Lied Animal Shelter spokesman Mark Fierro said that not all of the 1,000 animals killed since Friday had been infected by one of the three canine and feline diseases found by experts from The Humane Society of the United States.
Some were euthanized because they had gone unadopted at the Las Vegas shelter for more than 120 days, and were contributing to overcrowding that Humane Society officials said helped the spread of distemper and Parvovirus in dogs and panleukopenia in cats.
"We're trying to run it like a rescue operation, a no-kill shelter," Fierro told the Las Vegas Review-Journal. "The HSUS told us the crowding caused by that causes animals to be stressed and more susceptible to disease."
Local activists gathered in a park across the street from the shelter Tuesday evening to memorialize what was thought to be the largest mass killing of animals in Las Vegas history.
"It's unforgivable in light of the fact that it was absolutely preventable," said Holly Stoberski, legal counsel for Heaven Can Wait Sanctuary, a group that has worked with Lied to find homes for impounded animals. "They were not properly vaccinating the dogs and cats in a timely manner."
Lied officials have said they did not realize animals were infected until the Humane Society team noticed animals with intestinal and respiratory problems during an inspection.
Fierro said the facility was adopting changes recommended by the national organization to improve conditions, including vaccinating animals as they arrive at the shelter.
The Animal Foundation, a private nonprofit group that operates Lied, also created a new position to oversee shelter operations in response to the outbreak.
The foundation contracts with Clark County and the cities of Las Vegas and North Las Vegas to handle abandoned, neglected and stray animals at Lied. The shelter annually adopts more than 7,000 dogs, cats, rabbits, gerbils, guinea pigs and other animals.