Causative agent
Canine influenza (CI) is caused by a type A Orthomyxovirus. The viruses are 80-120 nanometers (nm) in diameter, and consist of a core of eight separate pieces of single-strand ribonucleic acid (RNA) surrounded by a spiked arrangement of glycoproteins. The canine influenza virus has been classified as H3N8, based on the amino acid composition of the hemagglutinin (H) and neuraminidase (N) glycoproteins in the lipid outer layer of the capsid. These glycoproteins facilitate entry into and release from host cells, and are important targets for antibodies against the virus (generated as a result of infection or response to vaccination). CI 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.

Natural distribution
The first recognized outbreak of canine influenza is believed to have occurred in racing greyhounds in January 2004 at a track in Florida. From June to August of 2004, outbreaks of respiratory disease were reported at 14 tracks in 6 states (Florida, Texas, Alabama, Arkansas, West Virginia, and Kansas). Between January and May of 2005, outbreaks occurred at 20 tracks in 11 states (Florida, Texas, Arkansas, Arizona, West Virginia, Kansas, Iowa, Colorado, Rhode Island, and Massachusetts). Florida, Colorado, and the New York City region have become endemic for CI, and Pittsburgh (PA) and Lexington (KY) may also be emerging as endemic areas. Sporadic seropositive dogs have been found in many other states.

Transmission
Canine influenza is spread via aerosolized respiratory secretions and contaminated inanimate objects and people moving between infected and uninfected dogs. The incubation period is usually two to five days. Infected dogs shed virus for seven to 10 days after clinical signs first appear. 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. 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.

Clinical signs
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 brochiseptica/parainfluenza virus complex. Clinical disease may be mild or severe.

Most dogs (80%) 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.

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 may reveal consolidation of lung lobes.

Diagnosis
There is no reliable rapid test for diagnosis of acute canine influenza virus infection. The most reliable and sensitive method for confirmation of infection is serologic testing. Antibodies to canine influenza virus may be detected as early as seven days after onset of clinical signs and paired acute and convalescent serum samples are necessary for diagnosis of recent infection. Convalescent samples should be collected at least two weeks after collection of the acute sample. If an acute sample is not available, a convalescent sample will indicate whether a dog has been infected 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.

The collection of pharyngeal swabs or tracheal wash samples from febrile dogs very early in the course of the disease may also provide confirmation via polymerase chain reaction (PCR) or virus isolation.

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. For detailed information on sample submission, visit http://www.diaglab.vet.cornell.edu/issues/civ.asp.

Treatment
As for all viral disease, treatment is largely supportive. Good husbandry and nutrition may assist dogs in mounting an effective immune response. In the milder form of the disease, a thick green nasal discharge most likely represents a secondary bacterial infection that usually resolves quickly after treatment with a broad-spectrum bactericidal antimicrobial. Pneumonia in more severely affected dogs responds best to a combination of broad-spectrum bactericidal antimicrobials (to combat secondary bacterial infections) and maintenance of hydration via intravenous administration of fluids.

Currently available antiviral drugs are approved for use in humans only and little is known about their use 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).

Morbidity and mortality
The morbidity rate (the number of exposed animals that develop disease) associated with canine influenza is estimated at 80%. The case fatality rate (the number of clinically ill animals that die from the disease) to date has ranged from 5 to 8%.

Prevention and control
At this time, no vaccine is available to protect against canine influenza. Routine biosecurity precautions are key to preventing spread. The kennel cough (Bordetella bronchiseptica/canine parainfluenza complex) vaccine does not provide cross protection for CI.

The canine influenza virus is an enveloped virus that appears to be easily killed by disinfectants in common use in veterinary, boarding, and shelter facilities, such as quaternary ammonium compounds and bleach solutions. Protocols should be established for thoroughly cleaning and disinfecting cages, bowls, and other surfaces between uses. Employees should wash their hands with soap and water before and after handling each dog; after coming into contact with dogs' saliva, urine, feces, or blood; after cleaning cages; and upon arriving at and before leaving the facility.

Isolation protocols should be rigorously applied for dogs showing signs of respiratory disease. Clothing, equipment, surfaces and hands should be cleaned and disinfected after exposure to dogs showing signs of respiratory disease. 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.

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