(NEXSTAR) – Between dog boarding, play dates and trips to the dog park, Thanksgiving presents additional risk for pets this year as a mystery canine illness continues to spread in multiple states.

Oregon, Colorado, California and New Hampshire are among the states that have seen cases of the illness, which has caused lasting respiratory disease and pneumonia and does not respond to antibiotics. In some dogs, the illness has been fatal.

Symptoms of respiratory illness include coughing, sneezing, nasal or eye discharge and lethargy. Some cases of the pneumonia progress quickly, making dogs very sick within 24 to 36 hours.

“Since we do not have all the answers to this disease, pathogen, and transmission, it is most prudent for dog owners to avoid [bringing] their dog into contact with any ill animals,” Dr. Karl Jandrey, associate dean and professor at U.C. Davis School of Veterinary Medicine, told Nexstar. “Also, avoid or limit travel, confined spaces, multi-dog locations (shelters, dog parks, etc.) and poorly ventilated areas.”

While there have not been documented cases of the illness in Davis, California, Los Angeles County officials announced Tuesday that veterinarians had reported 10 cases since Nov. 16 that tested negative for known viral and bacterial infections.

Jandrey advised dog owners who notice the following signs to contact a veterinarian: increased respiratory rate and effort, malaise or lethargy, less interest in food, coughing or a runny nose.

“We don’t know exactly what we are seeing, and we don’t know if it’s something new,” Baldwin told The Denver Post.

While it’s not unheard of for an unknown respiratory illness to break out among dogs, the number of animals affected is new, according to Maggie Baldwin, a state veterinarian with the Colorado Department of Agriculture. She estimates that the number of cases is twice the amount that the state normally sees during such outbreaks.

It’s not yet clear if a virus or bacteria – or a combination of both – is sickening dogs, according to a Nov. 16 news release from Colorado State University. Veterinarians with the university are encouraging dog owners to keep their vaccines updated, which can help the animal’s immune system stay healthy and fight off illnesses that they may not be vaccinated against. Pet owners should also keep a close eye on their dogs for any of the symptoms mentioned above, and to avoid gatherings of dogs.

In Oregon, where the Department of Agriculture has documented more than 200 cases of the disease since mid-August, Kurt Williams, director of the Oregon Veterinary Diagnostic Laboratory at Oregon State University, says it’s hard to know how many dogs have died from a severe form of the infection without a way to define or test for the bug.

Williams had a simple message for dog owners: “Don’t panic.” He also said dog owners should make sure that their pets are up to date on vaccines, including those that protect against various respiratory illnesses.

The Associated Press contributed to this report.