Canine Respiratory Infection: What CIRDC Actually Is

Last reviewed · 8 August 2026 · Sources linked throughout

Canine respiratory infection and kennel cough are the same thing. The proper name is canine infectious respiratory disease complex — CIRDC — and the word complex is doing real work: this is not one disease caused by one germ, but a group of viruses and bacteria that produce a similar cough, very often more than one at a time. Most otherwise healthy dogs recover in about seven to ten days with no treatment beyond rest. The things that change that picture are a dog who is flat, off their food, feverish, breathing faster than usual at rest, or working visibly hard to breathe — those need a vet rather than more waiting.

Kennel Cough, CIRDC, Tracheobronchitis: Which Is Which?

Four names circulate for this illness and the overlap causes real confusion, including the belief that a dog has been diagnosed with something new when the vet simply used a different word.

The short version: the first three are synonyms. The fourth is a wider phrase that usually means the same thing in everyday use. Nothing about your dog’s illness changes depending on which one appears on the invoice.

“Kennel cough” is the name most likely to mislead you, and it is worth saying why. It attaches the illness to a place, which quietly implies that a dog who has never been boarded is not at risk. Dogs catch this at daycare, at training classes, in the waiting room, at shows, in the park, and from the new dog that has just moved in next door. Anywhere dogs breathe the same air will do. The clinical names avoid that trap, which is part of why the profession has drifted towards CIRDC — and why this page, which used to be called canine infectious tracheobronchitis, now leads with the phrase owners actually use.

One genuine distinction is worth keeping. CIRDC describes the upper-airway illness. Pneumonia is what it can turn into in a minority of dogs, and that is a different and more serious problem — covered in pneumonia or kennel cough. Canine influenza is likewise a specific virus that sits inside the complex rather than being another word for it.

What Actually Causes It?

Most affected dogs are carrying more than one organism

This is the finding that explains everything else on this page, so it goes first rather than last.

A five-year American study tested 459 dogs with respiratory signs between 2018 and 2022. A single pathogen was found in 31% of cases. Two or more were found in 24% (Yondo et al., Pathogens 2023). Put the other way round: among the dogs where anything was found at all, getting on for half were carrying more than one organism at the same time.

That is what “complex” means, and it is not a technicality. A dog with kennel cough frequently has a virus damaging the lining of the airway and a bacterium taking advantage of the damage. Neither is “the cause” on its own. It also means the honest answer to “what has my dog got?” is often “some combination of these, and it usually does not matter which”.

The organisms involved, and how often each turns up

Prevalence figures from 459 dogs with respiratory signs, United States, 2018–2022.

Two cautions about reading that table. First, “found in” is not the same as “caused it” — and Mycoplasma canis is the clearest example. It sits at the top of the list, but a systematic review of four mycoplasma species found no significant association between M. canis and lower respiratory disease, while M. cynos was significantly associated (Journal of Veterinary Internal Medicine). Some of these organisms are passengers. Second, prevalence varies by country and by population, so treat the ordering as a guide rather than a league table.

Why the list keeps getting longer

If you look this up in an older book you will find a shorter list — often just Bordetella, parainfluenza and adenovirus. A page written in 2021 might name seven organisms. Current panels test for nine or more, and the number has crept up steadily.

That is not because dogs are meeting new diseases every few years. It is mostly because the testing changed. Culturing a fastidious organism from a swab is difficult and often fails; PCR looks for genetic material and finds things that were always circulating but rarely proven. The mycoplasmas are the clearest case — they are now among the most frequently detected organisms in coughing dogs, and for most of the history of this illness they were barely mentioned.

Two things follow. The first is a reason for calm: a longer list is a sign of better instruments, not a worsening threat. The second is a reason for humility — if the list has grown every decade because we looked harder, the current list is unlikely to be the final one. That is worth holding in mind whenever a cluster of unexplained cases makes the news, which is exactly what happened in 2023 and is dealt with further down this page.

For a fuller walk through each organism, see what causes kennel cough. What the cough itself looks and sounds like is covered in signs of kennel cough.

Why “Complex” Changes What You Can Expect

Three consequences follow from the pathogen picture, and between them they answer most of the questions owners actually have.

No single vaccine covers it. The “kennel cough vaccine” typically targets Bordetella bronchiseptica, and often parainfluenza and adenovirus alongside it. That is three organisms out of a field of at least nine. A vaccinated dog can still catch kennel cough from something the vaccine was never aimed at — which is the usual explanation when an owner is told their vaccinated dog has it. Our guide to kennel cough in vaccinated dogs works through what the vaccine does and does not promise.

