Map shows where mysterious dog respiratory illness has spread in U.S.

A mysterious illness that causes fever, cough and lethargy in dogs has now spread to at least 16 states, the Louisiana State University School of Veterinary Medicine said earlier this month. 

Over the last year, there's been a rise in the number of dogs getting infected with what vets have named Atypical Canine Infectious Respiratory Disease Complex, or aCIRDC.

Not much is known about the origin of the disease but dogs who are believed to have caught it present severe symptoms that in some cases can lead to death. 

Here is what we know about the mystery illness:

Where have cases of dog respiratory illness been confirmed?

According to the Louisiana State University School of Veterinary Medicine, the illness has reached a total of 16 states: California, Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Maryland, Massachusetts, New Hampshire, Nevada, Oregon, Pennsylvania, Rhode Island, Vermont and Washington. 

In Oregon alone there have been more than 200 cases reported, officials said.

There is no official case count nationwide.

What are the symptoms of the mysterious dog illness?

Clinical signs of the illness include chronic tracheobronchitis, which is inflammation of the trachea and bronchi, and can last six to eight weeks or longer, according to officials. Symptoms of tracheobronchitis include coughing, sneezing and nasal discharge. 

Pet owners are also advised to look out for chronic pneumonia that does not respond to antimicrobials. In some cases the pneumonia progresses rapidly, leading to death within 24 to 36 hours, although those cases are rare, officials said. 

"Some states have reported dogs staying ill for long periods, not responding to treatment, and rarely, dogs with rapid onset of severe respiratory signs that have progressed to death," Pennsylvania Department of Agriculture spokesperson Shannon Powers told CBS Philadelphia in a statement. "Dogs with preexisting chronic respiratory illness may be more likely to develop pneumonia. Veterinarians are working to pinpoint the cause and identify effective treatments."

The mystery illness appears to pose a more serious threat to dog breeds with flat faces or short snouts, like pugs or French bulldogs, which are predisposed to respiratory illnesses, CBS Boston reported. 

Are there treatments for dogs who have the mystery illness?

Because so little is known about what causes the illness, veterinarians have yet to pinpoint how it spreads and how best to treat it, Animal Humane Society's director of veterinarian medicine, Dr. Graham Brayshaw, told CBS Minnesota. 

Most veterinarians simply advise that pet owners do their best to keep their dogs healthy, avoid other dogs that may be ill, and make sure they are up on their vaccinations. 

"Make sure they stay hydrated. They can get secondary bacterial infections, so sometimes antibiotics are necessary. If you are worried about your dog having these signs, reach out to your veterinarian, we have treatments we can do to support your dog through the virus," he said.

The United States Department of Agriculture is working with multiple states and labs to investigate what's causing the illness, CBS Minnesota reported. 

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• Dogs