‘Mystery’ Dog Illness May Not Be a Mystery at All, Experts Say (2024)

Scientists and health officials are scrambling to find an explanation for the outbreaks of severe respiratory illness that have been sickening dogs across the United States.

Respiratory infections are not uncommon in dogs, but some veterinarians have reported a surge in very sick pets with symptoms that last an unusually long time and do not respond to treatment.

Experts have not been able to identify a clear cause, fueling concerns that a mysterious new pathogen might be stalking the nation’s dogs. That is one possibility, and a team of scientists in New Hampshire is investigating whether a newly discovered bacterium might play a role in some of these cases.

But there is little evidence that a dangerous new canine superbug is sweeping the nation, and that prospect remains unlikely, experts said.

“Current disease patterns aren’t consistent with the emergence of a new highly transmissible pathogen,” Dr. Rena Carlson, the president of the American Veterinary Medical Association, or A.V.M.A., which has been monitoring the situation, said in an email.

Although the scientific investigation is in early stages, what has become known as the mystery illness may not be all that mysterious — or even a single illness, scientists said. Instead, a variety of run-of-the-mill viruses and bacteria could be driving the current outbreaks. “We cannot know or even assume that all cases being reported have the same cause,” Dr. Carlson said.

Respiratory diseases have always waxed and waned in dogs, and outbreaks often flare up unpredictably. And the Covid-19 pandemic — when many Americans not only adopted new dogs but also hunkered down at home with them — could have left many dogs without pre-existing immunity to disease, experts said.

“Since the pandemic, we’ve been seeing clusters of respiratory disease in dogs that spreads easily and has peaks where a large number of dogs are affected,” said Dr. Deborah Silverstein, a veterinarian at the University of Pennsylvania who specializes in emergency and critical care. “And anytime the numbers go up, we do tend to see more dogs with severe disease.”

Here’s what to know:

What symptoms have veterinarians been seeing?

Commonly reported symptoms include a cough, fever, lethargy and intermittent loss of appetite. Some dogs have gone on to develop pneumonia, and a small number of deaths have been reported.

None of these symptoms are novel; they have long been associated with a common canine condition often known as kennel cough. (The more technical name is canine infectious respiratory disease complex.) Kennel cough is a broad term for a contagious respiratory illness that can be caused by a variety of viruses and bacteria. Cases are typically mild, with symptoms that clear up on their own in one to three weeks, but they can lead to pneumonia or even, in rare instances, death.

The new cases have caused concern because they fall at the severe end of the spectrum, with symptoms that last six weeks or more and do not respond to treatment, including antibiotics. Moreover, many of the dogs have tested negative for the pathogens that commonly cause kennel cough.

Still, whatever is making the dogs sick does appear to be infectious. Dr. Lindsey Ganzer, a veterinarian and the chief executive at North Springs Veterinary Referral Center in Colorado Springs, said that all of the dogs she had treated for the illness had spent time in places with high concentrations of dogs, such as boarding facilities, dog day care or dog parks.

How widespread is the problem?

It’s unclear. There is no formal national reporting process and no official case count. According to some news accounts, hundreds of cases have been identified across more than a dozen states, including Oregon, Colorado, California, Florida, Massachusetts, New Hampshire, Rhode Island, Illinois and Maryland.

Some states have well-documented cases and investigations. Oregon’s Department of Agriculture, for instance, has logged more than 200 reported cases since August. But in some places, reports of a new mystery illness have been anecdotal or even contradictory, Dr. Carlson said, and the A.V.M.A. has stopped providing a list of potentially affected states. “Accurate numbers of cases or states affected are impossible to find or predict,” she said.

In the absence of more systematic surveillance, some experts have looked elsewhere for clues. According to data from Trupanion, a pet insurance company, claims related to respiratory illness have risen in a handful of U.S. states, including Oregon and Colorado, and several Canadian provinces. But elsewhere, there has been no notable increase, according to the data, which were presented last week at a webinar organized by Trupanion.

