Aspiration Pneumonia

Secondary bacterial infection after aspiration of gastrointestinal or oropharyngeal contents is a common underlying reason for bacterial bronchopneumonia in adult dogs, but is rare in cats. Chemical damage to the lungs results from inhalation of acidic gastric fluid and particulate matter (“aspiration pneumonitis”). In some cases this is followed by development of bacterial bronchopneumonia. Conditions that predispose dogs to aspiration pneumonitis include brachycephalic obstructive syndrome; laryngeal paralysis, laryngeal surgery, or laryngeal foreign bodies; esophageal disorders such as megaesophagus, decreased esophageal motility, or esophageal foreign bodies; hiatal hernia; decreased mentation, seizures, or recumbency secondary to neurologic disease, sedation, or anesthesia; tracheostomy tube placement; vomiting; and swallowing disorders such as cricopharyngeal achalasia.11,13–15 More than 30% of dogs in one study had more than one abnormality that predisposed to aspiration; the combination of esophageal and neurologic disease was most common.¹¹ In cats, most bacterial pneumonia follows viral infections or hematogenous spread of bacteria to the respiratory tract; aspiration pneumonia is rarely reported.^4,6,12 Aspiration pneumonia has been described rarely in association with dysautonomia¹⁶ and arytenoid lateralization for treatment of laryngeal paralysis.¹⁷ Systemic immune compromise can predispose to life-threatening bacterial infection after aspiration in both dogs and cats.

In a study of 47 dogs with aspiration that had airway lavage specimens cultured (primarily by transtracheal wash), 77% of specimens yielded bacterial organisms.¹³ The most prevalent species was Escherichia coli. Other frequently isolated organisms from dogs with aspiration pneumonia were Mycoplasma spp., Pasteurella spp., Staphylococcus spp., Klebsiella spp., and Enterococcus spp.¹³ Overall, gram-negative aerobes are more likely to be present than gram-positive aerobes in dogs with aspiration pneumonia. Anaerobes are also frequently isolated, and many dogs have mixed bacterial infections. Infections with Pseudomonas aeruginosa, Acinetobacter baumannii, and Corynebacterium species such as Corynebacterium ulcerans have also been described.^13,18 Some of these infections may be acquired from the hospital environment.

Tracheobronchitis and Bronchopneumonia Secondary to Impaired Local Airway Defenses

Conditions that impair local airway defenses predispose to opportunistic invasion of the tracheobronchial tree and pulmonary parenchyma by commensal bacteria. These conditions include ciliary dyskinesia, viral infections (especially persistent viral infections such as canine distemper virus or feline calicivirus infection), tracheobronchial collapse, tracheal stent placement, eosinophilic bronchopneumopathy, airway foreign bodies or neoplasia, chronic bronchitis, and bronchiectasis.5,7,9,12,18–21 Chronic bronchitis and bronchiectasis either may be sterile inflammatory processes that predispose to opportunistic bacterial invasion, or alternatively they occur secondary to chronic bacterial infections of the airways (e.g., as a result of chronic low-grade aspiration secondary to laryngeal paralysis). However, in many dogs with disorders such as tracheobronchial collapse, eosinophilic bronchopneumopathy, or chronic bronchitis, concurrent bacterial infection is not present.

In one study of dogs with tracheal collapse that underwent bronchoscopy and bronchoalveolar lavage (BAL), Pseudomonas was the most prevalent opportunistic bacterial species isolated (see Figure 87-1).⁹ In contrast, Pseudomonas is an uncommon isolate from dogs with opportunistic lower respiratory infection.¹⁰ In dogs with airway foreign bodies (usually grass awns), anaerobes, Pasteurella, gram-positive cocci, and Actinomyces predominate.⁷

Complete Blood Count, Serum Biochemistry Profile, and Urinalysis

Most dogs and cats with bacterial bronchopneumonia have leukocytosis due to a neutrophilia, often with bandemia and toxic neutrophil changes.4,5,12,13,22 Leukopenia may also occur, but some animals have a normal leukogram. Anemia of inflammatory disease may also be present. The serum biochemistry profile and urinalysis can be unremarkable or may have changes that reflect the presence of concurrent systemic disease.

Arterial Blood Gas Analysis

Arterial blood gas analysis provides the most accurate assessment of oxygenation status in animals with bronchopneumonia. Hypoxemia (PaO₂ ≤80 mm Hg at sea level) can be detected in around 75% of dogs with aspiration pneumonia,13,22 with a mean PaO₂ of 69 to 77 mm Hg.^5,13,22 Hypocapnia may also be present. The alveolar-arterial (A-a) oxygen gradient may be increased (up to 83 mm Hg with a median around 41 mm Hg; for room air, values >15 mm Hg indicate lung dysfunction).^13,22

Plain Radiography

Thoracic radiographs in dogs with bacterial bronchitis may reveal a diffuse bronchial pattern and there may be evidence of underlying airway disease, such as tracheobronchial collapse or bronchiectasis. Plain thoracic radiographs are insensitive for diagnosis of bacterial bronchitis, and dogs with severe bronchitis can have normal findings.

