The bird flu outbreak that started last year and has led to the deaths of more than 5 million birds in Nebraska has now been linked to the deaths of at least three cats in the state.

Two of the cats came from a home in Madison County and were the subject of a case report published in February by Dr. Sarah Sillman, an assistant professor at the University of Nebraska-Lincoln and veterinary diagnostic pathologist at UNL's Nebraska Veterinary Diagnostic Center. The third cat was from Buffalo County and became ill last month.

In an interview, Sillman said bird flu in domestic cats and other pets "is quite rare," with the Veterinary Diagnostic Center having seen just a few cases in cats since the outbreak in birds more than a year ago.

"The current HPAI outbreak in birds has been particularly severe, and it is not surprising that we see these occasional 'spillover' events in mammals, including pets," Sillman said in an interview. "Cats are particularly vulnerable, in large part because many cats are completely outdoor free-roaming and frequently hunting wild birds."

She said she's not sure why half the bird flu deaths in cats in the U.S. so far have occurred in Nebraska, but it could be due to the fact that the state is a main flyway for migrating birds and also has many farms and rural residences with outdoor cats.

"The cases diagnosed in our lab were all outdoor cats," she said.

It's not certain how the cat's became infected, but Sillman said it's likely they ate infected birds.

"Oral ingestion is an important route of infection for pet cats out in the natural environment when they consume infected wild birds," she said.

Though it is possible for the cats to transmit the disease to other animals in close contact, such as other household pets, the Madison County cats were around other household cats and did not appear to transmit the virus to them.

In addition to the three Nebraska cats, two cats died of bird flu in Oregon and another one in Wyoming. There have been sporadic reports of pet deaths in other countries, including a cat in France and a dog in Canada.

There have been a little more than 150 confirmed cases of bird flu in mammals in the U.S. since the current outbreak began, mostly in wild skunks and foxes, although it has affected bears and even marine animals such as seals, sea lions and dolphins.

An outbreak at the Riverside Zoo in Scottsbluff in December killed two mountain lions and two tigers and also infected a grizzly bear. The source of the outbreak was determined to be tainted goose meat used as food.

Sillman said the best way to protect cats and other domestic pets from bird flu is to avoid exposure to wild birds and poultry.

"It is advised to keep cats indoors to avoid contact with wild birds, waterfowl and domestic poultry," she said. "If there are sites known to have had infected birds, keep pets away from the virus-contaminated area. Avoid feeding raw poultry and keep pets from consuming wild bird carcasses."