Canine respiratory mystery: What we know about the outbreak

What’s going on with the canine respiratory illness in the news? Is it viral or bacterial—or possibly overblown media coverage? NEWStat talks to veterinarians facing the outbreak and looks into the research being done to solve the mystery.

By Tony McReynolds

“The ones who go down, who die, it comes on rapidly, very rapidly. You start supportive care and they don’t respond,” says Nicole Trenholme, DVM, MS, DCVECC, DACVAA. “It’s unnerving.” 

Trenholme, medical director at AAHA-accredited Evolution Veterinary Specialists in Lakewood, Colorado, told us that she first noticed the uptick in the number of dogs coming in with a cough last September: “It was maybe seven to 10 dogs a week, at the start.” 

Just what’s going on with these clusters of coughing dogs across the country? 

What is CIRDC? 

Colorado is one of at least 10 states to have reported cases of canine infectious respiratory disease complex (CIRDC), including Oregon, Rhode Island, Florida, and New Hampshire. The number of cases are estimated to be in the thousands nationwide, although official numbers aren’t available because currently there’s no definitive test for the illness and because symptoms often mirror those of many other ailments. 

CIRDC is a syndrome of diseases that can be caused by several different—and often highly contagious—bacterial and viral pathogens in which coinfections are common. Clinical signs are frequently mild and self-limiting but some individual cases progress to severe disease and, in instances, death. That’s what’s happening in these clusters, and that’s what’s got veterinary professionals and pet owners worried. 

Research into the outbreak 

Rgesearchers at Colorado State University (CSU) College of Veterinary Medicine and Biological Sciences, who are studying the possible virus, report that the cluster outbreaks have been linked to cases of severe pneumonia in some dogs, which has led to some fatalites 

Michael Lappin, DVM, PhD, DACVIM (SAIM), a professor at CSU, said this about the difficulty in diagnosing what’s behind the current outbreak: “Contagious respiratory disease in dogs—kennel cough or CIRDC—is common and there are multiple viral and bacterial causes.  

A board-certified internal medicine veterinarian, Lappin also notes that, “It’s currently unknown whether these unusual cases are caused by a virus or a bacterium, or a combination of both types of infection.” 

Clinical symptoms

In Colorado alone, veterinarians report that they’re currently seeing double the number of cases that are typically seen during a canine infectious respiratory disease outbreak, according to the Colorado Department of Agriculture.  

Nationwide, reported cases appear to fall primarily within three general clinical syndromes: 

  • Chronic mild to moderate tracheobronchitis with a prolonged duration of 6 to 8 weeks or longer that is minimally or not responsive to antibiotics.
  • Chronic pneumonia that is minimally or not responsive to antibiotics.
  • Acute pneumonia that rapidly becomes severe and often leads to poor outcomes in as little as 24 to 36 hours.

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Treating this new infection

 If a bacterial cause is identified or strongly suspected, then antibiotics may be indicated. Initial therapy is prescribed based on likely respiratory pathogens and information about the coverage provided by various antibiotics.  

Dogs with more severe symptoms may need more aggressive treatment, such as hospitalization (with isolation) and provision of IV fluids, oxygen support, nebulization, and coupage 

What’s different about CIRDC 

Nicole Trenholme’s staff at Evolution Veterinary Specialists tried all of these. And still, some dogs died. 

When the first few cases were dogs who’d been hanging around other dogs at dog parks or in boarding kennels, Trenholme says they figured it was likely kennel cough. “Then it was a matter of deciding if the dogs just needed time to allow their immune systems to kick in and take care of it naturally or to start them on a course of doxy.”  

Doxy—or doxycycline—is an antibiotic that historically has proven effective in treating common bacterial causes of kennel cough. After three to five days on doxy, and assuming the infection was bacterial, most dogs would have shown marked improvement. 

But that wasn’t happening to Evolution’s patients.   

Trenholme and her colleagues were stumped. They decided it was time to reach out to other local hospitals. They learned that this uptick in respiratory cases was not isolated to Evolution patients; it was happening all over the Denver area. And it wasn’t going away. “We were seeing a lot of dogs still coming back in weeks later with persistent cough or progressive cough.” 

