Cases of a fatal respiratory condition in dogs have appeared in Oregon, Indiana, Illinois, Washington, Idaho, California, Nevada and throughout the Northeast as veterinarians scramble to determine what is causing the mysterious illness. 

The first symptom, experts say, is a pervasive cough that can last for several weeks and is resistant to traditional antibiotic treatments. 

"It seems to happen very, very quickly," Dr. Lindsay Ganzer, a veterinarian at North Springs Veterinary Referral Center in Colorado Springs, told TODAY. "[Dogs] go from this cough that just won't go away… then all of a sudden they develop this pneumonia." 

'MIRACLE DOG' RECOVERS AFTER SURVIVING 10 WEEKS IN COLORADO MOUNTAINS FOLLOWING OWNER'S DEATH

Executive Director of the Washington Animal Disease Diagnostic Lab Kevin Snekvik, whose facility has been researching the condition, described the illness's symptoms in an interview with KIRO7. 

"Your dog will run a fever and they won't feel good," Snekvik said. "They'll become lethargic, meaning they want to lie around more when normally they'd be wanting to play outside… and the coughing part of it, that becomes more productive like a wet cough, like a hacking cough."

Per the Oregon Department of Agriculture, symptoms also include sneezing, eye or nose discharge, fatigue, blue or purple gums from oxygen deprivation, trouble breathing and negative tests for other common respiratory illnesses. 

'KELCE' BECOMES A TOP TRENDING DOG NAME IN AMERICA, PET COMPANY'S DATA SHOWS

Since mid-August, the Oregon Department of Agriculture has received over 200 reports of the unidentified condition, per a November 9 press release. 

However, Ganzer told Today, her hospital has been sending samples from dogs that contract puzzling, severe pneumonia to a research lab in New Hampshire that has been studying the illness since 2022.

Although the American Veterinary Medical Association told Today that they are monitoring cases of the condition in other states, it is unclear how many pets have suffered nationwide. 

Dogs are most likely to contract the condition by being in close contact with numerous other dogs at daycare, groomers, boarding kennels and dog parks, Ganzer told the outlet. 

EXOTIC BIRD FINALLY RELEASED FROM AIRPORT AFTER IT WAS HELD FOR 3 MONTHS DUE TO PAPERWORK ISSUE

Dr. Amanda Cavanaugh, head of urgent care services at Colorado State University James L. Voss Veterinary Teaching Hospital, told Today that, usually, cases of other contagious coughing conditions in dogs tend to decrease in the fall as temperatures cool and fewer canines congregate in parks. 

"But this year, the spike has stayed high," Cavanaugh told the outlet. 

Kurt Williams, director of Oregon Veterinary Diagnostics Lab, has been researching the illness for about a month and a half and says researchers are "trying to pin down a potential cause or causes for the entity."

Williams speculates that the condition could be a virus: "The way the cases are presenting, the way that they're apparently spreading - anecdotally, of course - it sounds infectious."

Dog owners should make sure that their dogs are up-to-date on their vaccines, including for canine influenza, Bordetella and parainfluenza, per the Oregon Department of Agriculture. Dogs should have health checks 12 to 24 hours before attending events with other dogs, the organization said, and owners should "consult [their] veterinarian[s] for dog-specific advice before attending any events where dogs are congregated."

Snekvik recommended that dog owners avoid boarding pets over the holidays if possible and avoid congregating with unfamiliar dogs.