Etiology of pneumonia in puppies
Puppies are often physiologically stressed by changes in ownership and new environments. In addition, poor nutrition, overcrowding,
poor hygiene, and concurrent diseases such as parasitism all predispose them to development of symptomatic contagious respiratory
tract infections. They can be exposed to a variety of infectious organisms, including viruses such as parainfluenza, adenovirus,
canine influenza and canine distemper virus; and bacteria such as Bordetella, streptococci, and Mycoplasma sp. Many of these organisms usually cause infectious tracheobronchitis (kennel cough), but can progress to pneumonia if the
load of infectious agents is high, or if the puppy is immunosuppressed. In addition to infectious pneumonia, puppies with
gastrointestinal tract disease caused by parasites or viruses can experience vomiting, which can predispose them to aspiration
The youngest, most immunosuppressed puppies, or those of breeds such as English bulldogs with congenital abnormalities including
brachycephalic airway syndrome or hypoplastic trachea, have a decreased ability to resolve respiratory tract infections. In
these patients, infectious pneumonia is a real and life-threatening risk when they are exposed to overwhelming loads of these
infectious pathogens. Puppies with infectious bronchopneumonia (i.e. infections of the lower respiratory tract) can be recognized
because they are usually systemically sick, often febrile, and they may have significant respiratory distress. These puppies
require aggressive and careful management.
In contrast, puppies that have infections confined to the upper respiratory tract (kennel cough) are usually clinically healthy,
eating and afebrile. Most of these less severely affected puppies will respond favorably to time, good husbandry, and antibiotic
Infectious tracheobronchitis "kennel cough" and Bordetella pneumonia
Acute tracheobronchitis is characterized by sudden onset of coughing, sometimes combined with fever, anorexia and depression.
The most common cause of acute coughing in dogs is infectious tracheobronchitis or "kennel cough". Dogs with kennel cough
frequently present with a sudden onset of clinical signs of paroxysmal coughing. Many have extremely viscid secretions, which
can be quite difficult to expectorate, causing them to gag as they cough. Most dogs with kennel cough are otherwise very stable,
and are not systemically ill. Many have a history of exposure to a stressful environment or potential infectious organisms.
The most severely affected animals are young puppies, but any other immunocompromised or older animal is also at risk.
Most cases of infectious tracheobronchitis are caused by infection with multiple organisms, rather than a single bacterium.
Most commonly, parainfluenza virus and Bordetella bronchiseptica are the culprits. Antigens of canine adenovirus and canine distemper virus are identified occasionally. Other organisms seen
include Mycoplasma spp, Pasteurella spp, and beta hemolytic Streptococcus spp. It is thought that any of these organisms can in fact cause disease alone, but that they act synergistically in most
Bordetella bronchiseptica is one of the few primary bacterial pathogens of the respiratory tract. It is a gram negative aerobic rod that is closely
related to Bordetella pertussis, the cause of whooping cough in people. Bordetella organisms are usually transmitted from dog to dog by aerosolization, but in heavily contaminated environments they may occasionally
be transmitted by fomites. The organism is fairly labile in the environment, and is destroyed by exposure to sunlight, heat,
and most detergents and disinfectants. Bordetella bronchiseptica possesses numerous fibrillar appendages that allow it to firmly attach to the cilia of the columnar epithelium lining the
nasal cavity and the airways after it has been inhaled. It then induces ciliary stasis, crippling one of the most important
defense mechanisms of the respiratory tract. Since the mucociliary escalator no longer functions, the organism can persist
in the airways for a protracted period of time, with isolates documented as late as 14 weeks post infection.
After inhalation of virulent Bordetella strains, an initial incubation period of 2-7 days is followed by sudden onset of a hacking, paroxysmal cough, which may be
dry rather than productive, and is exacerbated by exercise or excitement. Some dogs may have fever, but most are normothermic.
Similarly, most dogs do not have increased white blood cell counts. Some animals also have a mucopurulent nasal discharge,
reflecting bacterial infection of the ciliated epithelium in the nasal cavity. In general, Bordetella infections in dogs are relatively mild and often self-limiting. Clinical signs usually resolve spontaneously within 1-2 weeks.
In immunocompromized or massively exposed hosts, infection with Bordetella can cause infectious bronchopneumonia. Bordetella is a very common bacterial pathogen isolated in puppies with severe pneumonia,
particularly immunocompromized or stressed puppies of pet store origin. Young puppies with bronchopneumonia caused by Bordetella can require extensive antibiotic therapy, oxygen supplementation, and intensive supportive care for up to two weeks.