In this Thursday, April 16, 2015 photo, Beverley Petrunich, co-owner of DoGone Fun, a day care and boarding facility, visits with some of her clients in Chicago. Experts say doggie day care contributed to an epidemic of dog flu in Chicago that is spreading in the Midwest. Petrunich says when the virus emerged in Chicago, her canine clients were hit hard. She consulted with vets, then closed for five days in late March to control the outbreak. (AP Photo/M. Spencer Green)

Experts say doggie day care contributed to an outbreak of dog flu in Chicago that is spreading in the Midwest.

The illness is believed to be caused by an Asian viral strain, which could arise in other U.S. urban areas. In Chicago, it gained a foothold in the doggie day care centers and got an extra boost from when pets were boarded in kennels.

Experts at Cornell University's College of Veterinary Medicine say the H3N2 dog flu virus likely arose from viruses circulating in live bird markets in Asia. Before now, the strain hadn't been seen in North America. That suggests a recent introduction from Asia.

Not all infected dogs show symptoms. Some get a cough, and fever. Severe cases can lead to pneumonia and death.