What to know about bird flu

Avian influenza viruses have been detected for decades in wild waterfowl and shorebirds, such as ducks and geese. These birds are considered the “reservoir species” for the virus, hosts that harbor and transmit the pathogen. Some influenza viruses, however, can spill over and infect other populations. The current H5N1 virus, scientifically known as H5N1 clade 2.3.4.4b, can infect wild birds, domestic poultry and mammals, including humans.

Commercial poultry operations have been hit particularly hard. Producers have lost entire flocks to the virus due to high mortality and culling, a response to stop the disease from spreading. In addition to dairy cattle, there have been sporadic infections in other mammals, and, according to the Centers for Disease Control and Prevention, 70 reported cases in humans since April 2024, including one death. The human cases have been the result of people interacting with infected animals, according to the CDC.

Since the dairy cattle outbreaks began last spring, the Colorado State University Veterinary Diagnostic Laboratory has been testing samples daily, running as many as 600 samples on a busy day. The lab works closely with state and federal agencies, including the Colorado Department of Agriculture, Colorado Parks and Wildlife and the Colorado Department of Public Health and Environment on testing submissions and reporting.

The CSU lab is part of the USDA’s National Animal Health Laboratory Network, a collection of more than 64 public veterinary labs across the country, and it works closely with the network on testing. SOURCE spoke with Pabilonia about the most recent influenza outbreaks. She is a microbiologist, veterinarian and an expert on poultry diseases. Pabilonia has dedicated a large portion of her career to studying avian influenza viruses.

Testing is critical to controlling spread of the virus, and Colorado State University’s Veterinary Diagnostic Laboratory, or VDL, is playing a key role, regularly testing samples from birds, cattle and other species. Understanding how disease is affecting wild and domestic animals is a mission of the lab in Fort Collins, and that information is essential for animal and public health officials, food-animal producers and others to respond effectively to halt the spread of disease and curb potential impacts to human health.

Recently, the lab has been detecting this version of avian influenza virus in domestic cats. “In Colorado, we’ve detected a lot of cat positives,” said Dr. Kristy Pabilonia, executive director of the VDL. “Understanding the spread of the virus in cats has been one of our focuses. It’s very concerning because these are people’s pets.”

Q&A with Pabilonia, an avian influenza expert at CSU in the College of Veterinary Medicine and Biomedical Sciences.

Q. What species is your laboratory detecting the virus in?

A. Since early 2022, we’ve detected highly pathogenic avian influenza virus, or HPAIV, in many species. We’ve detected outbreaks in commercial poultry and backyard poultry flocks, dairy cattle, domestic pet cats, wild birds and a variety of wild mammal species, including mountain lions, foxes, mice and many others. Recently, we’ve had many positive tests on wild raptor birds, and our number of pet cat positives continues to increase. Luckily, we have not detected HPAIV in Colorado cattle for many months, due to the hard work of our state and federal partners and the dairy producers.

Q. How high is the mortality rate in birds?

A. It depends on the bird species that get infected. Commercial poultry can have a mortality rate approaching 100%. With commercial poultry, if a flock is infected, we depopulate the whole flock as a means to prevent spread and also because the birds have very high mortality.

In wild birds, the story is a bit different. We know that traditionally wild birds are the reservoir of influenza viruses. As a reservoir, they will get infected and shed the virus into different environments, but they typically don’t show clinical signs of disease. The currently circulating avian influenza virus is different. One of the reasons this is such a pesky virus is that it does kill wild birds, depending on the species. Certain birds are more susceptible, and in some species, you can see really large mortality events. In Colorado, we have experienced large mortality events in some wild bird populations, such as snow geese.

