Kennel cough is caused by a mix of bacteria and viruses that infect a dog’s upper respiratory tract, not just a single germ. The most common culprit is a bacterium called Bordetella bronchiseptica, but it frequently teams up with other pathogens to make things worse. Understanding the full picture helps explain why some dogs get a mild cough that clears on its own while others end up seriously ill.
Bordetella: The Primary Bacterial Cause
Bordetella bronchiseptica is the pathogen most closely associated with kennel cough. Once a dog inhales or ingests the bacteria, they latch onto the lining of the trachea and bronchi (the main airways leading to the lungs) and begin damaging those tissues. This damage triggers the hallmark symptom: a harsh, honking cough that sounds like something is stuck in the dog’s throat. In more advanced cases, the bacteria can spread deeper into the lungs themselves.
What makes Bordetella particularly tricky is how long an infected dog can spread it. Even after symptoms resolve, dogs can continue shedding the bacteria for two to three months. Unvaccinated puppies in one study were still shedding bacteria four weeks after exposure, and that study didn’t even continue past the four-week mark. This extended shedding period is a major reason the illness moves so quickly through shelters, boarding facilities, and daycare groups.
Viruses That Set the Stage
Several viruses can cause kennel cough on their own or, more commonly, weaken the airways enough for Bordetella to take hold. The main viral players include canine parainfluenza virus, canine adenovirus type 2, and canine respiratory coronavirus. Canine distemper virus can also cause respiratory symptoms, though vaccination has made this far less common.
Canine influenza is a separate concern worth distinguishing. Two strains circulate in U.S. dogs: H3N8, which originally jumped from horses, and H3N2, which originated in birds. Canine flu tends to cause more severe illness than typical kennel cough, with some dogs developing pneumonia. Most dogs recover within two to three weeks, but a small percentage die. Vaccines exist for both strains, and your vet can help you decide whether your dog’s lifestyle warrants them.
Why Coinfections Are So Common
Kennel cough is rarely a one-pathogen problem. A large systematic review found that roughly 47% of dogs with Bordetella infections also carried at least one other pathogen. The most frequent co-infecting agents were Mycoplasma (another type of bacteria) and canine respiratory coronavirus.
This matters because coinfections tend to produce longer-lasting illness and can change how well a dog responds to treatment. A dog fighting off Bordetella alone might cough for a week or two and bounce back. A dog dealing with Bordetella plus a virus plus Mycoplasma is more likely to stay sick longer and potentially develop complications. It also helps explain why kennel cough can look so different from one dog to the next, even dogs exposed at the same facility on the same day.
How It Spreads
The pathogens behind kennel cough travel through airborne droplets produced when an infected dog coughs, sneezes, or even breathes heavily. Dogs can also pick them up from contaminated surfaces like shared water bowls, toys, or kennel walls. Close confinement is the biggest amplifier. Veterinary hospitals, boarding kennels, doggy daycares, dog parks, and grooming salons all create the kind of nose-to-nose contact that lets these germs jump from dog to dog efficiently.
Environmental Factors That Raise the Risk
The pathogens alone don’t tell the whole story. Environmental conditions play a significant role in whether exposure actually leads to illness. Poor ventilation is one of the biggest risk factors, because stagnant air allows airborne droplets to linger. Extremes of temperature and humidity, both too high and too low, also increase a dog’s susceptibility and can make symptoms more severe once infection takes hold.
Stress is another major contributor. Dogs that are anxious from being boarded, recently transported, or adjusting to a new environment have weakened immune defenses. Poor nutrition compounds the problem. Even during recovery, stress and bad environmental conditions can trigger a relapse, which is why vets often recommend keeping a recovering dog calm and comfortable rather than rushing back to normal activity.
How Vets Figure Out What’s Causing It
In most cases, vets diagnose kennel cough based on the characteristic cough, recent exposure history (boarding, grooming, a new dog in the house), and a physical exam. They don’t typically run lab tests for straightforward cases. PCR respiratory panels exist and can identify exactly which pathogens are involved, but the University of Wisconsin’s Shelter Medicine Program recommends against routine testing for every coughing dog, noting that the funds are better spent elsewhere when the illness looks and behaves normally.
Testing becomes worthwhile when something is off: an unusually large number of sick dogs, symptoms progressing to pneumonia, dogs failing to improve with standard care, or a known outbreak in the area. In those situations, identifying the specific pathogen can guide treatment decisions and help contain spread.
When It Gets Serious
Most healthy adult dogs recover from kennel cough within one to three weeks without complications. The concern is when the infection moves past the upper airways and into the lungs, causing pneumonia. Warning signs include labored breathing, a wet or moist-sounding cough (as opposed to the dry honk of uncomplicated kennel cough), high fever, and thick nasal discharge. Puppies, senior dogs, and dogs with weakened immune systems or flat faces (brachycephalic breeds like bulldogs and pugs) face a higher risk of this progression.
If your dog’s cough shifts from dry to wet, if they stop eating, seem lethargic, or start breathing harder than normal, those are signs the infection has moved beyond a simple upper respiratory irritation and needs veterinary attention promptly.