Merck Manual

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Professional Version

Canine Influenza (Flu)


Caroline C. Tonozzi

, DVM, DACVECC, Mission Veterinary Partners

Reviewed/Revised Feb 2022 | Modified Oct 2022

Canine influenza is spread by dog-to-dog by aerosolization of two strains of the canine influenza virus (H3N8 and H3N2), as well as by contaminated objects and fomites. Most infections are mild with a persistent cough. Infected dogs can also be asymptomatic and shed virus. Treatment is supportive, with control measures consisting of routine infection control practices, hygiene measures, and vaccination.

Etiology, Epidemiology, and Transmission of Canine Influenza

Two strains of the canine influenza virus (CIV) have been identified, H3N8 and H3N2. The H3N8 strain was first identified in the USA in 2004 and had evolved from equine influenza. The H3N2 strain was identified as originating from Asia as an Asian avian influenza. An outbreak of H3N2 was identified in Chicago in 2015. Outbreaks are most common when dogs have not been vaccinated and are in close contact, eg, kennels, shelters, dog parks.

CIV is transmitted dog to dog by aerosolization of the virus, via contaminated objects (eg, water bowls), and by other fomites (people). The incubation period is 1–5 days after exposure. Dogs are most contagious 2–4 days after infection and can be contagious before showing clinical signs. Approximately 20% of infected dogs remain asymptomatic but can still shed virus. Dogs should be considered contagious up to 4 weeks after exposure.

Clinical Findings and Diagnosis of Canine Influenza

  • Clinical signs

  • Serologic testing

Dogs of any breed or age can be affected. Most exposed dogs (approximately 80%) develop mild infection, with a cough that persists 1–3 weeks that may be similar to the cough of canine infectious tracheobronchitis Kennel Cough Kennel cough results from inflammation of the trachea. It is a mild, self-limiting disease but may progress to bronchopneumonia in puppies or to chronic bronchitis in debilitated adult or aged... read more . Other possible clinical signs include ocular and nasal discharge, sneezing, fever, lethargy, and anorexia. Some dogs become severely ill, with high fever (104º–106ºF), pneumonia Pneumonia in Dogs and Cats Pneumonia is an inflammation of the pulmonary parenchyma that results in respiratory disturbance. It can be caused by a variety of organisms, including viruses, bacteria, fungi, parasites, and... read more , and secondary bacterial infection. The mortality rate is 1–5%.

There is no rapid test for specific diagnosis of canine influenza. Nasal or pharyngeal swabs from dogs ill for < 3 days can be submitted for PCR testing. After 4 days of illness, PCR testing may result in false-negatives, because the time of maximal virus shedding has passed. Serum antibodies to CIV may be detected as early as 7 days after onset of clinical signs. The best method for confirmation of infection is serologic testing with acute and convalescent serum samples. Thoracic radiographs may or may not have changes unless there is a secondary bacterial pneumonia.

Treatment, Prevention, and Control of Canine Influenza

  • Supportive care

  • Infection control practices

  • Vaccination

Treatment is largely supportive; most dogs recover in 2–3 weeks. Antibiotics have no effect on CIV but may be used to treat secondary bacterial infections. Antiviral therapy (oseltamivir) is neither approved nor recommended for use in dogs with CIV.

Vaccines for CIV are all killed vaccines and specific for the H3N8 or the H3N2 strain of influenza A and can be administered separately. A vaccine combining the two virus strains into a single vaccine is also available. Two doses of vaccine are necessary to develop an effective immune response.

Routine infection control practices and good hygiene within facilities are key to preventing spread. Proper isolation precautions should be used, such as keeping animals at a minimum of 20 feet from other animals, as well as using proper handwashing techniques. CIV can persist in the environment for 1–2 days but is readily killed by common disinfectants. There is no evidence of transmission of CIV (H3N2) from dogs to people.

Key Points on Canine Influenza

  • Appropriate isolation practices must be used with a dog known or suspected to have CIV. Personal protective equipment must be worn when handling dogs with CIV. Hands should be washed and a new gown and gloves worn between handling each patient.

  • Although most patients with CIV are dogs, cats have also been infected with H3N2.

  • Clinical signs (cough) may persist for up to 30 days.

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