It’s Not Just “Flu Season” for People…but for dogs, too!!!

 Dwight E. Cochran, D.V.M., MS
Member: American Academy of Allergy Asthma and Immunology

 

Each of the CIV epidemics ultimately derives from a single cross-species transfer event, with the H3N8 subtype from horses, while the H3N2 subtype is assumed to have arisen directly from a virus in an avian reservoir. A recent study showed that in Southern China dogs may be frequently infected by various swine, canine and other influenza viruses.1 


Both H3N8 and H3N2 CIV infections in dogs are associated with mild upper respiratory tract disease, often including frequent coughing and fever. Where it has been examined infection of the lungs may occur, and that is associated with more severe disease and sometimes death. The more severe disease is likely associated with mixed infections by other viruses or bacteria. The natural risks to other animals, including humans, are largely unknown. The H3N8 CIV appears to be restricted to dogs, and has not been reported to infect other animals, and does not infect horses. The H3N2 CIV may have a broader host range, and has been shown to infect ferrets, guinea pigs, and cats after experimental exposure. Experimental inoculation of Korean or USA strains of H3N2 CIV into swine resulted in poor replication, suggesting that sustained transmission of the virus after a canine-swine transfer is unlikely, despite swine being a common host of other H3N2 IAVs. Natural infection of cats by H3N2 CIV transferred from dogs has been documented in Korea and in the USA, but the cat outbreaks were largely confined to the shelter populations where they emerged. Despite high levels of viral replication and shedding the viruses have not undergone prolonged transmission among household cats, and the relatively unsocial nature of and lack of contact between populations of household cats suggests that a cat-adapted influenza virus would not transmit for very long. 

No infections by either CIV subtype has been reported in humans. However, human seasonal IAV subtypes may infect dogs occasionally, including both H1N1 (both seasonal and the 2009 pandemic strains) and H3N2 variants. None of these infections appears to result in much onward transmission among dogs, but this may provide opportunities for human IAVs to re-assort with CIVs during natural co-infections in dogs.


Vaccination Against Canine Influenza Viruses

Vaccines to both strains of influenza virus in dogs are available. Those are inactivated virus vaccines that are given as two doses, three or more weeks apart. Combined (bivalent) vaccines that contain both the H3N2 and H3N8 strains are also available. The most important virus is currently the H3N2 strain as that is still circulating widely in the USA. Vaccination is most recommended for use in dogs that attend doggy day cares, travel widely, spend time in kennels or frequently go to dog parks. NOTE: 2 DOSES OF VACCINE ADMINISTERED AT LEAST 2 WEEKS APART ARE NECESSARY TO CONFER ANY PROTECTIVE IMMUNITY. I ADMINISTER THE BI-VALENT VACCINE TO MY PATIENTS….


Summary and Conclusions

The influenza viruses that infect dogs are a threat to canine health, and it now appears that dogs are a susceptible host, so that we should consider influenza viruses in cases of contagious respiratory disease in dogs. The two CIVs both cause relatively uncomplicated upper respiratory tract diseases in dogs, with relatively short periods (3 to 4 days) of high level viral shedding, but with clinical symptoms continuing for several days or more. The viruses appear to require relatively close contact to give transmission, so that in most populations of household dogs the viruses are not maintained over long periods. However the viruses may be able to continue to spread for longer periods within large animal shelters, kennels, or in meat dog farms in Asia. The H3N8 virus is now only present in dogs at very low levels if it is present at all. The H3N2 virus has caused a number of widespread outbreaks, apparently due to the reintroduction of the virus from Asia. Vaccinations provide a level of protection against CIV infection or disease, and should be given to animals that are particularly likely to be exposed, or which might suffer more severe disease.


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