NEW YORK (AP) -- Scientists have spent years studying the health of search-and-rescue dogs that nosed through the debris at the World Trade Center site, and to their surprise, they have found no sign of major illness in the animals.

They are trying to learn why the dogs have not contracted illnesses similar to what thousands of rescue workers who toiled at ground zero after the September 11, 2001, attacks have reported.

"They didn't have any airway protection, they didn't have any skin protection. They were sort of in the worst of it," said Cynthia Otto, a veterinarian at the University of Pennsylvania, where researchers began a study of 97 dogs five years ago.

Although many ground zero dogs have died -- some of rare cancers -- researchers say many have lived beyond the average life span for dogs and are not getting any sicker than average.

The dogs' owners dispute the findings, saying there is a definite link between the toxic air and their pets' health.

Miss Otto has tracked dogs that spent an average of 10 days after the 2001 terrorist attacks at either the trade center site, the landfill in New York where most of the debris was taken or the heavily damaged Pentagon.

As of last month, she said, 30 percent of the dogs deployed after September 11 had died, compared with 22 percent of those in a comparison group of dogs who were not pressed into service. The difference was not considered statistically significant, Miss Otto said.

But she added: "We have to keep looking."

A separate study that a doctor at New York's Animal Medical Center will soon publish focused on about two dozen New York police dogs. It reveals similar conclusions...