How Dog Flu Works

Greyhound Dreamboat, a retired greyhound from a track outside Tijuana, Mexico, gets a bath from a greyhound adoption group. Public health authorities believe that canine influenza originated at a Florida greyhound racetrack.
Greyhound Dreamboat, a retired greyhound from a track outside Tijuana, Mexico, gets a bath from a greyhound adoption group. Public health authorities believe that canine influenza originated at a Florida greyhound racetrack.
© STRINGER/MEXICO/Reuters/Corbis

A mysterious illness struck racing greyhounds at a Florida track in 2004. They were coughing and had runny noses and fevers. Many of them were dying [source: Iowa State]. The symptoms said "viral respiratory infection," but the source was unknown [source: Payungporn et al.].

Within eight months, the illness had spread to 14 greyhound tracks in six states [source: AVMA]. By mid-2005, it was in a dozen states and had moved beyond the track, showing up in shelters, boarding facilities and veterinary clinics, and affecting multiple breeds [source: AVMA]. By then, authorities had identified the pathogen: the H3N8 virus, a form of influenza type A.


Somehow, these dogs had come down with the flu.

It was the world's first known occurrence of canine influenza [source: AVMA]. More than a decade later, it's not exactly common, but it's around [source: Bowen]. A few Asian countries and most U.S. states have reported cases, with about 2,100 confirmed infections in America between January 2005 and March 2015 [sources: Landolt, VCA, Cornell]. The U.S. Department of Agriculture approved an H3N8 vaccine in 2009 [source: AVMA].

But then, in April 2015, a massive outbreak of dog flu in the U.S. Midwest gave authorities another surprise: These dogs had a different strain of the virus, one derived from avian flu that had never been seen outside of Asia [source: AVMA].

"As this [2015] outbreak demonstrated, influenza is a very unpredictable virus," says Gabriele Landolt, D.V.M., an assistant professor at Colorado State University School of Veterinary Medicine. "Influenza A viruses can change quite easily."

The original, H3N8 canine influenza virus (CIV) is a mutation of the H3N8 equine influenza virus; it jumped from horses to dogs [sources: VCA, Iowa State]. The newer strain, H3N2, jumped from birds to dogs [source: Iowa State]. It was discovered in 2007 in South Korea and may prove more problematic than H3N8. "It appears that the H3N2 strain might spread more efficiently between dogs," says Landolt. It can also infect cats [source: Iowa State].

The two CIV strains are distinct, but you can't tell the difference without lab analysis. They're "the flu." And if you've ever had the flu, you know just what these dogs are going through.


Like Our Flu, But for Dogs

In dogs as in humans, influenza attacks the upper respiratory tract, is highly contagious and typically presents with a persistent cough. It spreads in respiratory secretions — the stuff that comes out in sneezes and coughs — and the severity of the illness can vary significantly. About 20 percent of infected dogs show no symptoms at all, while others are obviously sick [source: VCA].

All dogs, regardless of breed or age, are susceptible to the flu — the virus hasn't been around long enough for any natural (innate) immunity to develop, says Landolt. Even dogs who have had CIV can get it again, because the acquired immunity that comes with exposure to the virus doesn't last forever [source: South Loop Animal Hospital].


"The exact duration of protection is unknown," adds Landolt.

CIV transmission thrives on close contact [source: ASPCA]. This means it doesn't spread particularly well in the general population, but in areas packed with dogs, it's like wildfire [source: Iowa State]. At shelters, doggie day cares, boarding facilities, dog shows and dog parks, all it takes is one infected dog to cough on, sneeze on or touch noses with another dog, and you're looking at a possible outbreak. With no natural immunity in the population, pretty much every dog exposed to the virus contracts it [source: VCA].

There are two general types of flu sickness. Most dogs have the mild form. They display the hallmark cough and often have nasal discharge (a runny nose) and a low fever. They might be sneezing, have runny eyes and lose their appetites [source: ASPCA]. The nasal discharge might turn into a thick, yellow-green mucous if they've developed a secondary bacterial infection of the upper respiratory tract. Dogs with the mild form usually recover easily [source: Iowa State].

In the severe form of canine flu, the situation is more serious. Here, CIV infection is complicated by pneumonia. Dogs who develop this type of illness are typically ones with weaker immune systems due to medical conditions or very young or old age [sources: Lewis, Iowa State]. In severe flu, fevers are high, coughing is more intense and breathing might be labored and/or fast [source: Landolt]. They might be coughing up blood [source: PetMD]. Severe canine influenza requires hospitalization [source: PetMD].

Despite knowing what to look for, identifying dog flu at home is unlikely. Bordetella bronchiseptica, adenovirus type-2 and parainfluenza virus all can cause similar symptoms to CIV [source: Landolt]. Accurate diagnosis requires a veterinarian.


Is It Really Dog Flu?

