Animals with bacterial pneumonia usually have a history of a productive cough, fever, tachypnea, and respiratory distress. However, some animals (and particularly cats) present with more vague signs of illness, such as malaise, depression, anorexia, and weight loss. An early clue to the diagnosis of bacterial pneumonia is a change in the respiratory pattern. Parenchymal infection with alveolar flooding by inflammatory debris leads to restrictive lung disease, and a rapid shallow breathing pattern results. Thoracic auscultation is typically abnormal with loud or harsh lung sounds, and crackles can be variably detected throughout the lung fields. Absence of lung sounds in an area is suggestive of lung consolidation. A mucopurulent nasal discharge can be observed when animals cough respiratory secretions into the nasopharynx or have concurrent nasal infection. Fever may or not be present. These general rules are followed much more closely in dogs than in cats, in which bacterial pneumonia can appear similar to chronic bronchitis.
A complete database will aid in establishing the severity of the pneumonia and identifying predisposing conditions. Leukocytosis with a left shift supports the diagnosis of bacterial pneumonia in an animal with appropriate signs; however, this is not always present. Neutropenia with a degenerative left shift can suggest fulminant pneumonia with pulmonary sequestration of neutrophils. A biochemical profile and urinalysis assist in the diagnosis of underlying conditions and systemic involvement. FeLV/FIV serology should be performed in cats with pneumonia, although a direct association has not been made between viral status and the incidence of bacterial pneumonia. Canine distemper virus, canine influenza virus, feline calicivirus and even feline herpesvirus-1 should be considered as predisposing causes of pneumonia.
Alveolar infiltrates with air bronchograms are considered the classic radiographic findings in bacterial pneumonia; however, diffuse, nonspecific infiltrates may also prevail, particularly in cats. Lobar consolidation occurs when alveolar infiltration coalesces to involve an entire bronchial tree.
Direct airway sampling through tracheal wash, bronchoscopy with bronchoalveolar lavage, or fine needle aspiration of the lung is indicated, when possible, to confirm the etiology of pneumonia and to obtain samples for Gram stain, culture/antibiotic sensitivity testing, and cytology. Gram staining characteristics and cytology can be useful for initiating antibiotic therapy as soon as possible. Florid pneumonia with gram-negative bacteria is best treated initially with fluoroquinolones. Additional drugs are needed to provide broad-spectrum coverage, particularly for anaerobic organisms or infection with gram-positive bacteria. Early therapy with the combination of a fluoroquinolone with a beta lactam antibiotic should be efficacious against most of the commonly encountered pathogens of the lower respiratory tract, including aerobes, anaerobes, and Mycoplasma.
Confirmation of pneumonia generally relies on airway cytology findings of intracellular bacteria within a neutrophilic response and culture of potential pathogens. Most bacterial infections in dogs are mixed infections, indicating the need to perform aerobic, anaerobic, and Mycoplasma cultures specifically.
Initial antimicrobial therapy should be modified, as needed, when final culture results are obtained. Ancillary treatment for pneumonia should focus on facilitating removal of respiratory secretions and supporting oxygenation as necessary. Intravenous fluid therapy is generally indicated to improve systemic, and thus respiratory hydration. Nebulization provides direct airway hydration when performed using appropriate equipment that will create small (2–5 microns) particles to penetrate the lower airways. Ultrasonic, compressed air, or vibrating mesh nebulizers can all be used. Some nebulizers require sterile saline, while others utilize sterile water. Careful attention to cleaning and disinfection of all tubing is recommended to ensure the life of the unit. Coupage after nebulization helps mobilize secretions.
Aspiration pneumonia is a serious and potentially life-threatening inflammatory lung process that results from inhalation of oropharyngeal or gastrointestinal contents into the respiratory tract. This triggers chemical, bacteriologic, and immunologic damage to the airways. Gastric acid aspiration alters surfactant function, resulting in loss of surface tension and atelectasis. Bronchoconstriction increases airway resistance and the work of breathing. Acid injury adversely impacts pulmonary resistance to infection, and the presence of oral or gastrointestinal bacteria in aspirated fluid can result in severe bacterial pneumonia.
