CIV represents a very rare event in adaptive evolution; the entire genome of the H3N8 equine influenza virus was transferred to dogs, and the virus adapted to the canine species to emerge as a new canine-specific virus.1 Although the virus spreads readily from dog to dog, there is no evidence to support that it can be transmitted from dogs to humans.
The incubation period is usually two to four days from exposure to onset of clinical signs. The highest amounts of viral shedding occur during this time; therefore, dogs are most contagious during this 2-4 day incubation period when they are not exhibiting signs of illness. 4 Virus shedding decreases dramatically during the first 4 days of illness but may continue up to 7 days in most dogs and up to 10 days in some dogs.4
Because this is a newly emerging pathogen, all dogs, regardless of breed or age, are susceptible to infection and have no naturally acquired or vaccine-induced immunity when first exposed to the virus. If the virus enters a kennel or other closed group, a high percentage of the dogs may become infected, and most of these dogs will be symptomatic.2 Approximately 20-25% of infected dogs are expected to remain asymptomatic, but can still shed the virus and spread the virus. Although most dogs have a milder form of canine influenza and recover without complications, some may develop severe pneumonia.1
Virtually all dogs that are exposed become infected with the virus, but approximately 80% develop clinical signs of disease. The approximately 20% of infected dogs that do not exhibit clinical signs of disease can still shed the virus and can spread the infection.5
Like other mammalian influenza viruses, CIV causes an acute respiratory infection in dogs. However, unlike human influenza, CIV is not a "seasonal" flu – infections can occur year-round.2,4 Canine influenza virus causes clinical disease that mimics kennel cough. As a result, infection with the virus is frequently mistaken for infections caused by Bordetella bronchiseptica/parainfluenza virus complex. Clinical disease may be mild or severe.6
The majority of infected dogs exhibit the mild form of CI. In the mild form, the most common clinical sign is a cough that persists for 10 to 21 days despite treatment with antibiotics and cough suppressants. Most dogs have a soft, moist cough, whereas others have a dry cough that is similar to that induced by Bordetella bronchiseptica/parainfluenza virus infection. Many dogs have a purulent nasal discharge and a low-grade fever. The nasal discharge is usually caused by secondary bacterial infections, including Pasteurella multocida and mycoplasma species.7
Some dogs are more severely affected with clinical signs of pneumonia, such as a high-grade fever (104°F to 106°F) and increased respiratory rate and effort. Thoracic radiography (chest x-rays) may reveal consolidation of lung lobes.1
Antibodies to canine influenza virus may be detected in the blood as early as seven days after onset of clinical signs, and the virus may be identified in nasal or pharyngeal swabs during the first 4 days of illness. The most reliable and sensitive method for confirmation of infection is serologic testing. Paired acute serum samples (taken within the first 7 days of illness) and convalescent serum samples (taken 10-14 days later) are necessary for diagnosis of recent infection. If an acute sample is not available, a convalescent sample will indicate whether a dog has been exposed to the virus at some point in the past. A diagnosis of CI is made based on a four-fold increase in antibody titer from the acute to the convalescent sample.7
If a dog has been ill for less than 4 days, nasal and pharyngeal swab submission for Polymerase Chain Reaction (PCR) testing can be performed. If the PCR indicates a positive result, the dog is most likely infected. Negative PCR results may be falsely negative if the swabs are not collected during the time of peak virus shedding. After 4 days of illness, PCR results are less likely to be reliable.4,8 Serology should be performed to confirm infection, especially if the PCR results are negative and the case is highly suspicious for CI infection.4
Other diagnostic options applicable to dogs that have died from pneumonia are viral culture and PCR analysis using fresh (not formalin-preserved or frozen) lung and tracheal tissues. Virus detection in respiratory secretion specimens from acutely ill animals by use of viral culture, PCR analysis, or rapid chromatographic immunoassay is possible, but usually unrewarding. The Cornell Animal Health Diagnostic Center is currently accepting samples for analysis.
Currently available antiviral drugs are approved for use in humans only and little is known about their use, efficacy and safety in dogs. Veterinarians who use approved drugs in a manner that is not in accord with approved label directions (e.g., use of an antiviral drug only approved for use in humans) must follow the federal extralabel drug use regulations of the Animal Medicinal Drug Use Clarification Act (AMDUCA).
Employees should wash their hands with soap and water:
Isolation protocols should be rigorously applied for dogs showing clinical signs of respiratory disease. Sick or exposed dogs should be isolated for two weeks. Clothing, equipment, surfaces and hands should be cleaned and disinfected after exposure to dogs showing signs of respiratory disease.2 Dog owners whose dogs are coughing or exhibiting other signs of respiratory disease should not participate in activities or bring their dogs to facilities where other dogs can be exposed to the virus.7
In May 2009, the USDA approved the licensure of the first influenza vaccine for dogs developed by Intervet/Schering Plough Animal Health Corporation. The canine influenza vaccine contains inactivated whole virus.8,9
The vaccine is intended as an aid in the control of disease associated with CI virus infection. Although the vaccine may not prevent infection altogether, efficacy trials have shown that the vaccination may significantly reduce the severity and duration of clinical illness, including the incidence and severity of damage to the lungs.4 In addition, the vaccine reduces the amount of virus shed and shortens the shedding interval; therefore, vaccinated dogs that become infected develop less severe illness and are less likely to spread the virus to other dogs.9 These benefits are similar to those provided by influenza vaccines used in other species, including humans.
The canine influenza vaccine is a "lifestyle" vaccine, and is not recommended for every dog.4 In general, the vaccine is intended for the protection of dogs at risk for exposure to the CI virus, which include those that either participate in activities with many other dogs or are housed in communal facilities, particularly where the virus is prevalent. Dogs that may benefit from canine influenza vaccination include those that receive the kennel cough (Bordetella/parainfluenza) vaccine, because the risk groups are similar.5 Dog owners should consult with their veterinarian to determine whether their dog's lifestyle includes risks for exposure to the CI virus, and if the vaccine is appropriate for their dog.6, 9
We would like to thank Dr. Cynda Crawford from the University of Florida College of Veterinary Medicine for her contributions to the AVMA's canine influenza resources.
Links to more information about canine influenza
Control of Canine Influenza in Dogs: Questions, Answers and Interim Guidelines (AVMA)
Canine Influenza: Podcast by Dr. Cynda Crawford (AVMA)
Key Facts about Canine Influenza (Centers for Disease Control and Prevention)
Canine Influenza: Frequently Asked Questions by Dog Owners (University of Florida College of Veterinary Medicine)
Canine Influenza Fact Sheet (Iowa State University)
Canine Influenza (University of California-Davis Shelter Medicine Program)
Canine Influenza Virus: Detection, Sampling and Statistics (Cornell University Veterinary Diagnostic Laboratory)
This information has been prepared as a service by the American Veterinary Medical Association. Redistribution is acceptable, but the document's original content and format must be maintained, and its source must be prominently identified. Please contact Dr. Kimberly May (800.248.2862, ext 6667) with questions or comments.