The canine influenza outbreak that has infected more than 1,000 dogs in the Midwest involves the H3N2 virus, a strain new to the U.S. and previously seen in southern Chinese and South Korean dog populations, scientists say.

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CHICAGO — The canine influenza outbreak that has infected more than 1,000 dogs in the Chicago area and the Midwest involves the H3N2 virus, a strain not previously seen in North America, researchers say.

The virus is now widely circulated in southern Chinese and South Korean dog populations, scientists at Cornell University and the University of Wisconsin say. There is no evidence that it can be transmitted to humans. But it’s not known if the current vaccine provides any protection from the new virus, they say.

Some Chicago dog day-care businesses are temporarily closing their doors to prevent the spread. Others are only accepting dogs with up-to-date flu vaccines.

“In 14 years doing this, I’ve never seen anything like it,” said Angie Lopez, owner of Happy Tails and Trails, which closed for two weeks and recently reopened.

Lopez said she hired a company to power wash walls, floors and vents after seeing several dogs get sick starting March 28. She reopened this week, but is requiring a note from a vet and a flu vaccine before taking back any dogs. So far, 15 of the usual 50 or so dogs she cares for have returned.

The outbreak had been attributed to the H3N8 strain of virus, which was identified in the U.S. dog population in 2004 and has been circulating since. The H3N2 virus, identified in 2006, suggests a recent introduction from Asia, researchers say.

Both Influenza strains can cause high fever, loss of appetite, coughing, nasal discharge and lethargy, Cornell scientists say. Symptoms may be more severe in cases caused by the H3N2 virus. Some infected dogs may not show symptoms at all.

H3N2 has caused infection and respiratory illness in cats.

Cook County officials issued a warning this month to pet owners, advising them to avoid dog parks, group dog-training facilities, dog day-care businesses and other areas where dogs congregate. At least five dogs have died from complications of the flu, officials said.

Brian Koester, co-owner of Bark Bark Club, plans to reopen Wednesday after being closed for about a week. Koester said his staff noticed a dog coughing last week and immediately called the owner, who picked up the dog within 20 minutes. He shut the center the next day, hoping boarded dogs didn’t get sick. So far, they haven’t shown any symptoms, he said.

“One of the real problems for people in our industry is that the dogs are asymptomatic for six to seven days,” Koester said.

Koester said he won’t require dogs to have a flu vaccine because it doesn’t protect them from the new strain of the virus. He said he gets updates on the health of their pets from owners and encourages them to take their dogs to the vet at the first signs of symptoms.