In South Philadelphia, housing officials remove 17 dogs from a house filled with feces. The stench is so revolting that workers wearing respirators immediately seal the place, and officials cite the owner for code violations.

In Swedesboro, N.J., a renter leaves behind 14 sick cats, along with a note telling the SPCA to take them. Four other cats are not going anywhere -- they are dead.

In Voorhees, N.J., animal control officers charge a man with cruelty after removing 50 mixed-breed beagles from his trash-filled home. Seven are paw-deep in their own waste in an outside pen.

These owners have long been seen as neighborhood eccentrics. But a growing number of veterinary and mental health professionals now use the term "animal hoarding" and are studying whether the behavior indicates untreated mental illness.

The first scientific study of this behavior took place in 1981 in New York. Since then, it has been examined by a consortium of Massachusetts researchers, including veterinarians, psychologists and social workers.

The group believes that hoarders may suffer from many psychological disorders, including dementia.

Researchers are developing a comprehensive profile. The more secretive and socially isolated people are, the more likely they are to be hoarders. Another factor is how actively they seek out new pets and how adequately they care for the pets they have.

"Some may exhibit a mental illness. Others, like the overwhelmed caregiver, may have some problems, but they're not mentally ill," said Gail Steketee, a Boston University social-work professor and member of the Hoarding of Animals Research Consortium.

Researchers are moving ahead to develop a treatment protocol, Steketee said. None exists.

Although no one knows how many hoarders there are, as many as 3,000 new cases crop up nationwide each year, said Randall Lockwood, vice president for research at the Humane Society of the United States. Hoarders fit a pattern -- often a solitary woman older than 60 for whom pets, more often cats than dogs, become a source of comfort and unconditional love.

"It's not about the animals at all, it's about fulfilling a human need," said Gary J. Patronek, a founding member of the consortium and an assistant professor at Tufts University School of Veterinary Medicine, in Massachusetts.

Animal hoarders generally start out as Good Samaritans who take in unwanted pets, the consortium says. Many are in the helping professions, such as nursing, and lead otherwise normal lives.

But as their homes fill with animals, pet care suffers. The homes evolve into run-down shelters lined with cages. Dogs go unwalked, cat litter boxes untended. Living conditions become squalid and unhealthy.

Many places lack plumbing and electricity. And some people supplement their hoarding by collecting inanimate objects, such as discarded containers, newspapers and guns.

As the behavior progresses, hoarders retreat into secrecy, keeping visitors away. But the odor of animal waste seeping through walls betrays them, spurring complaints and investigations by humane officers and code enforcers.

Experts are not sure what prompts a person to start hoarding. They do think that many hoarders form a parallel society of animals, much like a family. "This fills a huge hole in their lives," Patronek said.

Hoarding is resistant to intervention; the relapse rate is near 100 percent, Steketee said. Hoarders typically move rather than stop collecting pets.

The mental health community does not view animal hoarding as a discrete disorder, because it has so many elements and possible causes, said Jerrold Pollak, a Portsmouth, N.H., psychologist specializing in obsessive-compulsive behavior.

The behavior may point to a personality disorder, a dementia involving the brain's front temporal lobes, or obsessive-compulsive disorder, he said.

Stephanie LaFarge, a clinical psychologist who directs counseling at the American Society for the Prevention of Cruelty to Animals in New York, believes the mental health community has been negligent in not recognizing animal hoarding as a problem that needs clinical attention.

"Mental health turns a blind eye because these people usually aren't a danger to themselves or others," LaFarge said. "They give these folks a pass."

But law enforcement does not. Although there is no law against hoarding, Pennsylvania pet owners who do not provide adequate food, shelter and sanitary conditions may be charged with a summary offense. Usually, they pay a fine and forfeit the animals.

Municipalities may set the number of animals a person may keep.

It is common for people collecting large numbers of animals to be cited for failing to take care of them.

Steketee thinks it will take a combined effort from police, SPCA agents, social workers and veterinarians to stop animal hoarding.

"There's a strong need for a coordinated management of the situation within communities because, as you can see, it's not a simple problem," Steketee said.