Many of the nation’s dogs are uninsured, so the data are not entirely representative, but they reflect the patchy nature of reports from the ground, said Dr. Scott Weese, an infectious diseases veterinarian at the University of Guelph in Ontario who was a panelist at the webinar. “I don’t think there’s a huge North America-wide outbreak,” he said. “We’ve got some areas where we’ve seen spikes.”

Although much remains unknown, severe — and especially fatal — cases of respiratory illness remain rare across the entire canine population, experts said.

Could a new virus or bacteria be responsible?

There is not yet compelling evidence for that, scientists said, but the possibility cannot be ruled out. “It’s always worth looking,” Dr. Silverstein said, “and we may discover something that is novel.”

Scientists at the University of New Hampshire recently reported that swabs from some of the sick dogs contained a previously unknown bacterium. The findings are preliminary, and the researchers have not detected the organism in all of the cases they have studied, which have come primarily from the northeastern United States.

But no signs of the bacterium have been detected in recent samples from healthy dogs or in stored tissue samples that were collected from dogs five years ago, said Dr. David Needle, a senior veterinary pathologist at the New Hampshire Veterinary Diagnostic Laboratory of the University of New Hampshire, who was part of the research team.

“The data keeps building slowly toward me really feeling that this is something new,” Dr. Needle said. “And that it is certainly associated with some proportion of what could be called atypical canine infectious respiratory disease complex.”

But it is not yet clear whether the organism causes disease or what role, if any, it plays in the recent outbreaks. It could be utterly benign or a secondary cause of disease, infecting dogs that are already sick and making them sicker. Even if the bacterium turns out to be a primary driver of disease, it is very unlikely to explain all of the recent cases, Dr. Needle said. “There’s no way that one thing is causing all of the respiratory disease,” he said.

What other causes could there be?

The outbreaks could stem from a surge in what scientists called “the usual suspects” — the suite of viruses and bacteria that are well-documented causes of kennel cough.

Although many dogs have reportedly tested negative for these pathogens, the results are far from definitive, experts said. Many dogs with kennel cough are not tested for specific pathogens unless — and until — their symptoms become severe. By that point, they may no longer be shedding enough virus to detect easily, scientists said. “If you’re a few days too late, even though the animal might be coughing, you might miss picking up the agent,” said Colin Parrish, a veterinary virologist at Cornell University.

Anecdotally, some experts said that they had noticed more respiratory disease in dogs over the last year or two, and the Trupanion data shows a modest but noticeable rise in respiratory disease claims in the United States and Canada over that time period.

Some experts suspect that this might be a byproduct of the Covid-19 pandemic, when dogs were less likely to spend time in the group settings — such as dog day cares and kennels — that might have exposed them to common respiratory pathogens, although the idea remains speculative. With less canine socializing — and veterinary appointments hard to come by — dog owners might have been less likely to vaccinate their dogs against these pathogens.

“As human behaviors change, dog behaviors change,” Dr. Weese said. “And the net result might be that we have more dogs that are just more susceptible, because they’ve had less exposure in the past and they’ve had less vaccination. And that would account for a gradual increase that we’ve been seeing.”

When periodic, local outbreaks erupt on top of this already elevated rate of disease, veterinary clinics might suddenly be overwhelmed by a surge of coughing dogs, some of which may become seriously ill.

The flurry of recent news attention might also make people more likely to report the dogs that do get sick, experts said.

And the explanations are not mutually exclusive. If America’s dog population has become more susceptible to disease, that could make a new pathogen, or a pre-existing but rare one that scientists have not detected in previous cases of kennel cough, common enough to notice.

What should owners do?

There’s no need to panic, experts said, but caution and common sense are warranted. Owners of dogs that spend time in group settings should talk to their veterinarians about vaccinating their pets against common respiratory pathogens.

And if dogs are showing symptoms, owners should be proactive.

“All of us have gone through Covid,” said Dr. Stephen Kochis, the chief medical officer for the Oregon Humane Society. “I would say if your dog is showing signs of respiratory disease, isolate them in the home, call your vet, get them seen.”

Rebecca Carballo contributed reporting.