An Australian silky terrier named Zoey get a canine flu shot from veterinarian Sarah Hormuth on April 15, 2015.
An Australian silky terrier named Zoey get a canine flu shot from veterinarian Sarah Hormuth on April 15, 2015.
© Laurie Skrivan/ZUMA Press/Corbis

Faced with a coughing dog, a veterinarian will perform a complete physical exam. In a dog with the flu, the exam will reveal findings consistent with influenza, but it won’t reveal an influenza diagnosis.

"The signs of CIV are clinically indistinguishable from those caused by other respiratory pathogens," says Dr. Landolt.


Standard blood tests can offer evidence of some kind of microorganism infection, and lung X-rays can identify pneumonia [source: PetMD]. But diagnosing the flu requires more.

Various tests can help identify influenza, and they follow two main approaches: looking for the CIV virus in respiratory secretions (from the nose or throat) and looking for anti-CIV antibodies in the blood [source: Landolt]. The immune system produces specific antibodies to fight off specific infections.

Timing is often critical in testing. A nasal-swab test is only accurate at the beginning of the illness, because the virus is only reliably present in nasal secretions for about four days after symptoms begin [source: VCA]. And blood tests for antibodies are only reliable later in the illness.

"The body needs several days or even weeks to produce the antibodies in response to infection," explains Landolt.

Both of these methods require collecting samples and sending them to a diagnostic laboratory, so they don't provide immediate results. A veterinarian might decide to run a rapid, "on-site" test, or immunoassay, to support a possible flu diagnosis, which reliably detects components of human influenza A in human respiratory secretions. The test is not as reliable in dogs, though [source: Landolt].

The point of all this is to establish an influenza diagnosis in a sick dog, which is important. It can signal the start of an outbreak in the area. It can help to stop the spread by letting the owner know to keep the dog isolated. And according to Landolt, it can help infectious-disease authorities keep tabs on the virus:

Getting specimens to the lab for testing allows for monitoring of the virus in terms of its genetic and antigenic makeup. While there is no evidence at this time that CIV can transmit to humans ... it is possible that at some time in the future the virus might acquire the capability to infect other species.

But in terms of determining treatment, a definitive influenza diagnosis is not essential.


The Road Back to Health

Dogs with the flu need to take it easy, too, just as humans do.
Dogs with the flu need to take it easy, too, just as humans do.
© M. Rutz/Corbis

There is no way to treat the canine influenza virus itself [source: ASPCA]. Treatment is supportive – easing symptoms to keep dogs comfortable and providing an environment that helps the immune system work effectively [source: AVMA]. In mild cases of dog flu without secondary complications, this means rest, a warm and comfortable environment, and good nutrition, possibly including supplements [source: ASPCA]. A veterinarian might prescribe a cough suppressant and a nonsteroidal anti-inflammatory drug like aspirin or ibuprofen [sources: AVMA, PetMD].In the case of secondary bacterial infection, the treatment is antibiotics [source: PetMD].

In almost all of these cases, the flu will just run its course. It can take up to a month for the cough to go away, but other symptoms are usually gone in a week or so [sources: AVMA, Lewis].


For dogs with severe flu, treatment is more involved and requires hospitalization. Depending on the dog's condition and on which secondary complications are present, treatment can include supportive care, powerful broad-spectrum antibiotics (for severe forms of pneumonia) and IV fluids (for dehydration) [source: PetMD].

The severe form can be fatal, but it's not technically the flu that causes death. It's a secondary complication, usually pneumonia [source: AVMA].

Overall, fatality rates for dog flu are pretty low. (They were much higher in the initially exposed racing greyhounds – 36 percent at one track -- than they are in the pet population [source: Iowa State].) According to most sources, 1 percent to 5 percent of infected dogs die, though some say it's as high as 8 percent [sources: VCA, Iowa State].

Of course, when we're talking about beloved pets, even 1 percent is a lot. There are things you can do to minimize your dog's risk, though, the most obvious being to vaccinate. But veterinarians don't recommend it for everyone.


Keeping Dogs Flu-free

A Missouri animal hospital encourages people to vaccinate their dogs in April 2015. The U.S. Midwest was particularly hard hit by canine influenza.
A Missouri animal hospital encourages people to vaccinate their dogs in April 2015. The U.S. Midwest was particularly hard hit by canine influenza.
© Laurie Skrivan/ZUMA Press/Corbis

The H3N8 vaccine is considered a "lifestyle vaccine" – it's mostly administered to dogs who spend time in group facilities [source: AVMA]. The vaccine won't stop a dog from contracting the flu, but it helps ensure it's the mild form and speeds up recovery. It also reduces the level of contagiousness, so it can help prevent a full-blown outbreak [sources: AVMA, VCA].

The vaccine is only for the original U.S. strain, though. As of 2015, there's no U.S.-approved vaccine for the H3N2 dog-flu virus (though there is one available in South Korea), and it's unclear whether the H3N8 vaccine will work against the new strain [sources: CDC, Iowa State].


