Dogs get respiratory infections by inhaling airborne pathogens shed by other infected dogs, typically through close contact in shared spaces. The germs spread through coughing, sneezing, and direct nose-to-nose greeting, and they can also linger on shared water bowls, toys, and surfaces. Most cases involve a mix of viruses and bacteria working together, which is why veterinarians refer to the broader illness as canine infectious respiratory disease complex rather than pinning it on a single germ.
How the Germs Spread
When an infected dog coughs or sneezes, it releases particles of varying sizes into the air. Larger droplets travel a short distance and settle quickly on nearby surfaces or another dog’s face. Smaller particles, those under about 5 micrometers, can stay suspended in the air much longer and drift farther through a room or enclosed space. This means a dog doesn’t always need to be standing right next to a sick dog to pick something up, especially indoors with poor ventilation.
Contaminated surfaces are the other major route. Bordetella bacteria and several respiratory viruses can survive on food bowls, kennel walls, leashes, and human hands for hours to days depending on the pathogen. A dog that drinks from a communal water dish or mouths a shared toy at daycare can easily pick up an infection that way.
The Pathogens Involved
Canine respiratory infections are rarely caused by a single organism. More often, one virus weakens the airway lining and a bacterial infection piles on. The American Veterinary Medical Association lists the following as common contributors:
- Viruses: canine parainfluenza virus, canine adenovirus type 2, canine influenza (H3N2 and H3N8 subtypes), canine respiratory coronavirus, canine herpesvirus-1, and canine distemper virus.
- Bacteria: Bordetella bronchiseptica (the classic “kennel cough” bacterium), Mycoplasma species, and Streptococcus equi subspecies zooepidemicus.
Bordetella and canine parainfluenza are the most frequently identified culprits in routine kennel cough cases. Canine influenza tends to cause more severe illness and spreads in regional outbreaks. Distemper is the most dangerous pathogen on the list but is relatively rare in vaccinated populations.
Where Dogs Are Most Likely to Get Infected
Any place where dogs congregate in close quarters raises the risk. Boarding kennels, doggy daycares, grooming salons, training classes, shelters, and dog parks are the most common settings. The combination of many dogs breathing the same air, sharing surfaces, and greeting face to face creates ideal conditions for transmission. Stress from being away from home can also temporarily suppress immune function, making a dog more susceptible right when exposure risk is highest.
Dogs that stay home and interact only with a small number of known, healthy dogs face a much lower risk. But even a single trip to a busy veterinary waiting room or a weekend at a boarding facility can be enough for exposure.
How a Dog’s Airway Defenses Work (and Fail)
A healthy dog has a surprisingly effective filtration system built into its respiratory tract. The nasal passages are lined with tiny hair-like structures called cilia, covered in a layer of mucus. When a dog inhales particles carrying bacteria or viruses, the mucus traps them, and the cilia sweep the trapped material toward the throat to be swallowed or coughed out. Below that, a cough reflex, immune cells called macrophages that engulf invaders in the lungs, and protective proteins like antibodies and antiviral compounds add further layers of defense.
This system breaks down in a few ways. Very small particles can slip past the upper airway filters and reach the deepest parts of the lungs. High concentrations of pathogens, the kind a dog encounters in a crowded, poorly ventilated kennel, can overwhelm the mucus-and-cilia barrier. And a viral infection that damages the airway lining first makes it much easier for bacteria like Bordetella to establish a foothold, which is why co-infections are so common.
Why Some Dogs Are More Vulnerable
Puppies and senior dogs face the greatest risk because their immune systems are either still developing or declining. Dogs with chronic health conditions or those on immune-suppressing medications are also more susceptible.
Flat-faced (brachycephalic) breeds like French Bulldogs, English Bulldogs, and Pugs have anatomy that works against them. Their narrowed nostrils, abnormally developed nasal structures, elongated soft palates, and often undersized windpipes all restrict airflow. Changes in the nasal cavity alone account for roughly 80% of airway resistance in affected dogs. Their tongues are also disproportionately large for their mouths, reducing the air-to-soft-tissue ratio by about 60% compared to dogs with normal muzzle lengths. All of this means less efficient air filtration, more turbulence and irritation in the airways, and chronic inflammation that makes infection easier to establish. The risk for airway problems increases sharply as muzzle length gets shorter.
How Soon Symptoms Appear
After exposure, it typically takes 2 to 10 days before a dog starts showing signs of illness. The classic symptom is a dry, honking cough that sounds like something is stuck in the throat. Many dogs remain otherwise alert and still want to eat. In straightforward cases, the cough resolves on its own within one to three weeks.
More concerning signs include thick nasal discharge, lethargy, loss of appetite, labored breathing, or fever. These can indicate that a secondary bacterial infection has taken hold or that the infection has progressed to pneumonia. In recent years, veterinarians across the U.S. have reported cases of atypical canine respiratory disease where some dogs progress to severe illness in as little as 24 hours and test negative for all the usual pathogens. Researchers at Johns Hopkins and other institutions are still working to identify whether a new pathogen is involved or whether these cases represent unusually aggressive presentations of known organisms.
Vaccination and Prevention
Vaccines exist for several of the key pathogens, including Bordetella, canine parainfluenza, canine adenovirus type 2, canine distemper, and both subtypes of canine influenza. The core vaccines that most dogs receive as puppies cover parainfluenza, adenovirus, and distemper. Bordetella and canine influenza vaccines are considered noncore, meaning they’re recommended based on lifestyle risk rather than given to every dog automatically.
For dogs that regularly visit boarding facilities, daycares, or dog parks, annual Bordetella and canine influenza boosters are the current guideline from the World Small Animal Veterinary Association. The Bordetella vaccine comes in intranasal, oral, and injectable forms. Most boarding and daycare facilities require proof of Bordetella vaccination before accepting a dog.
Vaccination doesn’t guarantee protection. These vaccines reduce the severity and duration of illness rather than providing a complete shield, partly because so many different organisms can cause respiratory disease. Practical steps matter too: choosing facilities with good ventilation and low dog-to-space ratios, keeping your dog away from visibly sick dogs, and avoiding communal water bowls all reduce exposure risk.