Managing puppies with pneumonia (Proceedings)

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 pneumonia.

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 therapy.

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 clinical cases.

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.

Antibiotic therapy and cultures

Representative cultures should be obtained from the respiratory tract prior to initiation of antibiotic therapy. In most puppies, cultures are best obtained by endotracheal lavage. Once samples have been obtained for culture, antibiotic therapy should be instituted immediately. The initial antibiotic should provide broad-spectrum coverage for the most likely organisms, bearing in mind the possibility of polymicrobial infection. Cytologic results may assist in choice of the best antibiotic, by documenting whether the bacterial organisms are gram positive or gram negative, rods or cocci. Although Bordetella bronchiseptica is often implicated especially when the puppy has a history consistent with possible exposure (obtained from a pet store or shelter), it is important not to forget that aspiration pneumonia can also occur in this population. Aspiration pneumonia is usually caused by gram negative enteric aerobes such as E. coli, Klebsiella, or Enterobacter.

As a general rule, oral antibiotics can be used if the pneumonia puppy is systemically healthy and is not dyspneic. Antibiotics should be administered by parenteral routes (ideally intravenously) in puppies that are dyspneic, febrile, debilitated, or depressed. Intravenous antibiotics are the best way of ensuring that adequate plasma concentrations are achieved, because there is no guarantee of adequate absorption of drugs from the gut in such sick animals.

Our experience suggests that the best initial antibiotic choice in puppies with severe pneumonia is a combination of ampicillin and an aminoglycoside (once dehydration has been corrected), in addition to azithromycin which provides optimal coverage for Bordetella. When ampicillin is combined with an aminoglycoside, a synergistic effect provides excellent broad spectrum coverage in serious respiratory infections. Other options such as enrofloxacin or tetracyclines should ideally be avoided because of their respective adverse effects on joints and teeth. Interestingly, the beta lactams such as amoxicillin, ampicillin and ticarcillin do not penetrate well into the mucus lining the bronchi, and therefore are often less effective in puppies with Bordetella pneumonia. Once culture and sensitivity results are available, a specific and narrow spectrum antibiotic can then be chosen for ongoing care.

Airway clearance

Clearance of secretions from the airways occurs via the mucociliary escalator and cough reflex, and is delayed if the secretions are extremely viscous and tenacious. In puppies with pneumonia, large amounts of viscous secretions are produced, and must be moved up through a very narrow airway. Attempts to resolve the infection must include attention to the character of the respiratory secretions. Productive coughing must be actively encouraged, and the secretions must be maintained as liquid as possible. More than 90% of the mucus in the respiratory tract is water, so even a mild degree of dehydration leads to drying of the secretions. The most important means to achieve this is by judicious parenteral fluid therapy. Unless extreme respiratory distress is present, these patients should not be allowed to become dehydrated, and diuretic use should be avoided. Nebulization is a technique in which tiny spherical droplets of water are generated and inhaled by the patient. The droplets then "shower out" at various levels of the respiratory tract, depending on their size, due to changes in direction of air flow, brownian motion, and gravity.

The tenacity of mucus also depends on the structure of the mucopolysaccharides that it contains. N-acetylcysteine can be administered orally, and acts as a mucolytic by opening disulfide bonds, thereby decreasing the viscosity of the mucus. It can also be administered by nebulization, but it can cause bronchospasm by this route, which is usually manifested by coughing. If coughing or dyspnea occurs, the patient may be pre-treated with bronchodilators prior to nebulization. N-acetylcysteine can also be given intravenously or orally. Drug therapy can also include a bronchodilator such as aminophylline or terbutaline, ideally administered parenterally.

Once the respiratory tract secretions have been moistened and increased in volume, clearance of the material depends on normal function of the other respiratory defense mechanisms. In particular, the cough reflex is a vital part of recovery from serious pneumonia. The simplest method of stimulating coughing is simply to stimulate an increased tidal volume during respiration, usually by mild exercise. Puppies with pneumonia should not be allowed to lie in one place for long periods of time. The amount of exercise needed to increase the tidal volume and respiration rate is variable depending on the severity of disease. Mild to moderate exercise often stimulates productive coughing, which should be encouraged by coupage. Coupage is the action of gently tapping the chest wall of the puppy with a cupped hand, which helps to stimulate the cough reflex and to "break up" secretions in the airways. Coupage should be performed several times daily.