No single test rules it out. PCR panels test for a defined list of organisms. A negative result means those particular ones were not detected in that particular sample on that particular day — not that your dog is clear.

Which is why most cases are never tested at all. In a bright dog with a classic cough, identifying the organism rarely changes what happens next: the great majority of uncomplicated cases settle on rest and time, and testing is reserved for dogs who are seriously ill, not improving, or part of an outbreak someone needs to understand. It is also why antibiotics are not automatic — when a dog actually needs antibiotics for kennel cough is a question with a more careful answer than most owners expect.

And it is why vaccination is still worth doing. Partial cover is not no cover. In a shelter study where intake vaccination was pushed to high coverage, the proportion of dogs coughing fell substantially compared with the baseline period — the vaccine cannot prevent every case, but at population level it visibly reduces how much disease there is.

Healthy-Looking Dogs Are Carrying It Too

Researchers swabbed 503 shelter dogs with no cough and no fever and tested them by PCR. 240 of them — 47.7% — were positive for at least one CIRDC pathogen (Lavan & Knesl, Journal of Small Animal Practice 2015 — a study whose two authors were both employees of Zoetis, a manufacturer of kennel cough vaccines). Not mildly unwell. Not incubating something obvious. Outwardly fine.

Carriage of more than one organism was common too. The same study reported two, three, four and five pathogens in 12.7%, 3.8%, 1.8% and 0.4% of the dogs respectively — so close to one healthy dog in five was carrying more than one. Mycoplasma cynos alone turned up in 29.2%.

This is the answer to “where on earth did he catch it?” Very often, nowhere new. The organisms are already circulating among perfectly well dogs, and something — stress, a move, a stay in kennels, a new dog in the house, a virus getting a foothold — tips a quiet passenger into an active infection. It also explains why outbreaks seem to appear from nothing, and why tracing a case back to one guilty dog at the park is usually a waste of effort.

There is a second reason it spreads so efficiently: dogs shed these organisms before they show any signs. Most CIRDC pathogens have an incubation period of about two to seven days, and Bordetella specifically runs from two to ten days, most often around six. A dog can be infectious at the park on Tuesday and start coughing on Friday. How long that window lasts afterwards is covered in how long kennel cough is contagious, and the practical steps for limiting spread are in how to prevent kennel cough.

Is kennel cough seasonal?

It looks seasonal, and vets do see clusters at predictable times of year — but the driver appears to be us, not the pathogens. Cases rise when boarding kennels fill for school holidays, when people travel, when shows and classes restart, and when shelter populations peak. The organisms themselves are present all year.

That reframing is more useful than a calendar, because it tells you when your dog’s risk actually rises: not in a particular month, but in the fortnight around a boarding stay, a house move, a new puppy, or a busy period at daycare.

Who Gets It Badly?

For most dogs this is a miserable fortnight and nothing more. Uncomplicated CIRDC in an otherwise healthy pet settles in about seven to ten days — how long kennel cough lasts goes through the recovery timeline properly. The dogs who get into difficulty are reasonably predictable.

• Puppies, particularly very young ones from high-density breeding or rescue settings — they are over-represented in the most severe pneumonia outbreaks.
• Older dogs and dogs with existing heart or airway disease, who have less reserve to spare.
• Immune-suppressed dogs, including those on medication that dampens immunity.
• Dogs in crowded group housing. One shelter study put the effect in unusually concrete terms: each additional day a dog spent in a crowded shelter raised the risk of developing CIRDC by roughly 3%.

How dangerous is it, honestly? The answer depends entirely on which dogs you are counting, and the honest starting point is that no case-fatality rate has been published for kennel cough itself. The figures that do exist come from dogs who had already developed pneumonia — a different and much smaller group. A series of 65 hospitalised puppies with community-acquired infectious pneumonia found 57 went home and eight did not (Radhakrishnan et al., J Am Vet Med Assoc 2007;230(10):1493–1497), while a referral series of 49 dogs admitted with pneumonia of any cause found 33 of the 49 survived to discharge (Conway et al., J Vet Emerg Crit Care 2021;31(4):490–497). At the far end sit the S. zooepidemicus outbreaks: one Korean shelter was reported with 70–90% morbidity and 50% mortality (Byun et al., J Vet Sci 2009;10(3):269–271). Those are three different populations — hospitalised puppies, a referral hospital’s whole pneumonia caseload, and a shelter epizootic — and the ordinary pet dog coughing on your sofa belongs to none of them. Can kennel cough go away on its own? works through this evidence in full.

