A mysterious respiratory illness is affecting dogs, but tests show no new pathogens: U.S. officials

Testing of the mysterious respiratory illness affecting dogs across the United States has not yet revealed any new pathogens or any common cause for the wave of infections that have alarmed pet owners, the agriculture department said.

Instead, common causes of canine infectious respiratory disease have been identified in many of these cases through genetic sequencing of samples, the U.S. Department of Agriculture (USDA) said in an emailed statement.

The illness has sickened dogs in at least 14 states. It can last four to six weeks and cause mild bronchitis, which could escalate to pneumonia.

Respiratory diseases in canines can typically spread from dog to dog through direct contact or contaminated surfaces, according to the U.S. Centers for Disease Control and Prevention.

So far there is little indication the disease can spread to humans, but veterinariansin Canada and the U.S. are warning dog owners to take extra care this holiday seasonwhen pets travel with families or are placed in kennels.

Though cases have been reported for months, the American Veterinary Medical Association has stepped up its public advisories recently for the holiday season.

"We've got more dogs that have a lower level of resistance because they've been exposed less over the last couple of years and they've had less vaccination,"Dr. Scott Weese, an infectious disease veterinarian at the Ontario Veterinary College in Guelph, said during an online briefing reported by NBC News earlier this month.

Laboratories within the National Animal Health Laboratory Network and USDA National Veterinary Services Laboratories are conducting gene sequencing to pinpoint cause of the illness, the USDA said.

The United States Department of Agriculture Animal and Plant Health Inspection Service (APHIS) is working with multiple state animal health officials and diagnostic laboratories regarding the reports, the USDA said, adding that APHIS and partners will "keep the public apprised on future developments and information."