Turn on the Animal Planet channel, and you see yet another case: Officers from a local Society for the Prevention of Cruelty to Animals arrive at a dilapidated home to find it overrun with cats, dogs or other pets.

Typically, the house is filthy, and the animals are suffering from various stages of malnutrition and disease. The human in charge tries—unconvincingly—to explain why so many animals are being kept in such an unhealthy state.

Such incidents make gripping television, but the sad truth is they’re not isolated events. According to Northeastern sociology professor Arnold Arluke, an expert in human-animal relationships, between 2000 and 2006 there was a five-fold increase in animal-hoarding cases investigated by U.S. agencies.

No one is sure whether this spike was triggered by a heightened attention to the problem or by an actual increase in the number of abuse cases out there. Regardless, Arluke says, “what is clear is that about 2,000 new cases of animal hoarding are investigated each year in the United States, affecting approximately 200,000 animals.”

In collaboration with co-author Celeste Killeen, Arluke examines an extreme case of canine mistreatment in “Inside Animal Hoarding: The Case of Barbara Erickson and Her 552 Dogs,” a book published by Purdue University Press in March. Erickson, who hoarded animals in both Oregon and Idaho, served prison time for her role in the deaths of 134 animals and the abuse and neglect of 418 others.

“Hoarding is a major problem,” says Arluke, “and not one that should be trivialized.” (He specifically criticizes a popular “Crazy Cat Lady” doll, which comes with what’s meant to be a light-hearted checklist of symptoms.)

Arluke’s research began 25 years ago as a broad examination of the human-animal connection. As his work developed, he started focusing more narrowly on issues related to cruelty and neglect. In the 1990s, he was invited to join the Hoarding of Animals Research Consortium, affiliated with the Tufts veterinary-medicine school, to study the pathological need some people have to collect animals.

Animal hoarders shelter a vastly inappropriate number of creatures, dozens or even hundreds of them. The animals are kept in poor living conditions, and often lack proper nutrition, veterinary care, and sanitation.

A typical hoarder is nearly always in complete denial that the animals are suffering, Arluke says.

It’s difficult to sketch out a general psychological profile for a hoarder, he adds. In fact, experts know very little about what motivates the hoarding impulse. A common lay hypothesis points to unmet needs and an absence of love during childhood. Arluke doesn’t buy that explanation. “It’s a much more complex phenomenon than that,” he says.

More research is needed, he says, to illuminate the sociological influences that enable the behavior.