The practical signal that a case has stopped being ordinary is not the cough getting louder — it is the dog changing. Lethargy, going off food, a fever, and a resting breathing rate that has climbed above about 30 breaths a minute all suggest the infection has moved beyond the upper airway. Counting that rate at home is the single most useful check an owner can do, and the normal respiratory rate for dogs explains how.

What Was the 2023–24 “Mystery” Dog Illness?

From mid-2023, vets in New Hampshire began reporting a respiratory illness that did not behave like ordinary kennel cough, and similar reports followed from Oregon, Colorado, California and Florida. The pattern described was a cough that dragged on for weeks, a poor response to the antibiotics that usually help, a higher-than-expected proportion of dogs progressing to pneumonia, and — in a small minority — rapid deterioration. Many affected dogs tested negative for the usual panel of pathogens, which is what generated the alarm.

Investigations found three things depending on the case: the familiar CIRDC pathogens after all, no identifiable pathogen at all, or — in one subset of samples — an unusual small bacterium that had not been characterised before. None of these was ever confirmed as the cause. The American Veterinary Medical Association’s position in December 2023 was that cases were being seen across several states, that laboratories were collaborating, and that no single agent had been identified.

The honest summary is that it looked different clinically, and the cause was never established. That is unsatisfying, and it is better than either of the tidier stories — that a new pathogen was found, or that the whole thing was imagined. Reporting was patchy and voluntary, testing practices varied between states, and heightened attention finds more cases of everything. Some of the signal was probably real and some of it was probably the looking.

What it means for you now: nothing has emerged since to suggest an ongoing novel threat to dogs, in the UK or elsewhere. The advice is unchanged, and it is the advice on this page — keep vaccinations current, be alert around periods of mixing, and take a cough that comes with lethargy, appetite loss, fever or faster breathing to a vet rather than waiting it out. A cough that is still there after two to three weeks deserves a second look regardless of what is circulating.

Owner FAQ

What is CIRDC?

CIRDC stands for canine infectious respiratory disease complex. It is the current clinical name for what most people call kennel cough: a contagious illness of the upper airway caused not by one germ but by a group of viruses and bacteria, often several at once. Most healthy adult dogs recover in about seven to ten days.

Is canine respiratory infection the same as kennel cough?

In everyday use, yes. Kennel cough, CIRDC and canine infectious tracheobronchitis are three names for the same illness, and “canine respiratory infection” is the plain-English phrase people reach for. Strictly the last is a little broader — it could cover pneumonia or canine influenza — but when an owner uses it, they almost always mean kennel cough.

How many germs cause kennel cough?

At least nine are regularly identified, split between viruses and bacteria, and the list grows as testing improves. More importantly, they frequently arrive together: in 459 dogs with respiratory signs, a single pathogen was found in 31% of cases and two or more in 24%. There is no single “kennel cough germ”.

Can a healthy-looking dog be carrying it?

Yes, and commonly. When 503 shelter dogs with no cough and no fever were tested, 47.7% were positive for at least one CIRDC pathogen. Dogs also shed these organisms before they show signs — incubation is usually two to seven days, and up to ten for Bordetella. That combination is why outbreaks seem to come from nowhere.

What was the 2023 dog respiratory illness?

A cluster of cases reported across several US states from mid-2023, in which the cough lasted longer than expected, responded poorly to standard antibiotics, and more often progressed to pneumonia. Testing frequently came back negative for the usual pathogens. Investigations turned up the familiar organisms in some dogs, nothing identifiable in others, and an uncharacterised bacterium in a subset — but no cause was ever confirmed. Nothing since suggests an ongoing novel threat.

Is kennel cough seasonal?

Not really, despite appearances. Clusters follow boarding, travel, shows and shelter occupancy rather than temperature or daylight — the pathogens circulate all year. The useful version of the question is not “what month is it?” but “has my dog just been somewhere busy?”

How long does it last?

About seven to ten days in an otherwise healthy adult dog, with the cough sometimes lingering a little beyond that. A cough still going strong at two to three weeks, or a dog who becomes lethargic, off their food, feverish or short of breath at any point, has stopped following the ordinary course and should be seen.