Canine influenza is a contagious respiratory infection in dogs caused by influenza A viruses. Two strains have circulated in dogs: H3N8, which jumped from horses to dogs and was first identified in the U.S. in 2004, and H3N2, which originated in birds and was first detected in dogs in South Korea in 2007. The H3N8 strain went extinct in 2016, making H3N2 the only strain currently affecting dogs.
Where the Virus Came From
Unlike human flu strains that have circulated for decades, canine influenza is relatively new. The H3N8 strain was discovered when dogs at a Florida racing greyhound track came down with an unexplained respiratory illness in 2004. Investigators traced the virus back to horses, where H3N8 had long been a known pathogen. The virus mutated just enough to infect dogs and spread between them.
H3N2 took a different path. It originated in birds and made the jump to dogs, first appearing in South Korea in 2007 before eventually reaching the United States by 2015. H3N2 proved more successful at spreading. The H3N8 strain stayed geographically limited to parts of North America and was last detected in the northeastern U.S. in 2016. Researchers at Oxford’s Virus Evolution journal confirmed no further cases have been reported since, even during a period of widespread canine influenza testing. H3N2 remains the active concern.
How It Spreads
Canine influenza spreads the same way human flu does: through respiratory droplets from coughing, sneezing, and barking, as well as through contaminated surfaces like water bowls, leashes, and kennel floors. Dogs in close quarters, such as boarding facilities, shelters, grooming salons, and dog parks, face the highest risk.
The incubation period is typically two to three days, though it can range from one to five days after exposure. A critical detail for dog owners is that infected dogs begin shedding the virus before they show any symptoms. By the time your dog starts coughing, it may have already been contagious for a day or two. Dogs generally stop shedding the virus within 10 days of infection, but some veterinary guidelines recommend considering dogs potentially contagious for up to four weeks after exposure to be safe.
Symptoms to Watch For
Most dogs with canine influenza develop a persistent cough that can last 10 to 21 days. It often sounds similar to kennel cough, which makes the two conditions easy to confuse. Other common signs include a runny nose (starting clear and sometimes turning thick and yellow-green), sneezing, reduced appetite, lethargy, and fever.
The illness ranges widely in severity. Some dogs develop only mild symptoms that resolve on their own. A smaller percentage develop no symptoms at all but can still spread the virus to other dogs. On the serious end, some dogs develop high fevers and secondary bacterial pneumonia, with signs like labored breathing, increased respiratory rate, and a noticeably sick appearance. Puppies, senior dogs, and flat-faced breeds like bulldogs and pugs tend to be more vulnerable to severe illness.
How Veterinarians Diagnose It
Because canine influenza looks so much like other respiratory infections, a cough alone isn’t enough for diagnosis. Vets use a PCR test on a deep nasal swab to detect the virus directly. Timing matters: the swab needs to be collected within four days of the first symptoms for the most accurate results. After that window, the amount of virus in the nasal passages drops and the test is more likely to come back negative even if the dog is infected.
If you miss the early testing window, your vet can run blood antibody tests instead. These require two blood draws taken two to three weeks apart to look for a rising antibody response, which confirms recent infection but takes longer to get a definitive answer.
Treatment and Recovery
There is no widely used antiviral treatment for canine influenza. While antiviral drugs like oseltamivir (the same drug used for human flu) technically exist for dogs, they are considered reserved medications to preserve their effectiveness in human medicine. They are only used in rare cases where the dog’s survival is at stake and other measures aren’t working.
For most dogs, treatment is supportive. That means rest, fluids, nutrition, and keeping your dog comfortable while its immune system fights off the virus. Dogs with secondary bacterial infections may need antibiotics. Dogs with pneumonia or severe dehydration sometimes require hospitalization for intravenous fluids and closer monitoring. The majority of dogs recover fully within two to three weeks.
Preventing the Spread
Vaccines are available for both the H3N8 and H3N2 strains. They don’t completely prevent infection, but they reduce the severity of symptoms and the amount of virus a dog sheds, which helps limit outbreaks. Vaccination is most useful for dogs that regularly visit boarding facilities, daycares, or dog shows. Your vet can help determine whether vaccination makes sense based on your dog’s lifestyle and local outbreak activity.
If your dog is diagnosed or suspected of having canine influenza, isolate it from other dogs for at least three weeks. The virus survives on surfaces but is relatively easy to kill. Standard household bleach diluted at roughly 1 part bleach to 32 parts water works well for cleaning bowls, crates, and hard surfaces. Alcohol-based cleaners at 70% concentration or higher also destroy the virus within about a minute. Wash bedding and soft items in a machine cycle of at least 60°C (140°F) for 30 minutes. Wash your hands and change clothes after handling a sick dog before interacting with healthy ones.
Risk to Humans and Other Pets
Canine influenza has not been shown to infect humans. The CDC does not currently consider dog flu a public health threat to people. However, H3N2 canine influenza can spread to cats, causing similar respiratory symptoms. If you have both dogs and cats in your home during an outbreak, keep them separated until the sick dog has fully recovered.