From New Hampshire to Oregon, researchers are trying to figure out what’s causing an infectious respiratory disease among dogs that has turned deadly in rare cases.

The mysterious illness is described as an “atypical canine infectious respiratory disease,” the Oregon Department of Agriculture said in a November 9 news release. Symptoms include coughing, sneezing, eye or nose discharge and lethargy.

Veterinarians in Oregon have reported more than 200 cases of the disease since mid-August. Other cases have been reported in Colorado, Illinois and New Hampshire.

“Based on the epidemiology of the cases reported at this point, the cases appear to share a viral etiology, but common respiratory diagnostic testing has been largely negative,” Oregon State Veterinarian Dr. Ryan Scholz told the American Veterinary Medical Association.

In other words, dogs with the unidentified illness show similar signs of an upper respiratory disease but generally don’t test positive for common respiratory diseases. And the disease is generally resistant to standard treatments, said Dr. David B. Needle, a pathologist at the New Hampshire Veterinary Diagnostic Laboratory and clinical associate professor at the University of New Hampshire.