Dr. Landolt, for one, is doubtful. "Genetically, the two strains are distinct enough that the vaccine is likely to induce less protection, if any."

Besides vaccinating high-risk dogs, the best approach is to educate yourself and be smart. Talk to your vet about prevention. If dog flu has been an issue in your area, maybe stay away from the dog park for a while. Ask the folks at doggie day care if they've seen any respiratory symptoms in their guests, and if they have, keep your dog home. Make sure any group facility has an infection-control plan in place, which involves checking for respiratory symptoms at admission, immediately isolating a dog that starts coughing, disinfecting everything that dog has come into contact with and notifying all owners of the incident [sources: Iowa State, AVMA].

It's not fool-proof. Dogs are most contagious in the first few days of infection, before symptoms even appear; they can spread the virus for up to two weeks, even after symptoms have disappeared; and about one in five dogs doesn't develop any symptoms at all [sources: VCA, Lewis]. So just because your dog's buddies aren't coughing doesn't mean they don't have the flu.

If your dog does start coughing, isolate him or her at once, and call the vet [source: ASPCA]. In general, the earlier you seek medical help, the better the prognosis [source: ASPCA].

And if it turns out to be the flu, keep your dog isolated for up to 14 days. Use a short leash on walks and cross the street if another dog is heading toward you (call to the owner "dog flu!" so she doesn't think you just hate her) [sources: Blum Animal Hospital, Chicago Tribune]. Make sure no one gets near his or her food dishes, beds and toys. And then disinfect [sources: Lewis, ASPCA].

Disinfect yourself, too. While your dog is sick, wash your hands and change your clothes before going near another dog [source: Landolt]. Humans can't catch canine influenza, but they can spread it.


Lots More Information

Author's Note: How Dog Flu Works

Before I started researching dog flu for this assignment, I hadn't heard of the April 2015 outbreak in the Midwest (I'm writing this in May), which appears to be quite serious and now has some people crying "epidemic." I chose not to dwell on this outbreak – this article is about solid information, not a still-unfolding situation with understandably mixed reports. But since I've been writing this, I've seen daily updates, and for some reason none of them has provided a very useful context that I now find myself able to offer, so here goes: In the initial weeks of the Midwest outbreak, it seems more than 1,000 dogs came down with the flu. I wasn't really sure if that was a lot until I tracked down CIV-infection data from Cornell University (which I did include in the article and admittedly was hiding in some distant Internet nook), and according to Cornell, in the 10 years leading up to March 2015 there were a maximum of 2,300 confirmed cases of dog flu in the United States. So it seems 1,000 dogs infected in a couple of weeks is, indeed, a LOT. (Good to know, right?)

Related Articles

More Great Links

  • The American Society for the Prevention of Cruelty to Animals (ASPCA). "Canine Influenza Virus/Canine Flu." (May 11, 2015)
  • American Veterinary Medical Association (AVMA). "Canine Influenza." April 22, 2015. (May 22, 2015)
  • American Veterinary Medical Association (AVMA). "Canine Influenza FAQ." April 22, 2015. (May 15, 2015)
  • Bowen, Melinda. "Dog Smarts: Understanding Canine Influenza in Advance with Dr. Bowen." April 16, 2015. (May 22, 2015)
  • Blum Animal Hospital. "Frequently Asked Questions: 2015 Canine Influenza Outbreak." April 21, 2015. (May 27, 2015)
  • Centers for Disease Control and Prevention (CDC). "Key Facts about Canine Influenza (Dog Flu)." April 22, 2015. (May 11, 2015)
  • The Chicago Tribune. "Editorial: Canine flu outbreak means no doggie Happy Hour." April 14, 2015. (May 27, 2015)
  • Cornell University College of Veterinary Medicine. "Test Summary for Canine Influenza Virus in Dogs." March 30, 2015. (May 22, 2015)
  • Iowa State University. "Canine Influenza." The Center for Food Security and Public Health. June 2014. (May 18, 2015)
  • Landolt, Gabriele. (Email interview.) Colorado State University School of Veterinary Medicine. May 20, 2015.
  • Lewis, Tanya. "Dog Flu Outbreak: What You Need to Know." Live Science. April 17, 2015. (May 11, 2015)
  • Payungporn, Sunchai, P. Cynda Crawford, Theodore S. Kouo, Li-mei Chen, Justine Pompey, William L. Castleman, Edward J. Dubovi, Jacqueline M. Katz, and Ruben O. Donis. "Influenza A Virus (H3N8) in Dogs with Respiratory Disease, Florida." Emerging Infectious Diseases (CDC). Vol. 14, No. 6. June 2008. (Accessed May 11, 2015) Available from :
  • PetMD. "Dog Flu." (May 11, 2015)
  • South Loop Animal Hospital. "CIV Epidemic Update." April 3, 2015. (May 28, 2015)