Update for Veterinarians, April, 2010
Canine Influenza Virus:
The Canine Influenza Virus subtype H3N8 was first identified in January,
2004 during an outbreak of respiratory infection amongst Florida 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 in 2007, highly pathogenic avian influenza
(HPAI) H5N1 in southeast Asia in 2006 and the new North American H1N1 in
New York State in 2009.
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 diagnosis of canine influenza subtype H3N8 infection at this time is
most reliably done by detecting antibodies to the virus. We recommend
testing acute and convalescent sera. Canine influenza serology testing
is performed at Cornell University. Collection of oropharygeal swabs or
tracheal wash samples on febrile dogs early in the course of the disease
may be submitted on ice pack to WADDL for detection of viral RNA by
polymerase chain reaction assay (PCR) for all Influenza A virus
subtypes. Tissues (lung, bronchiolar lymph nodes) from dogs that have
died acutely can 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.
There is a
newly released 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 risks associated 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. The duration of protective immunity (DOPI) and
frequency of boosters have not yet been well defined for the canine
influenza vaccine. However, an annual booster is recommended by the
manufacturer (Intervet data, 2009). This vaccine is not
cross-protective to the other influenza subtypes infecting dogs, and
there are no vaccines currently available for the other subtypes
available for use in dogs.
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.
Crawford PC, et al. Transmission of equine influenza virus to dogs.
Science 310: 482-485, 2005.
Daly JM, et al. Transmission of equine influenza virus to english
foxhounds. Emerg. Infect Dis 14: 461-464, 2008.
Deshpande MS, et al. Experimental reproduction of canine influenza virus
H3N8 infection in young puppies. Vet Therapeutics 10: 29-39, 2009.
Dubovi EJ and Njaa BL. Canine Influenza. Vet Clin No Amer. Small Ani
Prac 38: 827-835, 2008.
Harder et al. Influenza virus infections in dogs and cats. Vet.
Imm. And Immunopath. 134: 54-60, 2010
Jung et al. Pathology in dogs with experimental canine H3N2 influenza
virus infection. Res. In Vet. Sci. Article in press, 2010.
Solomon R and Webster RG. The influenza virus enigma. Cell 136:402-410,
Songserm et al. Fatal avian influenza A H5N1 infection in a dog. Emer.
Inf. Dis. 12:1744-7, 2006
Yamanaka T, et al. Interspecies transmission of equine influenza virus
(H3N8) to dogs by close contact with experimentally infected horses. Vet
Microbiol 139:351-355, 2009.
Compiled by J. Evermann and C. Huntsberry, WADDL