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CANINE INFLUENZA VIRUS
AND THE NEW CANINE INFLUENZA VACCINE

By

ERIC M. DAVIS, DVM, DIPL. AVDC

The following information has been compiled from responses by several veterinary virologists, internists, and infectious disease experts from across the United States on the Veterinary Information Network to questions raised about canine influenza virus (CIV) and the new vaccine marketed by Intervet/Schering-Plough Animal Health.  Additional background information was provided by the American Veterinary Medical Association.

The canine influenza virus, or H3N8, is believed to have mutated from an equine influenza virus to a virus capable of infecting dogs approximately 5 years ago.  There has been no evidence to date that the virus can infect other mammals including humans.  Because this is a newly emerging infection, virtually all dogs that get exposed to CIV become infected, but approximately 20% of infected dogs exhibit no clinical signs, and of the remaining 80% of infected dogs, most exhibit only mild signs of disease.  Even if no clinical signs are observed, the virus is still shed in nasal and respiratory secretions and the disease can be spread to other dogs by aerosolized spread of respiratory secretions or via contaminated food dishes, leashes, kennels, and human hands. The virus can remain infective on inanimate surfaces for up to 48 hours and on human hands for approximately 12 hours. Dogs with mild disease have a persistent cough for 10-21 days.  Some patients may also have a low-grade fever and nasal discharge. Fatalities are uncommon but have been reported, and mortality is usually associated with the subsequent development of bacterial pneumonia. 

Intervet/Schering-Plough has recently introduced, and is heavily marketing, a new vaccine that has been conditionally licensed by the USDA, meaning that preliminary data suggests that the product may be effective.  The license has been granted based upon the manufacturer’s claim that the vaccine “lessens the frequency and severity of lung lesions, reduces viral shedding, and reduces duration of coughing”.  In other words, vaccinated dogs may still become infected by CIV, but their signs of illness may be milder and shorter in duration compared to non-vaccinated dogs.  The vaccine is made from killed Canine Influenza virus particles, and two doses are required in order to be protective.  The first dose introduces the viral particles to the dog’s immune system so that antibodies can be made to recognize, seek out, and inactivate the virus should CIV exposure ever occur.  The second dose of vaccine, given three weeks from the first dose, stimulates the so-called, anamnestic response or “remember me” response, so that a lot of antibody is made, ready to pounce on and inactivate any live CIV particles that enter the dog’s body.  Vaccinated dogs should have protection conferred by the vaccine, approximately ten days after the second injection of the vaccine has been given.

At the present time, the only scientific data regarding the effectiveness of this vaccine comes from the manufacturer.  No independent scientific study has corroborated the findings of Intervet-Schering-Plough as to the efficacy of the vaccine or its safety in a large number of dogs.

You can be certain that I will be vigilant in monitoring for any reports of CIV cases in the Central New York area.  However, given that Canine Influenza Virus infection causes mild illness in the majority of patients, and given that the vaccine does not prevent CIV infection, but lessens severity and duration of illness, I am not recommending vaccination against CIV at this time.  That opinion is subject to change as more information about this new, emerging disease becomes available.