The symptoms got worse. “In the past couple of months, we started getting these really terrible coughing cases where the usual history was three to seven days of coughing that seemed nonproductive and then sudden onset of respiratory distress.” 

Very sudden, almost out of nowhere, she says. And very unnerving. 

Identifying CIRDC symptoms 

She says dogs who had purple or cyanotic mucous membranes were really struggling to breathe. And their pulse oximeter was in the 80s.  

“It was probably only or two cases a week that were extremely severely affected.” In addition to having an abnormal set of vitals, these patients also typically had a very elevated heartrate, “Usually 105 or above.” 

Trenholme says that usually when temperatures are that elevated, “We start worrying if is this a severe viral infection or is this a patient that’s actually septic?” With these patients, they wound up doing full lab work, including a complete blood count chemistry panel looking for overwhelming inflammation.  

“When you’re choosing antibiotics, if you feel like they’re necessary for a case, you don’t want to breed that antimicrobial resistance.”  

Trenholme said staff would give these patients oxygen support, nebulization to break up any secretions in the airway, and, in cases of severe dehydration due to elevated temperature, IV fluids.  If the lab work showed overwhelming inflammation, they considered administering antibiotics as well, just in case. “We didn’t know what it was. Is it bacterial? Is it viral? Is it some other weird, protozoal thing?” 

Baffled, they soldiered on, but despite aggressive supportive care, including high flow oxygen, some dogs simply succumbed. 

“We were looking at trying to tailor the care. But if it’s viral, then antibiotics won’t help,” she adds. “It just has to run its course.”  

Waves of hospitalizations 

Cases of dogs who were sick enough to need hospitalization were sporadic and seemed to come in waves, Trenholme says. There was no particular pattern. Some had been around other dogs in kennels and dog parks, or around other sick and coughing dogs, but others hadn’t. Age didn’t seem to matter, either; patients ranged in age from 10 months to 10 years.  

Trenholme says it was frustrating. “I’d tell clients, ‘Since I don’t know what this is, I recommend a diagnostic workup.’ But that’s expensive, and [I understand why] some owners were reluctant to go that route.” Especially when, after spending all that money, the tests typically came back negative. “We began to worry this was some kind of novel virus.”  

Evolution sent a few of the dogs who died to Colorado State University (CSU) post-mortem so researchers could gather samples in the hope of figuring out just what was going on. “I’m hoping people smarter than I am at university settings are able to isolate and figure it out,” she says. 

Perhaps the worst part was being unable to tell anguished owners just why their beloved pets died. She says that telling them that “some patients seem to succumb to [the infection] because they can’t get enough oxygen” was poor comfort. 

Equally frustrating was their inability to advise clients on how to fully protect their pets. “It was kind of similar to what humans were told when COVID started,” she says. “We’d tell them to try and limit their dogs’ interactions with unfamiliar dogs, even though we vaccinate against canine influenza and kennel cough complex.”  

Gloves, double-gloving, and booties were mandatory for staff selected to work exclusively with the coughing dogs, who were kept in isolation, as was the use of dedicated stethoscopes for each animal, all to try and minimize the risk of spread.   

But despite their best efforts, they lost a few patients.   

“I can think of three or four dogs that have died with some sort of respiratory signs in the past three months,” Trenholme says. “That’s not to say that they definitively had this type of complex. Again, there’s lots of causes for respiratory distress, but two of those victims were young, otherwise healthy dogs. So that makes me think it’s likely they did.” 



Calling in a respiratory specialist 

Trenholme even took the extraordinary step of calling in a specialist to work at the hospital on a temporary basis: Aida Vientos–Plotts, DVM, PhD, DACVIM (SAIM). Vientos-Plotts, an assistant professor of small animal internal medicine at the University of Missouri Veterinary School—and a veterinary school classmate of Trenholme’s—came to Evolution to put in a brief stint examining dogs who came in with respiratory complaints.  

Trenholme says that Vientos-Plotts has been able to convince some clients to do more diagnostic testing to facilitate that gathering of more information to aid researchers in figuring out what this condition is.  And she says that having a specialist on the premises helped reassure clients that the staff at Evolution is doing everything in their power to care for their pets.  