Supportive care

Oxygen supplementation should be delivered as required to keep the puppy comfortable. Many of the sickest puppies require oxygen supplementation for prolonged periods of time, sometimes as long as 2-3 weeks. Puppies with severe respiratory distress may not be able to sleep because they need to remain sternal with their head and neck stretched out in order to breathe. In these puppies, the provision of a pillow or teddy bear can help them to find a comfortable position in which to sleep.

Nutritional support must also be considered in these puppies, particularly if they are too dyspneic to eat sufficient calories to support their resting energy expenditure. Ideally, if voluntary food intake is possible, they should eat a high calorie diet designed for recovery from illness or a high quality puppy food. Careful calorie counting should be performed to ensure that they are eating enough, and the puppy should be weighed daily to confirm that growth is occurring. Alternatively, a feeding tube may be placed, although this can be risky in these small patients who present significant anesthetic risks. Nasoesophageal feeding tubes are not usually a good choice in these dyspneic patients. Total parenteral nutrition is another possible option when it is available.

Radiographic monitoring

Puppies with pneumonia must be monitored carefully to ensure that they are continuing to respond appropriately to therapy. Radiographs of the chest should be obtained periodically during hospitalization (about every 4-7 days) to confirm that the alveolar disease is resolving. Failure to achieve clinical or radiographic improvement should prompt reconsideration of antibiotic therapy, repeat tracheal wash culture, or repeated attempts to resolve the underlying cause of the pneumonia.

Lung function testing

Lung function should be repeatedly evaluated by monitoring arterial blood gases or pulse oximetry. The pulse oximeter can be used for intermittent monitoring of oxygen saturation, or alternatively it can be used to provide a continuous real-time read-out, which is particularly useful for monitoring general anesthesia or sedation. The pulse oximeter can also be used to monitor changes in saturation when stressful procedures are being performed, for example transtracheal washes or radiographs. This technique allows determination whether a need exists for oxygen supplementation, and also to objectively assess the response in terms of an increase in oxygen saturation. Pulse oximetry readings of < 90% are clinically significant, and should be addressed immediately with oxygen supplementation. Desaturation in dogs that are already on oxygen supplementation is a serious situation.

Arterial blood gas analysis is the gold standard for direct assessment of pulmonary function, and it also provides information about the metabolic acid-base status of the body. Normal partial pressure of oxygen is expected to be 90-100 mmHg when the animal is breathing room air, and results < 80 mmHg are clinically significant. Normal partial pressure of carbon dioxide is 35-45 mmHg, and clinically significant hypoventilation occurs when carbon dioxide is > 50 mmHg. Many puppies with bacterial pneumonia experience hypoventilation and slightly high carbon dioxide concentrations. Sequential analysis of arterial blood gas results is the most accurate tool for objectively assessing trends in response to treatment.

Follow-up care and duration of therapy

Once the lung function has returned to normal, the radiographs are improving, and the puppy is feeling better, eating well, and is active and alert, oral antibiotic therapy can be instituted and discharge from the hospital can be considered. In most adult dogs, this occurs 3-14 days from hospital admission, but in puppies it can be a very protracted process sometimes requiring as long as 3-4 weeks of hospitalization and oxygen supplementation.

The puppy should be re-examined about one week after discharge with chest radiographs to confirm that the pneumonia is continuing to resolve. In severe cases, several weeks or even months of therapy are required for complete resolution of radiographic signs of pneumonia. As long as the animal is doing well clinically, it should be radiographed approximately every 2 weeks until the radiographs are normal. Oral antibiotic therapy should be continued for a further 2 weeks after radiographic resolution of the disease, in order to assure that the bacterial infection has been completely eliminated. The total duration of antibiotic therapy may be as long as 3-6 months in severely affected puppies.

References available on request