Dani Moore of Hesperia, Calif., owes much of what she can do in her life to a rat.
The rat, named Hiyo Silver, has the unique ability to feel when the 56-year-old Moore's body is just starting to shake because of muscle spasms. Because she suffered injuries to her spinal nerves, she can't feel those spasms until they become extremely bad. By then, it's sometimes too late to avoid a serious injury.
"Since I have osteporosis, if the spasms get too bad, they can fracture vertebrae, which has happened to me before."
When Hiyo licks her neck or face, Moore knows it's time to take action either by stretching her muscles or taking medication to stop the spasms.
She keeps Hiyo on a leash atop her shoulder wherever she goes because she never knows when she'll get spasms.
"Before I got my service rat, I would sometimes spend weeks in bed because the spasms would not let up. I was so much more limited to where I could go or what I could do," Moore said.
Rainette Murphy Lopez trains service dogs to respond to various human illnesses and is also a friend of Moore's. As a former New York City resident who saw her fair share of sewer rats, she was skeptical of Moore's claims about her rodents. But over time, she witnessed just how right Moore was.
"I saw how they would come up and they would lick her cheek or reposition themselves -- sort of roll up into her neck -- when she started having spasms."
Despite the freedom she's able to enjoy now, she wasn't always able to take Hiyo or her other rats with her anywhere. The Americans With Disabilities Act only recognizes dogs and miniature horses as service animals, meaning that businesses are only required to allow these animals onto premises.
But back in March, Moore's home city of Hesperia voted to allow all species of service animals into local businesses provided they behave appropriately.
Hiyo isn't the only animal believed to be able to sense a human's physical ailments. Studies have shown that dogs may be able to sense when their diabetic owners are having trouble with their blood sugar. Other research has shown that dogs can sniff out cancer.
Some animals have made headlines with their uncanny abilities. A dog in Michigan bit off his diabetic owner's infected toe, and a visit to the hospital led to a diagnosis of type II diabetes. A cat living in a Rhode Island nursing home seemed to be able to predict when patients would die just by sniffing them.
Experts say what animals sense is most likely a smell or some type of body language that alerts them that something is amiss.
"It can be as minute as a slight change in respiration, or the person just moves at a different pace, or he has a different look, or a smell -- it can be any of those things," said Dr. Marty Becker, a veterinarian in Idaho and author of "The Healing Power of Pets."
Despite the reputation of their sewer rat cousins, pet rats make wonderful companions. Veterinarians who specialize in the care of rodents say rats kept as pets are very intelligent and easy to train.
"Rats have been used for all kinds of psychological studies and they can be trained to respond to whatever signal you're trying to get them to respond to," said Dr. Laurie Hess, owner of the Veterinary Center for Birds and Exotics in Bedford Hills, N.Y.
In Hiyo's case, that signal is shaking from the very beginning of a muscle spasm. Moore said she first started using a service rat when her daughter was training them to provide therapy, meaning they're trained to assist with healing and rehabilitation.
"She noticed that one of her therapy rats was extremely sensitive to my spasms, so she trained him to tell me when I was starting to have them," Moore said.
Like any other animal, rats can be successfully trained by using rewards.
"An animal will perform behaviors if they get something good out of it," said Hess.
In Hiyo's case, that reward is a strong bond with a human who's grateful to have him.
"I never thought a rat would be this helpful," said Moore. "But I certainly can't say I would trade them for anything nowadays."