Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066



Influenza viruses of an assortment of varieties have been the subject of concern for humans, wildlife, and domestic animals for many decades. The dog was largely felt to be exempt from “the flu” until 2004 when a new canine influenza virus, clearly stemming from the equine influenza virus, was isolated from several groups of Florida racing greyhounds. The problem seemed confined to the racing industry until 2005 when cases began appearing in boarding facilities involving pet dogs. In 2015, a new strain of canine flu entered the U.S. from Asia this time more virulent and more contagious among dogs than the flu from the racing industry. This epidemic (more correctly called an "epizootic") centered around Chicago, leading to sickness in over 1000 dogs despite widespread quarantine. At present, it is these two strains of canine influenza that concern dogs: H3N8 (from the racing industry) and H3N2 (from Asia). The following information includes a basic primer on what they are and how to keep your pets safe.
(Photo Credit: Alex Lapuerta via Wikimedia Commons)




Let's start with what an influenza virus is. There are actually three types (genera) of influenza viruses: type A (including the canine influenza viruses), type B, and the less closely related Type C. They all produce fever, joint pain, and respiratory signs with which we are all familiar. Death is unusual but stems from respiratory complications and is most common in the very old and very young. Most human flu viruses are type B or C.

The Type A influenza virus has on its surface an assortment of proteins which determine its “strain” or subtype, and it is against these surface proteins that our bodies mount an immune response. Of particular note are the hemaglutinin proteins and neuramindase proteins for which specific strains of influenza viruses are named. For example, in 2009, there was a human epidemic of "H1N1" virus. The canine influenza virus is "H3N8." As human influenzas emerge, these terms may be in the news and it is of interest to know what they refer to. If a viral strain mutates and sufficiently changes its surface proteins, a new strain is created. A new strain is one where the susceptible population has no immunity and infection can spread rapidly.

Group of influenza viruses

(Photo Credit: CDC Public Health Image Library)


Influenza viruses have a unique ability to mutate and form new strains. This process of change is called "antigenic drift" and produces new strains of influenza that infect the same host species. (It is because of antigenic drift that we need a new vaccine for flu each flu season.  Sometimes these mutations, lead to a strain being able to cross over into a new species.)  Type A influenza viruses can do more; however. They can actually reorganize their surface proteins to facilitate crossing from one species to another and creation of a vastly different different new flu virus. This process of change is called "antigenic shift" and usually occurs within the body of a bird or pig. The new viruses that are created by this process are more likely to be able to jump to a new species than those created by the simple mutations of antigenic drift (which is why "swine flu" and "avian flu" are portrayed as particularly ominous in relation to human infection).

Despite all this mutation and recombining, influenza viruses generally keep to the same host species.  Neither of the two canine influenza viruses is transmissible to humans. The H3N2 (Asian) canine influenza virus is able to infect cats as can the H7N3 (bird) influenza virus. The H3N8 canine influenza virus has not been known to infect cats. The H1N1 influenza virus that was involved in the human pandemic of 2009 can also infect cats but cats do not shed virus back in amounts likely to infect humans. As a general rule, humans infect other humans and household pets are not involved in transmission of flu to humans.


Infection rate is high but (depending on which report one reads) 20-50% will simply make antibodies and clear the infection without any signs of illness at all.

The other 50-80% will get symptoms of the “flu:” they will have fevers, listlessness, coughing, and a snotty nose. Most dogs will recover with supportive treatment (antibiotics, perhaps nebulization/humidification, etc.). A small percentage of dogs will actually get pneumonia. These dogs are those at risk for death and support becomes more aggressive: hospitalization, intravenous fluid therapy etc. Most of these dogs will recover as well, as long as they receive proper care. Mortality rate is 5-8%

The incubation period is 2-5 days and the course of infection lasts 2-4 weeks. Because this is an emerging disease, few dogs will have immunity to it unless they have received one of the new vaccines. This means that any dog unvaccinated for influenza is a candidate for infection.

  • The point is not to ignore a coughing dog.
  • Do not allow your dog to socialize with coughing dogs. If your dog develops a cough, see your veterinarian.
  • If your dog develops a snotty nose, listlessness, and cough do not be surprised if your veterinarian wants to look at chest radiographs and considers hospitalization. More routine forms of Kennel Cough do not produce fever or listlessness.




