Black, brown and white puppy sleeping on the floor

What’s new about dog flu?

Dog flu is a highly contagious respiratory disease. Learn about the illness and how to protect your dog.

Unlike a lot of canine infectious diseases, dog flu has been around for a relatively short time. The first strain, H3N8, first appeared in dogs around 2004, and the newest strain, H3N2, started to appear in dogs in 2015 and spread throughout the U.S.1

What are flu symptoms in dogs?

Coughing and sneezing, a runny nose or runny eyes, a lack of energy, decreased appetite, and a fever over 103 degrees (a dog’s normal temperature is between 100 and 102.5 degrees) may all indicate the presence of flu.2

These symptoms can last from a few days to several weeks depending on the severity of the disease. If your dog shows such signs, don’t hesitate to make an appointment with your veterinarian; early treatment can result in a better outcome. Treating an advanced case can cost thousands of dollars. Fortunately, the mortality rate for dogs with canine influenza virus (CIV) infection is low.1

How does a dog catch the flu?

Canine influenza is highly infectious and spreads very quickly from dog to dog, most commonly via direct contact (sniffing, licking, nuzzling) with an infected dog, through the air (coughing, sneezing, barking), and via contaminated surfaces (sharing water bowls or toys). A human who’s been in contact with a sick dog can transmit the virus on his or her hands or clothing.1

How can your veterinarian tell it is flu?

A veterinarian can gently swab inside your dog’s nose and/or throat and send these samples to a veterinary diagnostic laboratory to determine the nature of the infection.

A baffling diagnosis

No one knew that the H3N2 virus had come to America until a number of dogs in Chicago fell sick with the same unusual respiratory illness early in March 2015 — and it spread fast.2 Preliminary testing showed that it wasn’t the familiar H3N8 strain of canine influenza — nor was it one of the other known canine respiratory viruses or bacteria. 2

What was also curious was that dogs showing signs of this new flu weren’t shelter dogs, but pets who belonged to individuals.

Identifying the new flu

In late March 2015, the Merck Animal Health Diagnostic Support Program began testing samples from more than 650 dogs. Multiple other laboratories contributed testing information as well, to allow for a more complete picture of the new flu’s distribution and activity.3

“We came to realize what was actually unfolding was the transmission of an influenza strain — H3N2 — never before seen in the United States,” said veterinarian Dr. Kathleen Heaney.

“Based on the highly contagious nature of the strain, the severity of clinical disease and the rate at which we were seeing newly diagnosed cases, we knew we needed to act fast — both to help veterinarians and pet owners contain the outbreaks and develop a vaccine to protect dogs against it.”

Thanks to the efforts of teams of veterinarians at university diagnostic laboratories, the H3N2 canine influenza virus was identified. Subsequently, Merck Animal Health research and development scientists went quickly to work on developing a safe and effective vaccine to protect canine patients.4

Is your dog at risk?

The more your dog socializes with other dogs, the higher the risk of contracting canine influenza and other contagious respiratory diseases. An infected dog can easily spread the virus to other dogs, even without direct contact. The infection can spread quickly at any location where multiple dogs mingle.1

Does your dog:

  • Spend time in doggie daycare
  • Overnight at a boarding facility or pet hotel
  • Attend training classes
  • Visit a dog run or park
  • Travel to dog-friendly events
  • Attend dog shows or competitions
  • Visit a groomer or a pet store
  • Greet other dogs during walks (such as nose-to-nose contact)
  • Come into contact with dog neighbors, even through a fence

If you answered “yes” to any of these questions, your dog is at a higher risk of contracting canine influenza and other respiratory diseases.

How can you protect your dog from the flu?

Dogs have no natural immunity to CIV and virtually 100 percent of those exposed to it become infected. Of those, about 20 percent will show no signs yet will still spread the disease, while the vast majority — 80 percent — will develop recognizable symptoms. Up to eight percent of infected dogs may die from complications of the disease, primarily from its advance to pneumonia.5

The best protection against the canine influenza virus for your dog and the dogs in your community is vaccination, which works by giving a dog immunity to fight off the virus.

The canine influenza vaccines consist of an initial vaccine followed by a booster two to four weeks later. Puppies as young as six to seven weeks of age can safely be vaccinated. After that, an annual dose protects a pet for a year.

Just like human flu vaccines, the vaccines may not completely prevent the flu but will make contracting it less likely. And if a vaccinated dog catches the flu, symptoms are likely to be milder and easier to treat and manage.

How do you care for a dog diagnosed with the flu?

There are no specific drugs to treat dogs that have already contracted canine influenza infections at this time. The only option is supportive care to make your pet as comfortable as possible. The illness must simply run its course.

Treatment options are focused on managing the symptoms, providing supportive care, and making sure the dog is as comfortable as possible, remains hydrated, and eats well. Those steps help boost a dog’s immune system so she can fight the virus on her own.

Dogs with canine influenza that have a thick nasal discharge or signs of pneumonia may be prescribed antibiotics because of the risk of secondary bacterial infection. Some dogs with severe illness may require hospitalization and intravenous fluids to combat dehydration. In the more severe cases of pneumonia, oxygen therapy may be necessary.5

And be patient; it can take several weeks for your dog to improve. In the interim, keep her away from other dogs, feed her well, wash your hands frequently, and once she recovers, keep up with regular preventative vaccinations for this and all infectious diseases.


  1. Key Facts about Canine Influenza (Dog Flu). Center for Disease Control and Prevention.
  2. Borland S,, Gracieux P., Jones M., Mallet F., & Yugueros-Marcos J. (2020). Influenza A Virus Infection in Cats and Dogs: A Literature Review in the Light of the “One Health” Concept. Frontiers in Public Health, 8.
  3. Addressing Canine Influenza. Merck Animal Health.
  4. Merck Animal Health Pioneers H3N2 Canine Influenza Vaccine. Merck Animal Health.
  5. Canine influenza. American Veterinary Medical Association.

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