Since a mysterious, potentially fatal respiratory disease started spreading through Colorado’s dog population in September, veterinarians from more than 15 states have reported cases.

The disease, which is unique in both how many dogs are being affected and how long symptoms last, has still not been identified and its unclear if transmission rates are slowing down for Colorado.

“Contagious respiratory disease in dogs … is common and there are multiple viral and bacterial causes,” said Michael Lappin, a board-certified veterinarian at the Colorado State University’s teaching hospital, in a Dec. 4 news release. “However, in recent months, cases are being diagnosed more frequently and the course of disease is different than usual, surprising both pet owners and veterinary health care providers.”

Experts still aren’t sure if the illness is viral or bacterial, but in the release CSU veterinarians said that the infection has been linked to cases of severe pneumonia, some of which were fatal.

Between September and November, Colorado State University’s Veterinary Teaching Hospital saw more than double the cases of canine pneumonia than it did during the same period in 2022, according to the release.

How this mystery disease is being transmitted is also not clear, but CSU veterinarians believe it’s contagious and most likely from direct contact with other dogs.

CSU researchers from the teaching hospital and its diagnostic laboratory are working with the Colorado State Veterinarian’s office, the USDA and the Colorado Veterinary Medical Association to collect diagnostic data and distribute testing kits to veterinary clinics.

Maggie Baldwin, state veterinarian with the Colorado Department of Agriculture, said the testing CSU is doing is a mixed bag — aimed at identifying any diseases veterinarians already know how to treat and looking at cultures to determine if the infection is viral or bacterial.

“CSU has been instrumental in helping increase diagnostic testing in hopes of finding a primary cause of the outbreak,” Gillian Gwartz, a program veterinarian in the State Veterinarian’s Office, said in the news release. “We still don’t have a single conclusive causative agent, but we’re working with select vet clinics to try to get more samples to see if we can find a pattern.”

By partnering with local veterinarian offices to test infected dogs and collect samples, CSU has found that some of the reported cases are common strains of respiratory diseases normally seen in dogs, such as Kennel Cough, Baldwin said.

However, there are still cases that remain a mystery, she said.

According to Baldwin, CSU expects to get additional testing results out in early 2024 that will hopefully help researchers identify what exactly is spreading and how to stop it.

Some veterinarians, such as Scott Weese at the University of Guelph-Ontario Veterinary College, believe the rise in cases is driven by COVID-19. During the pandemic, dog ownership increased, veterinary and boarding care declined and vaccination rates may have dropped, all of which could have resulted in a lower level of immune resistance and more opportunities to catch regular respiratory diseases post-pandemic, he said on a Nov. 30 virtual panel.

In addition to Colorado, cases of this mystery canine respiratory disease have spiked in Oregon, California, Florida, New Hampshire and parts of Canada. Veterinarians from Georgia, Idaho, Illinois, Indiana, Maine, Maryland, Massachusetts, Nevada, Pennsylvania, Rhode Island, Vermont, and Washington have also reported cases, according to data collected from Louisiana State University’s School of Veterinary Medicine.

The disease is definitely spreading across the country, but Baldwin said she wasn’t sure if transmission rates were still high within Colorado.

“It seems like the call volume to our office of veterinarians reporting these cases has decreased, but whether that’s because there’s actually fewer cases or because we’re spreading this information and it’s available online, I’m not sure,” Baldwin said.

Anecdotally, some veterinarians are saying they’re seeing less cases and others are saying they’re seeing just as many, so it’s hard to say for sure where Colorado lies, Baldwin said.

For now, Baldwin said she recommends staying overly cautious and limiting your dog’s interaction with others, even during the holiday season.

“We know that during the holidays there’s going to be a lot of travel and boarding,” Baldwin said. “We just recommend that you work with your vet, keep your dog up to date on their vaccines, and keep sick pets home instead of letting them interact with others.”

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