A potentially deadly illness in dogs is spreading throughout the U.S.

The illness starts out as a cough that can last for several weeks but does not respond to typical treatments, such as antibiotics, TODAY reported. The illness can develop into severe pneumonia. Cases have been reported in California, Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Massachusetts, New Hampshire, Oregon, Rhode Island and Washington.

Some of the dogs died from the illness, veterinarians told TODAY.  Most of the dogs with the illness were in close contact with numerous other dogs, such as day care, dog parks or boarding kennels.

The cause of the illness has not been determined.

Symptoms include:

• Coughing that doesn’t get better on its own after a week
• Sneezing
• Nasal or eye discharge
• Lethargy
• Trouble breathing
• Blue or purple gums (due to not getting enough oxygen

What dog owners should do

From the Oregon Veterinary Medical Association:

• Seek medical attention for a dog displaying symptoms
• Make sure all dogs are up-to-date on all their vaccines including canine influenza, Bordetella and parainfluenza.
• For event organizers: Dogs should have a health check 12-24 hours before the event. Have a DVM onsite checking dogs for health issues (mild nasal discharge, cough, elevated temperature, being off food).
• Consult with your veterinarian for dog-specific advice before attending any events where dogs are congregated.