Hundreds of cases of a mysterious fatal respiratory illness in dogs have been reported throughout the country as veterinarians rush to find out what is behind it.

With instances recently reported in Oregon, Indiana, Illinois, Washington, Idaho, California, Nevada, and throughout the Northeast, experts say that the first symptom is a “pervasive cough that can last for several weeks and is resistant to traditional antibiotic treatments,” Fox News reports.

The Oregon Department of Agriculture received “over 100 reports of illness meeting the criteria from veterinarians” since mid-August, with the majority centered around Portland and Salem.

The department listed symptoms including sneezing, eye or nose discharge, fatigue, blue or purple gums from oxygen deprivation, trouble breathing, and negative tests for other common respiratory illnesses. 

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

Ganzer estimated her hospital has seen nearly 30 dogs with the condition within the last month, adding that the cases are “really not slowing down.” Two to three dogs are coming in with the illness every day, with most requiring hospitalization. 

Four or five of the dogs her practice has seen so far have died due to the respiratory distress caused by the pneumonia, Ganzer said.

The Washington Animal Disease Diagnostic Lab is also researching the outbreak, with Executive Director Kevin Snekvik describing the painful symptoms to KIRO7:

Your dog will run a fever and they won’t feel good. 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.

Snekvik recommended that owners of dogs experiencing any of those symptoms should take them to the vet and make sure to get sampling done so it can be tested.

“What we’re seeing is respiratory disease, so again, the coughing, dogs not feeling well, and presenting as kind of a kennel cough,” he said.

Snekvik also recommends that owners avoid dog parks, dog boarding, or anywhere else where their pets may congregate with strange dogs.

The American Veterinary Medical Association told TODAY that it is monitoring cases nationwide, though it does not have a set number on how many dogs have been impacted nationwide.