A new, highly contagious and occasionally fatal canine influenza is spreading among the nation's racing greyhound tracks, and also has infected dog boarding and training kennels, pet shops, pounds and shelters, and potentially other venues where dogs gather. The extent of spread is unknown but cases so far have been identified in pet dogs from the eastern and western regions of the country.

The outbreaks of severe respiratory disease in racing greyhounds have been determined to be caused by an influenza virus closely related by adaptive evolution to the H3N8 equine influenza virus. Dr. Cynda Crawford, University of Florida College of Veterinary Medicine, first recognized the disease at a Florida greyhound track in early 2004. The influenza virus cause was then identified by Dr. Ed Dubovi, Cornell University Animal Health Diagnostics Center from specimens taken by Dr. Crawford. Genomes of this canine influenza virus form a monophyletic group, consistent with a single interspecies virus transfer. It is not related to typical human influenza strains or to the H5N1 avian flu that has killed people in Asia. According to Dr. Ruben Donis, Chief of Molecular Genetics, Influenza Branch of the Centers for Disease Control and Prevention, who is tracking the canine illness, equine influenza virus has been in horses for > 40 years, but there has been no case of human infection with this virus. There also have been no known cases of the canine influenza virus infecting humans. Dogs affected by this virus have rising antibody titers associated with respiratory disease, and the disease can be transmitted by experimental inoculation studies. It is likely that this virus will spread quickly in the pet dog population throughout the United States.

Dr. Crawford indicates that the influenza spreads most easily by aerosol transmission where dogs are housed together, but that it could also be passed on the street, in dog runs, or even by humans transferring it from one dog to another. Kennel workers have carried the virus home to their own pets. Affected tracks and kennels have been forced to shut down for about three weeks during disinfection. Quaternary ammonium or 10% bleach should kill the virus, but it is highly contagious, so any dogs suspected of a kennel cough-like disease should be kept isolated.

Because dogs have no natural immunity to this newly emerged virus, virtually every exposed animal becomes infected. The incubation period is 2-5 days, and infected dogs may shed virus for 7-10 days after onset of clinical signs. About 80% of infected dogs will likely develop symptoms such as cough and nasal discharge for 10-21 days, often mistaken for a more severe form of "kennel cough". The remaining 20% are asymptomatic, although they may still shed virus.

Both the kennel cough respiratory disease complex and canine influenza can cause coughing and gagging for up to three weeks, but dogs with canine influenza can spike fevers as high as 106¡F and have persistent nasal discharge. Some dogs have subclinical or inapparent infections and some have severe infections, despite use of antibiotics and cough suppressants. A few will progress to develop bronchopneumonia; radiographs may show consolidated lung lobes; and some of those cases will be fatal. Prompt treatment with antibiotics and fluid reduce the pneumonia fatality rate. The fatality rate is ~1-5%, but could be as high as 10% among puppies and older dogs.

Diagnosis is based upon detecting rising canine influenza antibody titers in paired sera collected during the acute (7 or more days after infection) and convalescent (2 weeks later) phases of infection. The lungs and distal trachea of dogs dying from pneumonia complications of the disease can be examined for influenza virus, however virus isolation, to date, has yielded many false negative results.

There is currently no vaccine for canine influenza, but one should be relatively easy to develop.

The secondary stages of pneumonia are treated with broad-spectrum antibiotics, fluid replacement, and supportive care.

References: Univ FL/College Vet Med Veterinary Advisory-Canine Influenza Virus, Aug 18, 2005; D.G. McNeil, Jr, Rubenstein, C. NY Times, Sept. 22, 2005; P. C. Crawford et al, Transmission of Equine Influenza Virus to Dogs, Science, On-Line, Sept. 26, 2005; CDC Media Briefing on Canine Influenza, Sept. 26, 2005.