As of January 2016, there are two canine primary influenza viruses in the dog population for owners and veterinarians to contend with. These include subtype H3N8 (2004, Florida) and subtype H3N2 (2015, Chicago).
The Canine Influenza Virus subtype H3N8 was first identified January, 2004 in Florida during an outbreak of respiratory infection among racing greyhounds. This virus is an Influenza A virus and is closely related to the equine influenza virus. In fact, canine influenza is now considered to have originated when the equine influenza virus “jumped” species during the 1990’s. Since the first identified outbreak, there have been reports of respiratory disease in racing dogs as well as shelter dogs in Florida, New York, New Jersey, Pennsylvania, Virginia, and Colorado. More recently there have been documented reports of additional influenza subtypes infecting dogs which include avian-origin influenza H3N2 in South Korea and China in 2007.
1. CLINICAL SIGNS
The most well characterized subtype in the United States, H3N8, can cause two clinical syndromes. The majority of dogs develop the milder syndrome, involving a cough that persists for 10 to 21 days despite therapy with antibiotics and cough suppressants. This syndrome can also include purulent nasal discharge and a low-grade fever. The more severe disease involves pneumonia, including a high fever (104º to 106º F) and increased respiratory rate and effort. Thoracic radiographs may show consolidation of lung lobes. Dogs with pneumonia often have a secondary bacterial infection and have responded best to a combination of broad-spectrum, bactericidal antibiotics and intravenous fluid therapy. Because this is a new pathogen almost all dogs are susceptible to infection and an estimated 80% of exposed dogs develop clinical signs. The case-fatality rate in the initial outbreak was high (8 of 22 ill dogs died, for a 36% case-fatality rate), but since then case-fatality has been reportedly low (1 to 5%).
Incubation and infectious periods (H3N8 subtype):
Clinical signs appear two to five days after exposure. Infected dogs may shed virus for seven to 10 days from the onset of clinical signs. An estimated 20% of infected dogs will not show clinical signs and can become asymptomatic sources of infection.
Other Influenza A viruses also affect dogs. Influenza subtype H3N2-induced disease is characterized as causing severe lower respiratory tract disease and appears to have a high mortality rate. Differences in pathogenicity are believed to be due to the whole virus adaptation from an avian virus into a naïve population. HPAI H5N1 has been associated with severe respiratory disease and fatal infection in a dog in Thailand after ingestion of infected chicken tissue. Lastly, influenza subtype H1N1 infection in New York State was reported in 2009 causing pneumonia but the dog recovered with supportive care. The owner of the dog was previously diagnosed with H1N1 and it is believed transmitted the virus to the dog.
The most reliable way to detect influenza virus in dogs is through a PCR test on either nasal swabs or respiratory tissue. WADDL employs a generic PCR test which detects the highly conserved matrix gene (Influenza A). While dogs
usually have either the H3N8 or H3N2 virus, tests unique to these viruses would miss a potential influenza infection caused by another flu strain such as H5N1. The WADDL PCR assay will detect H5N1, H3N8, H3N2, or any other novel strain
which might circulate in dogs.
If possible, please use a plastic shaft dacron or rayon swab rather than a wood shaft cotton swab when collecting nasal samples (wood and cotton may inhibit PCR). Do not submit swabs in Amies or Port-a-Cul swabs for PCR, as these contain PCR inhibitors. Swabs may be submitted either dry or in approx. 200 ul of saline in a clean container such as a red top tube. Tissues (lung, bronchiolar lymph nodes) from dogs that have died acutely can also be submitted to WADDL, a portion fixed in buffered formalin, and fresh tissues submitted on ice pack for PCR. Contact WADDL at (509) 335-9696 if any questions arise.
Collection of serum samples may be desirable in case clients want to determine the subtype of canine influenza. We recommend collecting acute and convalescent sera, and refrigerating these. If influenza virus is detected by PCR, the serum can be sent to Cornell University for the subtyping assays, which detect the H3N8 or H3N2 strains only.
NOTE: Remember that the standard canine respiratory disease pathogens should still be part of your differential diagnosis. We recommend collecting and submitting additional samples for a more complete diagnostic work‐up that would include cultures for bacteria and Mycoplasma. You can write on the Accession Form “Please perform aerobic and Mycoplasma cultures if PCR is negative for canine influenza.” Bronchoalveolar lavage, transtracheal wash or fresh lung samples from necropsies are preferred for this Bacteriology testing.
There is a vaccine available for the protection against clinical signs associated with canine influenza subtype H3N8. At this time it is regarded as a “non-core” vaccine in the Pacific Northwest, meaning that the vaccine is not routinely recommended
unless there were associated risks with horses, racetracks, greyhounds, etc. Dog owners travelling with their dogs to shows in the aforementioned states would be advised to see their veterinarian to decide if the risks warrant vaccination. The infection
rate and disease potential with canine influenza are much lower than canine distemper, canine parvovirus, and canine adenovirus, which are considered “core” vaccines for all dogs. It is important that dogs be updated on their vaccinations
with these core vaccines, since canine distemper, canine parvovirus, and canine adenovirus infections can cause profound immune suppression and add to the severity of any dog disease, including canine influenza. It is also recommended that dogs
be vaccinated for canine parainfluenza and Bordetella bronchiseptica, which can commonly lead to severe kennel cough in housed/comingled dogs.
There is a conditionally licensed vaccine available for H3N2, which means that it is safe to administer to dogs, but its true protective efficacy is not known at this time. Contact the Consulting Microbiologist at WADDL if questions arise at (509) 335-9696.
4. PUBLIC HEALTH CONCERNS:
There is no evidence that canine influenza subtype H3N8 and H3N2 can infect humans and thus does not pose a public health risk. H5N1 and H1N1 have been shown to infect humans however there have been no reports of dog to human transmission of the viruses.
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Revised by J. Evermann, D. Bradway, D. Diaz, WADDL
Updated for Veterinarians, June 2019