Dogs across the US are being sickened and killed by a mystery illness that veterinarians cannot identify or cure.

But now, researchers believe they may have finally identified what’s causing the respiratory disease, which has caused the deaths of numerous dogs nationwide.

The culprit is “a funky bacterium,” Dr. David Needle, pathology section chief at the College of Life Sciences and Agriculture at the University of New Hampshire, told NBC News.

“It’s smaller than a normal bacterium in its size and in the size of its genome,” he said. Its small size might allow it to evade a dog’s immune defenses in the upper respiratory tract and get down into the lungs.

“Long story short, it’s a weird bacterium that can be tough to find and sequence,” Needle added.

These just-discovered bacteria are “new as a potential cause of disease, but it is likely to be — or to have evolved from — a component of the dog microbiome,” Needle added, referring to the microorganisms that naturally live in and on a healthy dog’s body.

The research team is sharing these results prior to publishing a research article, in the hope that their preliminary findings will give struggling veterinarians some information as they deal with this and other respiratory illnesses.

The illness — it doesn’t even have a name yet — first shows up as a cough that can last for six to eight weeks, which antibiotics seem unable to control. Other symptoms may include sneezing, discharge from the eyes or nose, fatigue and difficulty breathing, according to the Oregon Department of Agriculture.

Afflicted dogs can then develop pneumonia that also doesn’t respond to antibiotics. In its final stages, dogs might develop acute pneumonia that quickly becomes severe.

After that, the disease may lead to death within a day or two. Most dogs, however, do not die, though the illness can sicken them for several weeks, during which they might infect other dogs.

Experts aren’t even sure yet whether the same germ is making dogs sick across the nation. The symptoms can also resemble other canine illnesses like kennel cough or influenza, which complicates estimating how widespread the disease really is.

It’s also unknown if any particular breeds are more susceptible to the illness. The only thing veterinarians agree on is that the mystery disease is something they haven’t seen before.

It’s unknown precisely how many dogs have been infected with the illness — and it has not yet been seen in Canada — but it’s been reported in the following US states:

Further research is needed to determine what kind of bacteria might be causing the disease, and how to treat dogs that come down with it. Needle and his colleagues haven’t yet found a way to grow the new bacteria in the lab.

But the structure of the bacterium offers some clues about which drugs might be the best choice to fight it, Needle suggested, adding that the antibiotic doxycycline may be effective.  

There have been no reports of humans or other animals getting sick from the illness, as the disease seems to be found only in dogs. However, if the bacteria continue to evolve, it might have the ability to jump to people or other animals.

Despite this being a busy holiday season with lots of families hitting the road, experts recommend dog owners keep their pets out of communal dog bowls, boarding kennels, groomers, dog parks and other areas where an infection is more likely to spread due to crowded conditions and close contact.

“When you congregate a bunch of animals together, there’s a larger likelihood of getting an infectious disease from other dogs,” Dr. Kurt Williams, director of the Oregon Veterinary Diagnostic Lab at the Carlson College of Veterinary Medicine at Oregon State University, told NBC News.

Other holiday options include having a dog sitter or dog walker come to your house to feed and exercise a dog, thus avoiding large groups of dogs.

Dog owners should also ensure that their pets are up to date on all vaccines and boosters — including canine influenza — and any dog that shows symptoms of a cough or other disease should be seen by a veterinarian as soon as possible.

“I’ve been telling people to work closely with their vet,” Williams said. “And to make sure their dog has all the vaccines available, in particular those for respiratory diseases we are familiar with.”