Clinical lycanthropy is a rare psychiatric syndrome with the delusional belief that one is a werewolf.

They grunt, claw, and feel their body is covered with hair and their nails are elongated — some people strongly believe they are in the process of metamorphosis into a wolf. There have been 13 case reports of such people since 1850, one psychiatrist has found.

Intrigued by treating a patient who thought he was a werewolf, Dr. Jan Dirk Blom, an assistant professor of psychiatry at the University of Groningen, in the Netherlands, mined the archives of psychiatry to find out just how common the condition is.

Blom found that since 1850, there have been 56 original case descriptions of people who believed they were metamorphosing into an animal. Among them, 13 reports met the criteria for clinical lycanthropy, the medical term for having delusions of being able to turn into a wolf. The adjective 'clinical' is used to emphasize that the condition doesn't mean actual lycanthropy, or the ability to metamorphose physically into a wolf, Blom said.

The remaining cases were variants of the condition, with patients having delusional convictions about being a dog, a boa snake, a frog or a bee, according to the study published in the March issue of the journal History of Psychiatry. [10 Controversial Psychiatric Disorders]

"I had expected to find more cases, because in textbooks the condition is mentioned quite often in passing," Blom said.

But such a low number of clinical lycanthropy cases reported in over 150 years suggests the condition may be even rarer than previously thought, Blom said. Doctors "should take heed not to cry wolf too often."

The wolf in the mirror

The idea of shape-shifting humans has been around since ancient times and remains an evocative theme even today. But less attention has been given to clinical lycanthropy, a condition that, although rare, does occur.

"In clinical practice, many cases are missed because mental health professionals are insufficiently aware of the existence and the uniqueness of this disorder," Blom told Live Science.

The condition is generally thought to be an unusual expression of another disorder, such as schizophrenia, bipolar disorder or severe depression.

Indeed, in reviewing all 56 cases of delusional metamorphosis into animal, Blom found that 25 percent of the patients were diagnosed with schizophrenia, 23 percent with psychotic depression and about 20 percent with bipolar disorder.

Among the patients, 34 were men and 22 were women, and their symptoms lasted anywhere from a single hour to decades.

The first case report on clinical lycanthropy was published in 1852, and described a man admitted to an asylum in Nancy, France, who was convinced that he had turned into a wolf. "To demonstrate this," Blom explained, the man "parted his lips with his fingers to show his alleged wolf's teeth, and complained that he had cloven feet and a body covered with long hair. He said that he only wanted to eat raw meat, but when it was given to him, he refused it because it was not rotten enough."

Other patients in the reports had similar delusions about changes in their appearance. One saw the head of a wolf when looking at himself in the mirror; another was convinced the bones in her body had been replaced by a pig's, and one felt claws growing in her feet.

The brain that sees a wolf

Although for millennia, explanations for lycanthropy were metaphysical, eventually, modern science raised the idea that brain diseases cause the condition.

Over the past decade, various brain imaging studies have pointed to specific brain areas that appear to be essential for creating the sense of physical existence, and perceiving our body schema, Blom said. [The 10 Biggest Mysteries of the Mind]

These brain regions include areas of the brain's cortex (outer layer) that are responsible for movement and sensation.

"We know that neural circuits in the brain — involving premotor and sensory cortical areas, and probably various subcortical areas as well — are essential to creating our body schema," Blom said.

In the cases Blom reviewed, patients perceived changes in their own physical appearance. For example, some thought their mouths and teeth had changed shape, or their chests had broadened; some experienced their bodies shrinking, and some felt burning sensations in the belly and thighs.

It is possible that in some patients these delusions originated from problems in related brain regions, which profoundly changed the individuals' sense of physical identity, Blom said. Now a forgotten diagnosis, this problem was called coenaesthesiopathy, by French neurologists in 1905.

Today, psychiatrists can useelectroencephalogram (EEG) or other brain imaging techniques to look for abnormalities in brain areas that give rise to the body scheme and sense of self, Blom said.

Still, because clinical lycanthropy tends to occur along with another major psychiatric disorders such as schizophrenia, psychotic depression or bipolar disorder, the best practice would be to treat that underlying disorder, he said.

Email Bahar Gholipour. Follow us @LiveScience, Facebook & Google+. Original article on Live Science.