Mystery illness affecting dogs across the United States

Researchers are still attempting to determine if the disease, which can be fatal, is viral or bacterial, and whether it could be a variant of the well-understood canine disease known as "kennel cough," the association said.

Symptoms including coughing that can last four to six weeks, which could be mild bronchitis or could escalate to pneumonia. Some acute cases have quickly become pneumonia within 24 to 36 hours, the association said.

So far there is little indication the disease can spread to humans, but vets are warning dog owners to take extra care this holiday season, when pets travel with families or are placed in kennels.

Though cases have been reported for months, the association has stepped up its public advisories recently for the holiday season.

Oregon has reported some 200 cases, and there have been an unknown number of cases in California, Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Maryland, Massachusetts, New Hampshire, Rhode Island, Vermont and Washington.

The Oregon Department of Agriculture is working with state and national diagnostic laboratories to identify the cause, and the Colorado State University College of Veterinary Medicine and Biological Sciences has linked the disease to cases of severe pneumonia and some deaths, the association said in its latest advisory updated on Wednesday.

Veterinarian Rena Carlson, president of the association, said so much remained unknown about the outbreak that it was simply being referred to as an atypical disease.

"We just don't know whether it can jump to other species, whether that would be humans or cats. We're watching that really closely," she said.

Experts in New Hampshire have focused their investigation on a new bacterium, but so far antibiotics have been largely ineffective as treatment, Carlson said.

Carlson recommended dog owners to get their pets current on vaccinations and to seek veterinary help as soon as dogs show symptoms.

Australian Associated Press