A mysterious illness is sickening dogs in several states. Some are dying.

Marie Heckemeyer had every reason to believe her dog was recovering. Six-year-old Thunder, the beloved Siberian husky she raised from a puppy, was back to digging holes, jumping around the house, playing and seeking back scratches.

Thunder had fallen ill after attending a boarding camp while Heckemeyer and her husband were on a trip to Italy for their 20th wedding anniversary. After about two weeks of treatment - and a veterinary bill topping $16,900 - his cough seemed to have retreated.

But within an hour of playing, Thunder was back in the emergency room with respiratory distress. And soon after that, Heckemeyer and her husband, both residents of Colorado, received a call from the vet to come in and say their goodbyes.

“He was so young,” Heckemeyer said through sobs. “. . . For this to just come on so quickly - it’s just so hard.”

Thunder, who died on Nov. 6, is one of hundreds of dogs across at least five U.S. states - Colorado, Rhode Island, Oregon, New Hampshire and Massachusetts - thought to have contracted a mysterious respiratory illness that experts are scrambling to understand.

Very little is known about the illness, but veterinarians say it usually starts with a cough that might last for weeks, then progresses to pneumonia (visible on X-rays) and severe respiratory distress. The disease generally does not appear to respond to antibiotics, and in acute cases of pneumonia, poor outcomes are seen in as little as 24 to 36 hours, according to the Oregon Veterinary Medical Association, which has received more than 200 reports of potential cases.

“We don’t know what’s causing it, and we can’t say definitively how it’s being transmitted,” said Lindsey Ganzer, a veterinarian who owns the North Springs Veterinary Referral Center in Colorado. “We just don’t know enough right now.”

Ganzer’s hospital has seen at least 35 cases since about Oct. 20, four of which were fatal “due to severe pneumonia,” she said. None of the dogs appears to be fully recovered yet, and her treatment plan so far includes testing to rule out common viruses and infections, supplementary oxygen and a cocktail of antibiotics - which may help with secondary infections, though she’s not convinced it addresses the root cause.

“We need to be treating sooner rather than later. It’s really important,” Ganzer said. “I’ve had cases where it’ll be two housemate dogs, and one of them will show signs, but I’ve gone ahead and started both on antibiotics. A couple days later, the other one will start to cough but is doing better than the first one.”

From what she has seen, symptoms include fever, cough, lack of appetite, eye and nose discharge, sneezing and difficulty breathing. Most cases start out looking like kennel cough, a common and highly treatable disease, she said.

“Where we’re seeing this end up differently is that the cough is very prolonged, it doesn’t resolve on its own, and it very quickly develops into pneumonia,” Ganzer said. “When it presents in that pattern, we are going ahead and being more aggressive with treatment.”

For Heckemeyer, 48, the illness has ravaged nearly her entire household. Three of her other dogs - 2-year-old Moose, 6-year-old Denver and 6-year-old Bronco - are still sick, and her elderly Jack Russell terrier is quarantining with her son. She has spent more than $15,500 on medical care for the three so far, she said, not including the cost of their prescription foods, over-the-counter medications, future X-rays and antibiotic refills.

Heckemeyer also purchased each of her dogs an oxygen chamber after she noticed the local veterinary office, inundated by possible cases, was running low on spares. To help power them, she pays about $1,500 a month to rent compressors, and one of her dogs - Denver - is in it round-the-clock, she said. Because the chambers overheat and become humid, she runs fresh ice packs to them several times an hour to ensure they are comfortable.

“It’s just a lot of work. . . . You can walk away for maybe only 10 to 15 minutes,” she said with a laugh. “It’s a good thing I work from home.”

More than a year ago, a veterinary researcher in New Hampshire started hearing stories about a respiratory syndrome that wasn’t responsive to treatment. He began driving out to nearby veterinary practices to collect swabs and examined them in partnership with the Hubbard Center for Genome Studies.

“We found no known viruses, bacteria or fungus that were known pathogens,” said David Needle, the researcher, who works as the pathology section chief at the New Hampshire Veterinary Diagnostic Laboratory. Even more intriguing, a colleague who examined the samples found that, “in 21 of 30 initial animals we sequenced, there was this kind of funky little bacteria,” Needle said.

The team has found evidence of the same bacteria in sickly dogs around New England but has only recently started to collect samples from other parts of the United States. There is currently no evidence that humans or other animal species can catch the illness from the dogs.

“We’re not sure what has happened in New England is what’s happening in the rest of the country,” Needle said. “. . . But in three weeks, we will know so much more.”

For now, the fatality rate of the mystery illness is unclear, but both Needle and Ganzer said it may not be very high. And given that the potential pathogen has been detected in nasal and throat swabs, dogs may be passing it to one another through close contact or by air, especially during play.

“I would strongly recommend that people avoid boarding facilities, doggy day care, anything that’s going to be a high volume of dogs in a space,” Ganzer said. “I know it’s going to be hard with the holidays coming up, but trying to find somebody that will come to your house and take care of your dog is a better option.”