Dogs with aspiration bronchopneumonia may have bronchointerstitial to alveolar changes in one or more lung lobes. Most often an alveolar pattern is present (Figure 87-2).^13,22 The most commonly affected lung lobe is the right middle lung lobe, and so a left lateral projection as well as right lateral and ventrodorsal projections in full inspiration are important to identify changes in this lobe. The left and right cranial lung lobes are also frequently affected.^13,22 Radiographic changes can lag behind the aspiration event for up to 24 hours. Clues to underlying causes of bronchopneumonia, such as megaesophagus or situs inversus (ciliary dyskinesia), may also be evident.

Dogs and cats with plant awn pneumonia may have a focal interstitial to alveolar pattern that tends to involve the caudal and accessory lung lobes, and to a lesser extent the right middle and left and right cranial lung lobes.²⁶ Less often, pneumothorax, consolidation of affected lung lobes, and/or pleural effusion (see Pyothorax) are identified.

Advanced Imaging

Because of increased sensitivity over plain radiography, computed tomography (CT) may be useful to evaluate the airways and pulmonary parenchyma when thoracic radiographs are unremarkable or underlying plant awn pneumonia is suspected. Airway thickening and bronchiectasis may be evident in dogs with bronchitis. Focal interstitial to alveolar pulmonary opacities and pleural thickening are the most common finding in animals with migrating plant foreign bodies; other findings include pneumothorax and/or pneumomediastinum, mild enlargement and contrast enhancement of thoracic lymph nodes, lung lobe opacification, mediastinal or thoracic wall masses, and/or pleural effusion (Figure 87-3).²⁶ CT may be useful to guide subsequent bronchoscopy and BAL fluid collection for animals with focal bronchopneumonia.

Bronchoscopy

On bronchoscopic examination, dogs and cats with bacterial bronchitis or bronchopneumonia may have edematous and hyperemic airways, accumulation of mucus or mucopurulent secretions, and/or mucosal irregularity.5,6 Other findings include airway collapse, airway stenosis, and bronchiectasis. Foreign bodies such as plant awns may be identified and removed (see Figure 87-3, B);⁷ if not, identification and localization of a focal accumulation of pus in an airway may direct subsequent surgical removal of a lobe affected by a migrating plant awn.

Cytologic Examination

Cytologic examination of airway lavage specimens or fine-needle aspirates of the lung in dogs and cats with bacterial bronchopneumonia usually reveals a mixed inflammatory response with a predominance of neutrophils. The presence of degenerate neutrophils should increase suspicion for bacterial infection. In some animals, one or more bacterial populations may be seen intracellularly and extracellularly, but culture is considerably more sensitive than cytologic examination for detection of bacteria in airway lavage specimens.

Culture

Ideally, airway lavage specimens should be collected and submitted for aerobic, anaerobic, and Mycoplasma cultures before treatment with antimicrobial drugs is commenced, although this may not be feasible if severe respiratory compromise is present. The need for anaerobic cultures is controversial because anaerobes are difficult to isolate and tend to have predictable susceptibilities. Anaerobes are most likely to be present in dogs with aspiration pneumonia or grass awn foreign bodies. Mycoplasma species may also be difficult to isolate (see Chapter 40). False-negative culture results can also occur if low numbers of bacteria are present, if the specimen is small, fastidious organisms are present, or if antimicrobial drugs have recently been administered.

Because a variety of gram-positive and gram-negative bacteria as well as Mycoplasma spp. can be found in low numbers in the lower respiratory tracts of healthy dogs and cats,3,27–29 a positive culture on a respiratory lavage specimen alone is not diagnostic for bacterial bronchitis or bronchopneumonia. However, a positive culture in conjunction with cytologic evidence of leukocytosis with intracellular bacteria supports the presence of infection. When quantitative bacterial cultures on BAL fluid can be performed, it has been suggested that growth greater than 1.7 × 10³ CFU/mL is likely to be clinically significant.⁵ BAL is more likely to identify clinically significant bacteria, but collection of specimens by transtracheal wash is sufficient if diffuse disease is present and bronchoscopy is not available or not affordable.³⁰ Care should be taken not to overinterpret the significance of few organisms, or minimally pathogenic organisms (e.g., Bacillus spp. or coagulase-negative staphylococci) that may represent airway contaminants.

Some animals with bacterial pneumonia may have positive blood cultures. If airway lavage is not possible because of severe respiratory compromise, blood cultures should be considered in order to obtain information about the identity and susceptibility of bacteria involved. Further study is required to determine the sensitivity and specificity of blood culture for diagnosis of bacterial pneumonia in dogs or cats.