Veterinary laboratories in several states are investigating an unusual respiratory illness in dogs, and encouraging people to take basic precautions to keep their pets healthy as veterinarians try to pin down what’s making the animals sick.

Oregon, Colorado and New Hampshire are among the states that have seen cases of the illness, which has caused lasting respiratory disease and pneumonia and does not respond to antibiotics. 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.

The Oregon Department of Agriculture has documented more than 200 cases of the disease since mid-August. It has encouraged 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 died, said Kurt Williams, director of the Oregon Veterinary Diagnostic Laboratory at Oregon State University. 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.