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.

What is Canine Influenza?

Group of influenza viruses

The 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. 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.

Unless a mutation occurs as described, influenza virus strains are specific to host species. Human influenza only infects humans. Equine influenza only infects horses. Canine influenza only infects dogs.

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 has largely been 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.

What Happens to the Sick Dogs?

Infection rate is high but (depending on which report you read) 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 those at risk for death and thus support becomes more aggressive: hospitalization, intravenous fluid therapy etc. Most of these dogs will recover as long as they receive proper care. 

Mortality (death) rate is 5 to 8 percent.

The incubation period is 2 to 5 days and the course of infection lasts 2 to 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.
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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?

Fevers are treated with anti-pyretic medications or cool water baths. 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 dogs is virtually always secondary in this way (meaning that an initial condition damages the lung and allows bacterial invaders to settle in) and treatment is similar regardless of the cause.

One treatment that might be different in this disease compared to 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.

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 three main ways to confirm canine influenza infection.

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 to 4 days after the onset of symptoms. Because timing is difficult, this method is not commonly recommended.
  
Serology
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 to 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.
  
ELISA 
ELISA 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 Intervet/Schering and the other from Pfizer Animal Health). 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.

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.

Intervet/Schering-Plough Animal Health has information on the Canine Influenza Vaccine, H3N8.