“Separating disease from the hype” 

There seem to be cluster outbreaks of canine respiratory illness every few years. What’s so different about this one?  

“We don’t really know,” J. Scott Weese, DVM, DVSc, DACVIM, told NEWStat. 

Weese, an associate professor in the Department of Pathobiology at the University of Guelph  in Ontario, Canada, and a co-author of the 2022 AAFP/AAHA Antimicrobial Stewardship Guidelines, adds, “Separating disease from hype is the challenge.”  

He notes that we’ve been seeing gradually increased rates of disease over the past couple years, with the typical up and down that occurs normally with respiratory disease: “There’s definitely something going on in some parts of North America, but it’s sporadic and it may just be typical outbreaks layered over a higher baseline rate of disease versus something really new.” 

Weese doesn’t think the various outbreaks of canine respiratory disease across the US are necessarily connected: “I suspect all our outbreaks are independent but related to the same general issues,” he said. “Most likely, increases in disease reflect changes in dogs and dog contacts, not pathogens.”  

Those changes, just in the past few years—many of them COVID-related—include: 

For example, more people working from home meant fewer dogs needed day care and kenneling, which are the typical drivers for getting dogs kennel cough vaccines. “[That helped lead] to a larger population of dogs with less overall immunity.” 

Some experts have said that the course of this disease is different than usual, but Weese is skeptical: “It’s hard to say since there are no data, but probably not,” he said.  

“We hear more about severe disease but that’s likely because people pay more attention to it. When you have more cases, you’ll get more severe disease since a small percentage of dogs with CIRDC get severe disease.” In other words, the more dogs who get sick, the more dogs who will get severe disease, even if the disease itself is unchanged. 

Viral or bacterial? 

Clearly, the biggest questions on everybody’s mind are: Is this a new disease, or a more pathogenic form of a known disease? And, once again, is it viral or bacterial in origin? 

Weese suspects the latter: “It’s probably the regular suspects doing their regular things, just more commonly in some areas.” 

So, how serious is the situation? Is it being blown out of proportion by the media? 

“There are definitely increases in disease in some areas, but not all,” Weese said. “These increases have been amplified by media and social media. Looking at Trupanion respiratory disease claims data, we’ve seen higher claims in areas where there’s been buzz about disease, like Oregon.”  

But, he added, we’re also seeing higher rates in areas where there’s been little discussion. 

Talking to clients

What should veterinarians be telling clients who are worried that their dogs might catch it? 

“It’s largely just basic disease control,” Weese said. “It’s just the variety of little day-to-day things that we do to limit risks. Vaccination against Bordetella and paraflu, two of our main endemic causes of kennel cough, is part of that.” Weese wrote about this in his excellent blog Worms and Germs. He also discusses it in a recent webinar.  

More research  

Meanwhile, researchers at the University of New Hampshire’s (UNH) Veterinary Diagnostic Lab have identified a small portion of the genome of a previously uncharacterized bacteria that may be causing the illness, which may support Weese’s hunch about the possible bacterial origins of the ailment. 

The UNH Researchers believe the bacterium may be host adapted and potentially part of the dog microbiome and has recently developed the capacity to cause disease. Symptoms of infection include a cough that can linger for several weeks, runny eyes, and sneezing. A very small subset of dogs have died after a long bout of this illness that is then complicated or superimposed with a severe acute pneumonia. 

One researcher made the critical breakthrough when he noted that a small segment of genetic material from one atypical bacterial species was common in 21 of the 30 initial samples. 

UNH researchers hope that identifying the bacterium can ultimately lead to determining the right course of treatment for infected dogs.  

An answer can’t come too quickly for the patients and staff at Evolution Veterinary Specialists. 

Until then, there’s a limit to how much the staff can do for dogs who come in coughing. Trenholme says that without knowing what’s causing the symptoms, targeted therapy isn’t really an option: “Right now, we’re just treating it supportively and hoping for the best.”  


Cover photo credit:  © sanjagrujic E+ via Getty Images Plus 

Disclaimer: The views expressed, and topics discussed, in any NEWStat column or article are intended to inform, educate, or entertain, and do not represent an official position by the American Animal Hospital Association (AAHA) or its Board of Directors. 



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