Managers of the Lied Animal Shelter, where the outbreak occurred, said the severity of the crisis came as a surprise. They had invited a team of inspectors from the Humane Society of the United States to tour the center this month because they thought they would be praised for their practice of euthanizing animals sparingly, in comparison with shelters of similar size.

Instead, the six-member Humane Society inspection group found a severely overcrowded shelter where many animals appeared very ill. Tests revealed that hundreds were suffering from one or more of three viruses and an aggressive bacterial infection.

By Wednesday night, the shelter chairwoman, Janie Greenspun Gale, tearfully faced critics at a hastily called public meeting and said that the center's policy was "misguided."

Ms. Gale said her organization had been operating the shelter like a rescue operation and had not been euthanizing enough animals to keep the space safe and sanitary for the adoptable ones. From now on, she said, unadoptable animals will be euthanized after 72 hours at the shelter, as the Humane Society recommends.

"Our policies were written to save every animal we possibly could," Ms. Gale said. "In that misguided policy, we caused animals pain."

Lied (pronounced leed) is the main shelter in the Las Vegas area, a nonprofit center that is contractually obligated to accept strays and animals turned in by animal control departments from the Las Vegas and North Las Vegas as well as the unincorporated areas of Clark County.

The shelter continued to do that during its shutdown but stopped its voluntary policy of accepting unwanted animals turned in by pet owners. When the shelter reopens on Friday, it will resume accepting unwanted pets, the spokesman for Lied, Mark Fierro, said Thursday.

About 1,000 of the 1,800 animals in the shelter were euthanized this week in an effort to reduce the population to a more manageable level. In 2005, the most recent year for which statistics are available, the shelter euthanized an average of 400 animals a week. It took in about 950 a week and about 250 were adopted. (Some animals were returned to their owners; others died without being euthanized.)

"People get upset when they hear that 1,000 animals are put down, and, yes, 1,000 is a high number, but these animals have been sick and dying for a while," said Kim Intino, director of sheltering issues for the Humane Society and the inspection team leader. "This was a unique and extreme situation."

Disease outbreaks in shelters are not unusual, but this one was especially gruesome because there were so many different illnesses at once, said Dr. Kate Hurley, head of the Shelter Medicine Program at the University of California, Davis, and one of two veterinarians on the Humane Society inspection team. The viruses were Parvovirus, canine distemper and feline panleukopenia; the bacterial infection was a fatal hemorrhagic, or bloody, pneumonia.

"I'm not aware of outbreaks of this magnitude," said Dr. Hurley, a leading national authority who coincidentally will present a daylong seminar on shelter outbreaks in Las Vegas on Tuesday at the Western Veterinary Conference.

"One of the challenges we had was that they had this unusual bacterial infection that's not been documented in a shelter before," she said. "There was some uncertainty of how to best manage that and what best to do. We were in new territory and found it in both cats and dogs."

The situation outraged the president of the Las Vegas Valley Humane Society, Karen Layne, whose group has expressed concern for years that Lied was overcrowded. "They've been operating a shelter for 11 years and now they're saying they don't know how to run a shelter," Ms. Layne said. She said that Ms. Gale and other shelter officials simply thought disease was a normal part of running a shelter. "Truthfully," Ms. Layne said, "sheltering is all about disease control."

Mr. Fierro said shelter workers knew there was overcrowding and some disease, but they were also tormented by the need to kill so many animals.