Canine Influenza (H3N8)

Influenza viruses of asssorted varieties have been the subject of concern for humans, wildlife, and domestic animals for many decades. Dogs were 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 the last weeks of September 2005 and continuing into October, numerous warnings to dog owners about a new lethal canine disease swept the Internet. Some of these warnings contained legitimate information while others contained half-truths or information that is simply wrong. We would like to take this time to sort out the facts from the theories from the misinformation. Here is what we hope is a helpful FAQ regarding this relatively new virus that has come to be considered part of the kennel cough (recently renamed CIRD for canine infectious respiratory disease) complex.

What is Canine Influenza?

Let's start with what an influenza virus is. Influenza viruses represent a specific type of virus. There are actually three types (genera) of influenza viruses: type A (including the canine influenza virus), type B, and the less closely related Type C. They 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 influenza virus has on its surface an assortment of proteins that 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. 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.

Molecular studies indicate that canine influenza represents a mutation from the equine influenza virus. Canine influenza was first confirmed in a racing greyhound in 2004 and was largely a concern of the racing greyhound industry, particularly in Florida.

Starting in April 2005, the canine influenza virus has been seen in the pet populations of many states outside of Florida, hence the recent barrage of email, news reports etc.

What Happens to the Sick Dogs?

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 get pneumonia. These dogs are the ones 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.

How is the Disease Transmitted?

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 should be considered contagious for 14 days.

How are Sick Dogs Treated?

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.

One treatment that might be different in this disease vs. other pneumonias or respiratory disease is oseltamivir (Tamiflu®). This is an antiviral medication used in treating human influenza and is helpful only if used early in the course of infection or in prevention of infection in exposed dogs. For more details on this medication click here.

Can Dogs get Reinfected?

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

How are Dogs Tested for Canine Influenza?

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 inion.

PCR Testing
PCR testing is a method of testing involving amplifying small samples of DNA to make them more easily detectable. A nasal 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 is not commonly recommended.

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.

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.

Does Vaccination against Kennel Cough (Bordetella) or Parainfluenza Offer any Protection against Canine Influenza?

No. These are all completely different infections; however, there are two canine influenza vaccines that have recently become available (one from Merck Animal Health and the other from Zoetis). Vaccination is recommended for dogs that board frequently, attend group training classes or events with other dogs, play regularly at the dog park or doggie daycare, or who go to the groomer consistently. If you think your dog is at risk, talk to your veterinarian about vaccination.

Why don't Dogs Need a Different Flu Shot each year like People do?

One of the defining characteristics of the influenza viruses is their ability to mutate. Influenza viruses not only have a high capacity to mutate but different strains can combine into a completely new strain within an individual who is infected with two different strains at the same time. Mutation can create enough genetic change within a strain to make the vaccine less effective. This phenomenon is called antigenic drift and, in people, would mean that last year's flu vaccine might afford some protection the following year and it might not. If a whole new strain is created, then the a whole new vaccine is needed and there will be no protection from previous immunization. A great deal of research goes into predicting the strain that will hit North America in every fall.

The dog situation is a bit different. Canine influenza is not nearly as common an infection in dogs as human influenza is among humans. The more infected individuals there are, the more mutation is happening. The relatively low incidence of canine influenza means less mutation (antigenic drift), minimal potential for concurrent infection with multiple strains (antigenic shift) and, best of all, the available vaccines will not require modification for a long time.

Can People get Infected?

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. You should not be concerned about getting an influenza infection from a dog, horse, or any other species other than a fellow human being.

Merck Animal Health has information on the canine influenza vaccine, H3N8.

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