Definitive diagnosis of aspiration pneumonia is challenging. It is often clinically recognized by the development of respiratory signs and radiographic infiltrates in an animal at risk. Therefore, recognition of underlying disorders associated with aspiration is essential to limit the occurrence of this complication in hospitalized patients and to provide preventive advice for owners of animals that have certain diseases. In a recent study, the most common underlying disorders included esophageal disease or dysfunction, vomiting, neurologic disorders including myasthenia gravis, recumbency or seizures, laryngeal disease including paralysis or surgery, and post-anesthetic aspiration.
Aspiration pneumonia is much more commonly recognized in the dog than the cat, perhaps because dogs develop a greater volume of aspirant or have less pronounced laryngeal response to possible aspiration. Most dogs are middle-aged to older at the time of developing the condition; however, any age animal can be affected. Large breed dogs may be affected more commonly because they have concurrent illnesses and predisposing factors associated with aspiration, such as laryngeal paralysis or chronic vomiting disorders.
Thoracic radiographs most commonly reveal an alveolar infiltrate, although, depending on the severity of disease and the time that has elapsed prior to aspiration, an interstitial pattern may be found. Many dogs will have a single lung lobe that is infiltrated and this is most commonly the right middle lung lobe. Involvement of this lung lobe in an aspiration event is most likely the result of the ventral orientation of this bronchus from the right mainstem bronchus, although the position of the animal at the time of aspiration will influence the radiographic pattern. Almost 50% of affected dogs will have 2 or more lung lobes affected and mainly the right cranial lobe or caudal segment of the left cranial lung lobe is infiltrated along with the right middle lung lobe. In general, the right lung is more commonly involved than the left. This is particularly important to consider when obtaining thoracic radiographs of a dog suspected of aspiration pneumonia. The left lateral projection would best depict the right lung, and therefore, 3-view thoracic radiographs should be obtained routinely. It is important to assess the esophagus in animals with cranial lung lobe or middle lobar infiltrates due to the commonality of esophageal disease as a predisposing disease for aspiration pneumonia. Esophageal dilation with air or fluid can be an early sign of dysfunction or megaesophagus, and gross or persistent distention indicates that a complete work-up for esophageal disease should be performed, including acetylcholine receptor antibody testing and possibly a resting cortisol or ACTH stimulation test for hypoadrenocorticism.
Resolution of aspiration pneumonia requires treatment of respiratory dysfunction, management of pneumonia, and treatment or control of the underlying condition. Firstly, a decision must be made on the advisability of additional testing. For example, performing a barium swallow in an animal with obvious megaesophagus could result in barium aspiration and worsened respiratory compromise. While collection of an airway sample would likely be of value in the animal with pneumonia, the benefit must be weighed against the risks of anesthesia or sedation for the procedure. Judicious use of broad-spectrum antibiotics and ancillary care for 2–3 days is often the wisest course of action. Use of bronchodilators in aspiration pneumonia or any form of pneumonia is somewhat controversial. Terbutaline could be beneficial in alleviating acute bronchoconstriction associated with aspiration of acidic stomach contents, and might be considered in cats because they tend to develop bronchoconstriction easily. Theophylline metabolism is inhibited by fluoroquinolones, which are commonly employed for treatment of pneumonia when gram-negative organisms are suspected, and toxic plasma levels might develop that worsen the animal's condition.
Overall, aspiration pneumonia has a relatively good prognosis with over 75% survival in one study of dogs. Survival does not appear to differ in dogs with esophageal, laryngeal, gastrointestinal, neurologic, or post-anesthetic underlying disease processes or in dogs that have one or many conditions. One study suggested that the severity of radiographic changes did not impact survival rates in affected dogs. What remains unknown is the percentage of dogs (or cats) that have chronic aspiration injury and the affect that this has on overall respiratory status.
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