(Information taken from AVMA website) 8/2017
Canine influenza (CI), or dog flu, is a highly contagious viral infection affecting dogs. Influenza viruses are able to quickly change and give rise to new strains that can infect different species. At present, two strains of canine influenza virus have been identified in the United States: H3N8 and H3N2. There is no evidence that either strain of canine influenza (H3N8, H3N2) can infect humans.
Canine H3N8 influenza was first identified in Florida in 2004 in racing greyhounds. It is thought this strain developed from an equine H3N8 influenza strain that jumped from horses to dogs. Since being detected in 2004, canine H3N8 influenza has been identified in dogs in most U.S. states and the District of Columbia.
Canine H3N2 influenza was first identified in the United States in March 2015 following an outbreak of respiratory illness in dogs in the Chicago area. In May 2017, canine H3N2 influenza was diagnosed in dogs in Florida, Georgia, North Carolina, South Carolina, Texas, Kentucky, Tennessee, Missouri, Louisiana, and Illinois. This was the same strain of H3N2 involved in the 2015 outbreak in Chicago.
Canine influenza is transmitted through droplets from coughing, barking and sneezing. Dogs in close contact (kennels, groomers, day care, shelters) are at increased risk of infection. The virus can remain alive for up to 48 hours, and spread indirectly through objects (e.g., kennels, food and water bowls, collars and leashes) or people that have been in contact with infected dogs. It is important to clean and disinfect objects that have been in contact with an infected dog to avoid exposing other dogs to the virus. Likewise, people who have been in contact with an infected dog should wash their hands and clean their clothing to avoid spreading the virus.
H3N8 has an incubation period of 1 to 5 days, with clinical signs in most cases appearing 2 to 3 days after exposure. Dogs infected with H3N2 may start showing respiratory signs between 2 and 8 days after infection. Dogs are most contagious during the incubation period and shed the virus even though they are not showing clinical signs of illness. Some dogs may show no signs of illness, but have a subclinical infection and shed the virus.
Canine influenza virus infects and replicates inside cells in the respiratory tract causing an inflammatory response (rhinitis, tracheitis, bronchitis and bronchiolitis) resulting in death of the cells. This predisposes the respiratory tract to secondary bacterial infections that contribute to nasal discharge and coughing.
Virtually all dogs exposed to CI become infected, with approximately 80% developing clinical signs of disease. The approximately 20% of infected dogs that do not exhibit clinical signs of disease can still shed the virus and spread the infection.
The majority of infected dogs exhibit the mild form of canine influenza. The most common clinical sign is a cough that persists for 10 to 21 days despite treatment with antibiotics and cough suppressants. Affected dogs may have a soft, moist cough or a dry cough similar to that induced by kennel cough. Nasal and/or ocular discharge, sneezing, lethargy and anorexia may also be observed. Many dogs develop a purulent nasal discharge and low fever (103-104oF). The nasal discharge is usually caused by secondary bacterial infections, including Pasteurella multocida and mycoplasma species.
Some dogs are more severely affected and exhibit clinical signs of pneumonia (diagnosed with chest x-rays), such as a high-grade fever (104°F to 106°F) and increased respiratory rate and effort. Although most dogs recover without incident, deaths due to H3N2 have been reported.
Canine influenza cannot be diagnosed solely by clinical signs (coughing, sneezing and nasal discharge) because these clinical signs also present with other canine respiratory illnesses. Tests are available to diagnose and identify the strain of canine influenza virus. Tests include: virus isolation, immunoassays to detect virus antigen, PCR to detect virus nucleic acid, and serology for antibodies specific to the virus.
Treatment for CI, as for most viral diseases, is largely supportive. Most dogs recover from canine influenza within 2-3 weeks. Secondary bacterial infections, pneumonia, dehydration, or other health factors may necessitate additional diagnostics and treatments including, but not limited to:
- Antimicrobials for known or suspected secondary bacterial infections.
- anti-inflammatory medications as needed to reduce fever and inflammation.
- Fluids to help correct dehydration or maintain hydration.
To prevent transmission of the virus, dogs infected with canine H3N2 influenza as well as other dogs in the household should be isolated for 4 weeks.
Canine influenza virus is not widespread in the dog population and many dogs have never been exposed to the virus. The morbidity rate (the number of exposed animals that develop disease) estimated at 80%. The mortality (death) rate is low; less than 10%. Deaths occur mainly in dogs with the severe form of disease.