Dogs that are infected will shed virus in body secretions whether or not they appear to be sick. Virus transmission can occur from direct contact with an infected dog or with its secretions. Kennel workers have been known to bring the virus home accidentally to their own pets. The virus persists on toys, bowls, collars, leashes etc. for several days. Infected animals intermittently shed virus for up to 24 days.



High fevers are treated with anti-pyretic medications or cool water baths, but generally antibiotics to control secondary infections are all that is needed. Pneumonia results from secondary bacterial infections (i.e. bacteria invading the lung after the virus has damaged the tissue and compromised its ability to defend itself). Pneumonia in the dog is virtually always secondary in this way (meaning that an initial condition damages the lung allowing bacterial invaders to settle in) and treatment is similar regardless of the cause. For details click here.



After a dog has recovered from canine influenza, immunity appears to last at least 2 years.



In a perfect world there would be a simple test that could be performed on a single sample and yield unequivocal results. There are two main ways to confirm canine influenza infection:

  1. PCR Testing
    PCR testing is a method of testing involving amplifying small samples of DNA to make them more easily detectable. A nasal or throat swab is used for the sample but timing is crucial; the sample must be obtained 3-4 days after the onset of symptoms. Because timing is difficult, this method may not be accurate.
  2. Serology
    Here, a blood sample is tested for antibodies against canine influenza virus and the antibody level is compared to that from a second sample taken later. The first sample is drawn within one week of the onset of symptoms and the second sample is drawn 2-3 weeks later. If the second sample shows a four-fold increase in antibody level, this indicates a true infection has occurred. This means, inconveniently, that diagnosis cannot be confirmed for several weeks after the dog has gotten sick. A single sample with antibodies present only indicates that the dog has been exposed to influenza and does not clarify whether the infection is current, recent or in the long past.
    Negative test results are not felt to rule out a diagnosis of canine influenza infection.
  3. ELISA test
    Test kits are available to detect human flu virus in a matter of minutes at the doctor's office. These kits will also work to detect canine influenza virus in a canine nasal swab. False negatives may be a problem but a positive result can be trusted. Some veterinary offices are using these kits for their coughing canine patients.



No. In fact, H3N8 requires a separate vaccine from H3N2. There is no vaccine that covers both strains though both Merck and Zoetis manufacture vaccine against H3N8 and Zoetis presently has conditional approval on a vaccine for H3N2. Currently, vaccination against H3N2 is of some controversy for, even though there was an outbreak of extreme scale in this country, many feel that now that containment has been achieved it is unlikely that it will occur again. (H3N2 had previously been limited to South Korea but was introduced through rescue programs that bring Asian dogs to the U.S. for better life and adoption).  Vaccination for H3N8 should be considered for dogs at increased exposure risk: dogs that attend doggie daycare, who regularly go to the grooming parlor, who travel for show purposes, or who regularly play with other dogs at the park. The flu is no fun for anyone and has potential to be life-threatening. In most influenza situations, it is the very young and very old who experience the most severe disease but during the H3N2 outbreak, most infected dogs were healthy and middle-aged.



Earlier in this article, we discussed "antigenic drift." One of the defining features of the influenza viruses is their ability to mutate. Every time the virus infects a new person, the virus rapidly replicates as part of the infection process. Every time the virus replicates, there is potential for mutation. Some mutated forms of virus are ineffective and die off while others are very effective and continue spread through the community. It is because of all this contagion and replication that the human influenza virus is able to mutate into a new version of itself each flu season. Each new version is different enough that the old vaccine is probably not going to work and a new vaccine must be developed.

The dog situation is a bit different. A human being can easily have contact with hundreds of people daily depending on the nature of their job and whether they use public transportation. This is a huge potential to spread disease. The average dog, however, lives in his home mostly and usually does not see other dogs unless he goes out into a public area where there are many other dogs. This amounts to far less potential for the virus to spread and mutate and so far the original H3N8 vaccines are still effective. As noted, the H3N2 vaccine is new and is still conditionally licensed.



People cannot get infected by this virus. Influenza viruses are specific for their host species and require a dramatic mutation in order to jump species.? One should not be concerned about getting an influenza infection from a dog, horse, or any other species other than a fellow human being.



For additional information, see the links below:

Merck Animal Health has information on the Canine Influenza Vaccine, H3N8 at:

Zoetis page for pet owners on Canine Influenza:

Page last updated: 2/22/2017