Fewer dogs are getting sick with a mysterious, potentially fatal respiratory illness that surged among Colorado canines this fall, but researchers are still trying to answer questions about the disease’s origins.

Colorado State University’s Veterinary Teaching Hospital and the Colorado Department of Agriculture reported double the number of respiratory illnesses among dogs this fall, mirroring similar outbreaks in Oregon, California, Florida and New Hampshire.

While respiratory illnesses among dogs are not uncommon, this particular strain caused dogs to have a cough that lasted for weeks and resulted in more cases of pneumonia. In some cases, dogs died from the disease.

In a news release Monday, Dr. Michael Lappin, an internal medicine specialist and director of the Center for Companion Animal Studies at CSU, said it’s not clear why cases decreased but it could be related to pets spending less time in social situations with other dogs.

CSU experts are partnering with clinics in Fort Collins, Denver, Colorado Springs and Grand Junction to research whether cases of the disease are connected to one of 10 viruses or bacteria known to cause canine respiratory disease, the university said in a news release.

Researchers are also trying to determine if those viruses or bacteria have changed or if there’s another cause for the spike in disease.

That work is expected to continue through February, and the university will release results when it’s finished, according to the news release.

In a separate study, CSU clinical microbiology resident Dr. Blaire MacNeill reviewed 87 cases of suspected infectious respiratory disease at the teaching hospital between September and mid-January.

But the review “revealed no obvious patterns of organisms previously recognized to cause infectious respiratory disease,” university officials said.

Dog owners should continue to follow earlier guidelines to protect their pets, including staying up to date on vaccinations, watching for a cough and being cautious about close contact with unfamiliar dogs, according to the university.