LOS ANGELES — A mysterious respiratory disease that has been detected in hundreds of dogs across the country has been reported in 10 canines in Los Angeles County, health officials said Tuesday, prompting an investigation into the origin of the illness and the extent to which it may have spread locally.

The unknown illness has been dubbed Atypical Canine Infection Respiratory Disease, and it has been detected in multiple states, including New Hampshire, Rhode Island and Massachusetts. Officials in Oregon have reported more than 200 cases, according to media reports in that state.

According to the Los Angeles County Department of Public Health, 10 cases of the illness have been received from veterinarians in the county since Thursday. The dogs were showing signs of illness, even though they tested negative for all normal respiratory ailments.

Symptoms include cough, nasal discharge, sneezing and lethargy.

County health officials said owners of the dogs believed to have the new illness are being contacted in hopes of tracking down where the animals may have been infected. Health officials are also coordinating with federal and other state agencies to coordinate the investigation.

Cases of the new illness are being defined as occurring in animals that test negative for normal respiratory illnesses but also have at least one of three other indicators:

• chronic respiratory infection lasting more than six weeks that does not respond to antibiotics
• chronic pneumonia that is resistant to antibiotics
• acute pneumonia that quickly becomes severe and leads to severe illness or death

Owners who believe their pet may be infected should contact their veterinarian and isolate the animal at home for at least 28 days from the time symptoms developed. Owners should also clean and disinfect surfaces and animal equipment. Dogs should also be kept away from day care, boarding kennels, grooming facilities and dog parks.

If a dog becomes sick after being boarded, owners should quickly seek veterinary care for the animal and notify the boarding facility about the illness.