

Brittany Beard takes her miniature horses dressed as unicorns to hospitals across Florida, including Johns Hopkins All Children's and Tampa General. She says the animals are trained with the needs of patients in mind.

"When we're training a new horse, we take them to a lot of pet-friendly public places, like the Home Depot, Lowe's, Tractor Supply, Petco," said Beard, whose ranch is in Clermont. "We have some local nursing homes that let us train the horses on how to ride elevators and turn in tight corners, too."

Beard said she also potty-trained her horses so they know to go in the trailer before entering the hospital -- though they do wear a diaper, just in case. They also wear rubber-soled tennis shoes so they don't slip on slick floors.

Aside from visiting hospitals and nursing homes, Beard said she works with Make-A-Wish and Give Kids The World. Many kids ask to meet a unicorn, she says.

"One girl in January wants to meet a pink unicorn," she said, "so Spirit is getting dyed pink."

Contact Justine Griffin at jgriffin@tampabay.com or (727) 893-8467. Follow @SunBizGriffin.