A potentially deadly dog illness is spreading in the US. No one knows what it is.

Authorities in several US states are concerned. Hundreds of dogs have gotten sick with a respiratory illness that, in some cases, is fatal. But experts have no idea what’s causing it...

While the exact numbers are unknown, the Oregon Department of Agriculture has detected more than 200 cases since mid-August, according to the American Veterinary Medical Association (AVMA).

Cases have also been detected in Colorado, Massachusetts, and Rhode Island, but as the New York Times reports, experts suspect that the illness is much more widespread.

Veterinarians warn pet owners to look out for symptoms of the mystery disease, which include coughing, sneezing, nasal or eye discharge, and lethargy. According to the AVMA, this can lead to mild to moderate inflammation of the trachea lasting six weeks or longer or chronic pneumonia.

Another form of the disease is devastating acute pneumonia which rapidly becomes severe and often leads to poor outcomes in as little as 24 to 36 hours, according to the AVMA.

While this mystery illness is still being analyzed, the New York Times spoke to one veterinarian in Colorado who treated around 35 dogs with the illness since late October. She said four of the dogs died or had to be euthanized.

The illnesses are not treated, or only minimally affected, by antibiotics or antimicrobials, according to the reports. Dr. Kurt Williams, a director of the Oregon Veterinary Diagnostic Lab, told the New York Times that dogs can usually recover from respiratory illnesses on their own or with the help of antibiotics.

This illness remains a total mystery, although various US agencies are working to find its cause. Oregon says it looks like it could be a virus, but common respiratory diagnostic testing has largely come back negative, according to the AVMA.

While there is quite a bit of debate over whether it's a virus or bacteria, the question remains a total mystery. “I’m open to it being either, and I’m open to it being something we’re not even thinking about,” Dr. Williams told the New York Times.

Those dealing with the outbreak say that they have seen cases in all breeds and ages of dogs, although the symptoms range from just a cough to pneumonia.

However, as the Colorado veterinarian explained to the New York Times, all the dogs had one thing in common — they spent time in places with a high concentration of dogs like boarding facilities, doggy daycare, or dog parks.

With experts fearing a spike in cases as more people travel for the holiday season, they say there are some things to do to protect your pooches while experts figure out what’s going on. "We suggest caution rather than worry,” the Oregon Veterinary Medical Association’s website says.

First of all, if you notice your dog has compatible symptoms, it’s important to contact your veterinarian. They could then be tested to see if it is indeed this mystery illness or if it is a more common and understood sickness like kennel cough. Besides that, if your dog is sick, keep them home so they don’t infect others.

Dog owners are also advised to keep their pets as immunologically protected as possible. That means making sure they’re up to date on vaccines, like those for canine influenza, Bordetella, and parainfluenza, according to the Oregon Veterinary Medical Association.

As we know from COVID, the best way to avoid contagion is to avoid exposure. And while veterinarians aren’t advocating for doggie lockdowns, they do suggest that people reduce contact with large numbers of dogs when possible. And if another dog looks sick, try to keep your dog away.

Finally, the last tip to keep your dog healthy is to avoid having them drink from communal water bowls. Since experts are still unsure about how the illness is spread, it's better to be safe than sorry on this front, of course, whenever possible.