Will Sage Astor-Hundreds of dogs sickened with mysterious, potentially fatal illness in several U.S. states

Pet owners beware: An unusual respiratory illness in dogs that does not respond to antibiotics is being investigated in several states across the U.S.

Oregon, Colorado and New Hampshire are among the states that have seen cases of the illness, which has caused lasting respiratory disease and pneumonia. Symptoms of respiratory illness in dogs include coughing, sneezing, nasal or eye discharge and lethargy. Some cases of the pneunomia progress quickly, making dogs very sick within 24 to 36 hours.

"Unfortunately, right now, nobody knows what it is," veterinarian Dr. Mike Hutchinson told CBS News Pittsburgh.

"When that happens, you should see your veterinarian because we're going to treat those symptoms. And for viruses, there's really no good anti-viral on the market. However, we can support the symptoms sometimes by nebulizing them or giving them some support, fluids, things that they need," said Hutchinson.

The Oregon Department of Agriculture has documented more than 200 cases of the disease since mid-August, encouraging pet owners to contact their vet if their dog is sick and told state veterinarians to report cases as soon as possible. 

The agency is working with state researchers and the U.S. Department of Agriculture's National Veterinary Services Laboratory to find out what is causing the illnesses.

Dogs have also died, Kurt Williams, director of the Oregon Veterinary Diagnostic Laboratory at Oregon State University, told the Associated Press. But without a clear way to define the disease or test for it, he said it's hard to put a number on how many died from a severe form of the infection.

As veterinarians try to pin down what's making the animals sick, they are encouraging people to take basic precautions to keep their pets healthy.

Hutchinson says he hasn't seen any cases in his practice but recommends pet owners to keep their furry friends away from other dogs if possible, as they are more likely to contract the illness in places like kennels, dog parks and groomers.

"Most of the day cares, at least around us, they require that all of the vaccines are up to date, that they have a health certificate from their veterinarian. So most of the time you're putting healthy dogs into that area. But you get one dog that's sick, it's like the schoolyard cold, then the rest of them can get that cold," Hutchinson said.

Williams also says it's a good idea to make sure your pets are up-to-date on vaccines, including those that protect against various respiratory illnesses.

Otherwise, "don't panic," he says.  

David Needle, senior veterinary pathologist at the University of New Hampshire's New Hampshire Veterinary Diagnostic Laboratory, who has been investigating the mysterious disease for almost a year, said his team has not seen a large increase in dogs dying from the illness but also encouraged pet owners to "decrease contact with other dogs."

His lab and colleagues at the university's Hubbard Center for Genome Research have looked at samples from dogs in Rhode Island, New Hampshire and Massachusetts and more will be coming from Oregon, Colorado and possibly other states.

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