BOISE, Idaho — On the outside, their houses and shelters may appear to be havens for dozens or hundreds of homeless and unwanted companion pets and other animals.

On the inside, the conditions are often horrific: Urine-soaked and feces-covered floors. Sick and diseased animals living on top of one another and among the carcasses of the dead. In some cases, the animals are so desperate they eat each other alive.

Why would people who profess to love animals so much keep them in such deplorable conditions? Researchers say there are more than 2,000 new cases of animal hoarding reported each year in the United States. Analysis of a national database of animal cruelty shows a five-fold increase in reported cases from 2000 to 2006, according to a Boston veterinarian who has studied the problem for 20 years.

Numerous hoarding cases have grabbed headlines in the past 10 years. Some involve dozens of animals, others hundreds. It's not the quantity of animals that's of concern. It's the lack of proper care.

"I think of these as little animal concentration camps. It's disturbing," said Dr. Jeff Rosenthal, executive director of the Idaho Humane Society. "The animals suffer horribly." One of the most notorious cases involved Barbara Erickson, an elderly woman arrested twice in seven years on animal cruelty charges. The first time, authorities seized nearly 300 dogs at a Midvale, Idaho, house rented by Erickson and her husband, Robert Erickson. The second time, they seized about 550 dogs at a house she rented in eastern Oregon.

Animal hoarders fail to see what most others find repugnant. Investigators found a decomposing cat under a sofa and 12 other cats in the freezer of Gale McVay's Garden City, Idaho, home during a seizure of about 136 cats in 2005. The home was covered in urine and feces, and many of the cats were gravely ill.

"I'm outraged. They're my life. I love my cats. I've done the very best for them," McVay said then.

Experts say a common misconception about animal hoarders is that they are people who simply took on too much and fell behind, or got overwhelmed, by the care and feeding of the animals they rescued from certain death.

In reality, hoarders compulsively collect animals and/or allow them to reproduce even as conditions deteriorate. They lack awareness and sympathy for the creatures they inadvertently torture in the name of rescue, said the Boston vet, Dr. Gary Patronek.