Q. When the virus was detected in cattle, was that the moment when people started paying more attention?

A. Veterinarians, scientists and public health officials have been paying attention to avian influenza viruses for many decades. I do think when we detected avian influenza in cattle there was a lot of attention from others because it was unique. Any time we have big outbreaks, I think people will pay attention because the response requires so many resources and because we’re always worried about influenza viruses and their ability to infect humans. The broader public tends to pay closer attention when there is something new or unexpected, like detection of the virus in a new species, but veterinarians, scientists and state and federal officials are fighting to control spread of influenza viruses all the time.

Q. What do you see as the most recent significant avian influenza event here in Colorado?

A. Our laboratory has had many positive detections in many animal species over the past few years. Every detection is meaningful and important. Our job is really impactful because it provides state and federal officials with the information they need to know where the virus is so they can respond. Our laboratory always works to support animal producers and protect the U.S. food supply. This outbreak has had a big impact on our producers, particularly our commercial egg layer flocks and our dairy cattle herds. We also work to protect wild birds and mammals and companion animals. They’re all important to our laboratory staff, and our team has worked many extra hours because they know the importance of their work.

We get positives from wild birds all the time that are meaningful. We’ve had many commercial poultry positives that have really affected our state over the past few years. A couple years ago, there was the same thing we’re seeing now where there were a lot of empty egg shelves. In Colorado, we’ve detected quite a lot of cat positives, and the numbers have been increasing. That’s been one of our focuses.

Q. How uncommon is it for this virus to infect cats in a significant way?

A. We’ve known for many years that cats can be infected with influenza viruses. Over the past few years, though, there have been many detections in these species. In Colorado, we’ve detected HPAIV in wild and captive large cats, such as mountain lions, tigers and domestic pet cats. We’re seeing an uptick in detections in pet cats. This is really concerning because those are people’s pets.

We also know that preventing cases in pets can prevent cases in their human owners. We know dogs are also susceptible to influenza viruses, but we’ve had very few detections of avian influenza globally in domestic dogs. There are many scientists studying the differences in susceptibility we see in different species. We are hopeful that our work in rapid detection of the virus will prevent as much spread as possible to people’s pets.

Q. What are the symptoms for cats? What should cat owners be watching out for?

A. Early on in the outbreak, all the samples we received were from cats with severe neurologic disease, causing incoordination and an inability to walk. They were mostly cats that lived on infected dairy premises. These cats were very sick, and they died from their disease. More recently, we’ve been detecting HPAIV in cats that have had more mild clinical signs, and a couple of cases where cats have been sick but have survived.

The course of disease in an individual cat may be dependent on their original infectious dose and their route of exposure. If you’re a cat that lives on a dairy, and there are hundreds of dairy cattle shedding virus in their milk that you have contact with, you may have a high exposure that results in severe illness. There may be other routes of exposure where cats don’t get as sick. We don’t really know this answer right now, this is just speculation. I’m very interested in studying more about this. There’s a lot more we need to learn, but we have tested some cats recently that have survived. I think this is an interesting story: We just don’t know what the case fatality rate is in cats.

Q. Your lab recently detected avian influenza in a cat that consumed raw pet food. How was that discovered, and what should pet owners know about this?

A. Our laboratory has detected the virus in cats that live on dairies, cats that have consumed raw milk, cats that have scavenged on dead birds, and we’ve even had some cats with no known exposure. It’s very interesting that we are seeing cats exposed to the virus by so many different routes. Our recent detection of a positive cat that ate raw pet food is very important because it gives us information that we can pass along to pet owners. It is really important that cat owners do not feed their cats raw pet food or raw dairy products. They should also minimize their cat’s exposure to any wild birds, as much as possible.

Q, With domestic cats, when should testing for HPAIV be considered?

A. If cats have neurologic symptoms, we always recommend testing for rabies. Now, we recommend also testing for avian influenza. If you have a cat with neurologic disease, unexplained respiratory disease or an unexplained severe illness, particularly if it’s consumed raw pet food or raw milk, or has had contact with a dairy or contact with wild birds, these could all be reasons to get influenza testing. We recommend you talk to your veterinarian.