Bacterial infections of the lower respiratory tract are an important cause of morbidity and mortality in dogs and cats, especially in hospitalized patients and in those animals with other underlying predisposing diseases. Pneumonia can be rapidly fatal if not promptly diagnosed and treated.

Etiology

In most cases, bacterial pneumonia is probably the result of a combination of 1) compromise of the host's respiratory defenses; 2) the pathogenic potential of the offending bacteria; and 3) the nature of the exposure to the bacteria. Failure of one or more of the host's lower respiratory tract defense mechanisms is usually required for bacterial colonization. Host defenses against bacteria include 1) nasoturbinate filtration; 2) protective airway reflexes (sneezing, coughing, bronchoconstriction); 3) mucociliary clearance (mucus properties, ciliary propulsion); and 4) phagocytosis and killing by alveolar macrophages.

The spectrum bacterial isolates from animals with lower respiratory tract infection (pneumonia) are similar to the isolates made from the upper airways of healthy animals. The resident microflora of the respiratory tract is thought to be derived from aspiration of oropharyngeal bacteria (e.g., Streptococcus, Staphylococcus, Pasteurella, Klebsiella, E coli, and various anaerobes). Thus, most bacterial pneumonias are caused by secondary opportunistic invasion of these oropharyngeal bacteria. The principal primary bacterial pathogen of the respiratory tract in dogs and cats is Bordetella bronchiseptica. Primary respiratory pathogens can also include mycoplasmas, Chlamydia, and fungi.

Routes of Infection

Bacterial infection of the lower respiratory tract can occur by either the inhalation route or the hematogenous route. The inhalation route of infection includes: 1) aspiration of contaminated oropharyngeal fluid (normal secretions and saliva) or exudate associated with dental, oro-pharangeal, or nasal infections; 2) aspiration of food (such as occurs with the regurgitation caused by esophageal and swallowing disorders); 3) aspiration of vomitus (especially in anesthetized, depressed, or recumbent animals); and 4) contaminated endotracheal tubes, tracheostomy tubes, and bronchoscopes that carry bacteria into the lower respiratory tract.

The hematogenous route of infection involves seeding of the lung from bacteremia associated with infection of any distant tissue site. This can include intravenous catheter-associated bacteremia, which might be an important cause of hospital-acquired pneumonia.

Predisposing Factors

There are numerous predispositions and high-risk clinical settings for bacterial pneumonia. Pre-existing primary respiratory infections can be complicated by secondary bacterial invasion; for example, canine distemper virus; kennel cough complex; feline infectious respiratory disease (herpesvirus, calicivirus, Chlamydia); systemic mycoses; and pulmonary parasites (lung worms, lung flukes). Other predispositions include pulmonary thromboembolism (including heartworm disease); esophageal dysphagia and regurgitation; thoracic trauma and surgery; migrating pulmonary foreign bodies (e.g., foxtail awns); smoke inhalation; pulmonary neoplasia; systemic sepsis; and immunodeficiency syndromes. Pneumonia can also be associated with prolonged recumbency, stupor, or coma from any cause.

Clinical Manifestations

The most frequent clinical signs of bacterial pneumonia are cough, tachypnea, dyspnea, and lethargy. Fever is variable, occurring in less than 50% of cases. Nasal discharge is an infrequent sign. Pneumonia can be complicated by dehydration. Auscultation may reveal regions of increased or decreased breath sounds, crackles, and wheezes.

Diagnosis

Routine thoracic radiography generally reveals alveolar pulmonary infiltrates (fluffy alveolar opacities with air bronchograms) distributed especially to the ventral regions of the cranial and middle lobes. A complete blood count reveals a neutrophilic leukocytosis with a left shift in approximately 60% of cases. Cytologic examination and culture of lower respiratory tract specimens is highly recommended to confirm the diagnosis and to guide therapy. Respiratory specimens can be obtained by aspirates, washings, or brushings.

To limit oro-pharyngeal contamination of culture specimens, transtracheal washings, sterile endotracheal tube washings, bronchoscopic bronchoalveolar lavage, or guarded microbiologic specimen brushes can be used. Typically, cultures are positive and cytologies reveal mucupurulent inflammation with visible intracellular bacteria. The offending bacteria are often observed in stained cytologic preparations. Bacterial shape and Gram staining characteristics can guide initial antibiotic choice. Cocci are usually streptococci or staphylococci. Rods are usually Gram-negative bacteria. Branching filamentous rods are probably Actinomyces or Noccardia.

Treatment

Antibiotics, re-hydration (i.e., fluid therapy), and elimination or control of predisposing factors, are the highest priorities in treating bacterial pneumonia. Any bacterial isolate from the lower respiratory tract of a pneumonia suspect should be submitted for a susceptibility test, especially since the offending organisms in dogs and cats are often Gram-negative bacteria, which are extremely variable in their antibiotic sensitivity. While awaiting the results of culture and susceptibility, the initial choice of an antibiotic may be guided by a cytologic determination of the shape of the bacteria (cocci are assumed to be Gram-positives and rods are assumed to be Gram-negatives). Mixed infections occur in approximately 40% of cases. Suggested antibacterial regimens for pneumonia are as follows. For initial treatment of Gram-positive cocci, consider amoxicillin-clavulanate, cephalosporin, or trimethoprim-sulfa. For initial treatment of Gram-negative rods, consider amoxicillin-clavulanate, fluoroquinolone, cephalosporin, or trimethoprim-sulfa. For life-threatening sepsis, consider amoxicillin-clavulanate and fluoroquinolone in combination, amoxicillin-clavulanate and amikacin in combination, cephalosporin and amikacin in combination, or a third generation cephalosporin. For Bordetella bronchiseptica, consider amoxicillin-clavulanate, doxycycline, fluoroquinolone, or aerosolized gentamycin. For mycoplasma, consider doxycycline or fluoroquinolone. The antibiotic regimen should be continued for at least one week beyond clinical and radiographic remission of the infection; thus, the usual minimum duration is two or three weeks.

Measures to facilitate clearance of exudate and secretions from the lower respiratory tract include: 1) parenteral fluid therapy to maintain optimal systemic hydration and thereby prevent inspissation and retention of airway secretions; 2) airway humidification (nebulizer, vaporizer) to reduce viscosity of airway secretions; 3) bronchodilation using theophylline, terbutaline, or albuterol; 4) physical therapy such as mild exercise and chest percussion/vibration (“chest clapping”) to promote cough, lung expansion, and mobilization of retained respiratory secretions and exudates; and 5) avoidance of antitussive drugs that would suppress the beneficial cough reflex. Over-the-counter expectorant drugs have limited activity and an objectionable taste. Supplemental oxygen (oxygen cage, nasal oxygen tube) can be beneficial for relief of severe hypoxemia and dyspnea, preferably guided by blood gas monitoring if possible.

The success of treatment is monitored closely and indicated by resolution of fever, auscultation abnormalities, hypoxemia, neutrophilia, and radiographic pulmonary infiltrates.