The canine influenza outbreak afflicting more than 1,000 dogs in Chicago and other parts of the Midwest is caused by a different strain of the virus than was earlier assumed, according to laboratory scientists at Cornell University and the University of Wisconsin.

Researchers at Cornell say results from additional testing indicate that the outbreak is caused by a virus closely related to Asian strains of influenza A H3N2 viruses, currently in wide circulation in southern Chinese and South Korean dog populations, since they were first identified in 2006. There is no evidence that the virus can be transmitted to humans.

The outbreak in the Midwest initially was attributed to the H3N8 strain of virus - which was identified in the U.S. dog population in 2004. This is the first time the H3N2 virus has been detected in North America. The Chicago outbreak suggests a recent introduction of the H3N2 virus from Asia.

Testing of clinical samples from the outbreak conducted at the New York State Animal Diagnostic Laboratory at Cornell revealed that the virus was influenza A. Further testing led researchers to believe there may be a new strain. Subsequent testing, carried out with the assistance of the Wisconsin Veterinary Diagnostic Laboratory, identified the new subtype as H3N2. The National Veterinary Services Laboratories in Ames, Iowa, is sequencing two H3N2 isolates, which were sent from Cornell, to facilitate rapid complete characterization of the viruses.

Both influenza strains can cause high fever, loss of appetite, coughing, nasal discharge and lethargy. The H3N2 virus may cause severe symptoms, though some infected dogs may not show symptoms at all.

H3N2 has caused infection and respiratory illness in cats.

It is not known if the current vaccine will provide any protection from this new virus, but it does protect against H3N8, which is also in circulation in some areas. Other preventive advice remains the same: In areas where the viruses are active, avoid places where dogs congregate, such as dog parks and grooming salons.

Owners of symptomatic dogs and cats should consult their veterinarians about testing and treatment.