THE MERCK VETERINARY MANUAL
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Pneumonia in Small Animals

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Pneumonia is an acute or chronic inflammation of the lungs and bronchi characterized by disturbance in respiration and hypoxemia and complicated by the systemic effects of associated toxins. The usual cause is primary viral infection of the lower respiratory tract.

Canine distemper virus, adenovirus types 1 and 2, parainfluenza virus, and feline calicivirus cause lesions in the distal airways and predispose to secondary bacterial invasion of the lungs. Parasitic invasion of the bronchi, as by Filaroides, Aelurostrongylus, or Paragonimus spp may result in pneumonia. Protozoan involvement, eg, by Toxoplasma gondii (see Toxoplasmosis) or Pneumocystis jiroveci, is rarely seen. Tuberculous pneumonia, although uncommon, is seen more often in dogs than in cats. The incidence of mycotic granulomatous pneumonias is also higher in dogs than in cats. Cryptococcal pneumonia has been described in cats. Injury to the bronchial mucosa and inhalation or aspiration of irritants may cause pneumonia directly and predispose to secondary bacterial invasion. Aspiration pneumonia (see Aspiration Pneumonia) may result from persistent vomiting, abnormal esophageal motility, or improperly administered medications (eg, oil or barium) or food (forced feeding); it may also follow suckling in a neonate with a cleft palate.

The initial signs are usually those of the primary disease. Lethargy and anorexia are common. A deep cough is noted. Progressive dyspnea, “blowing” of the lips, and cyanosis may be evident, especially on exercise. Body temperature is increased moderately, and there may be leukocytosis. Auscultation usually reveals consolidation, which may be patchy but more commonly is diffuse. In the later stages of pneumonia, the increased lung density and peribronchial consolidation caused by the inflammatory process can be visualized radiographically. Complications such as pleuritis, mediastinitis, or invasion by opportunistic organisms may occur.

Analysis of bronchoalveolar lavage fluid is valuable for the diagnosis of bacterial infections. Cytologic examination can demonstrate the animal's immune response and indicate the intracellular or extracellular location of bacteria. Bacterial culture and sensitivity testing is required and may include anaerobe and mycoplasma culture, especially in refractory cases. A viral etiology generally results in an initial body temperature of 104°–106°F (40°–41°C). Leukopenia, often expected, may not be seen in many viral respiratory infections (eg, canine infectious tracheobronchitis, feline calicivirus pneumonia, feline infectious peritonitis pneumonia). A history of recent anesthesia or severe vomiting indicates the possibility of aspiration pneumonia. Acutely affected animals may die within 24–48 hr of onset. Mycotic pneumonias are usually chronic in nature. Miliary nodules seen at necropsy may suggest protozoal pneumonia.

The animal should be placed in a warm, dry environment. Anemia, if present, should be corrected. If cyanosis is severe, oxygen therapy may be used, administered by means of an oxygen cage, with a concentration of 30%–50%. Empirical antimicrobial chemotherapy should be initiated and changed if needed based on results of culture of bronchoalveolar lavage fluid. Supportive therapy should be instituted as needed and may include oxygen supplementation, pulmonary physiotherapy (nebulization and coupage), and bronchodilators. If no response is seen after 48–72 hr of therapy, the treatment plan should be reassessed. Antimicrobial chemotherapy should be continued 1 wk after clinical and radiographic signs resolve.

Animals should be reexamined frequently. Chest radiographs should be repeated at regular intervals to monitor recurrence or note a primary underlying disease process and to detect complications such as lung consolidation, atelectasis, or abscessation.

Last full review/revision November 2013 by Ned F. Kuehn, DVM, MS, DACVIM

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