Overview of Aspiration Pneumonia
(Foreign-body pneumonia, Inhalation pneumonia, Gangrenous pneumonia)
Aspiration pneumonia is a pulmonary infection characterized by inflammation and necrosis due to inhalation of foreign material. The severity of the inflammatory response depends on the material aspirated, the type of bacteria aspirated, and the distribution of aspirated material in the lungs.
Inappropriate administration of therapeutic agents is a common cause of aspiration pneumonia in large animals and less common in dogs and cats. Liquids given by drench or dose syringe should not be delivered faster than the animal can swallow. Drenching is particularly dangerous when the animal’s tongue is drawn out, when the head is held high, or when the animal is coughing or bellowing.
In sheep, poor dipping technique with repeated immersion of the animal’s head may cause aspiration of fluid. Calves and lambs may inhale inflammatory debris if affected with diphtheritic stomatitis/laryngitis. The muscles of deglutition may be affected in lambs with nutritional myopathy. Pigs fed fine particulate food in dry environments may inhale feed granules. Aspiration pneumonia in cattle after treatment for milk fever is usually fatal. Cervids affected with chronic wasting disease may develop aspiration pneumonia due to CNS dysfunction. In dogs and less frequently in cats, aspiration pneumonia is generally associated with inhalation of oral ingesta, regurgitated material, or vomitus. Common risk factors for dogs and cats include pharyngeal abnormalities (cricopharyngeal motor dysfunction), esophageal diseases (megaesophagus, gastroesophageal reflux disease, esophageal obstruction), weakened clinical condition, and anesthesia or heavy sedation. Bacteria in aspirated material may initiate acute infection or secondary infection later in disease.
A history suggesting recent foreign body aspiration within the past 1–2 days is of greatest value for a diagnosis of aspiration pneumonia. Affected animals separate from the rest of the group and present with pyrexia 104°–105°F (40°–40.5°C), a painful expression, arched back, inappetance, depression, toxic mucous membranes, and an increased respiratory rate (>40–60 breaths/min) with a shallow abdominal component. This is often associated with a purulent nasal discharge that sometimes is tinged reddish brown or green. Milk yield is greatly reduced to zero in lactating animals. Thoracic auscultation reveals reduced lung sounds over affected consolidated lung, with increased breath sounds over normal lung. In cows that aspirate ruminal contents as a consequence of becoming cast with hypocalcemia, toxemia is usually fatal within 1–2 days.
Superficial consolidated lung and overlying lesions of fibrous pleurisy can readily be identified on ultrasound examination using either linear or sector probes connected to 5-MHz machines; pleuritic friction rubs are not audible on auscultation. In dogs and cats, clinical signs may be peracute, acute, or chronic. Cough, dyspnea, tachypnea, or exercise intolerance are seen most frequently. Thoracic radiographs generally show a bronchoalveolar pattern in gravity-dependent ventral lung lobes (right cranial and middle and left cranial lobes); however, radiographic changes may not be seen until 24 hr after acute aspiration.
Aspiration pneumonia is usually in the anteroventral parts of the lung; it may be unilateral in animals in which lateral recumbency was the cause of aspiration, or bilateral and centered on airways. In early stages, the lungs are markedly congested with areas of interlobular edema. Bronchi are hyperemic and full of froth. The pneumonic areas tend to be cone-shaped, with the base toward the pleura. Suppuration and necrosis follow. The foci become soft or liquefied, reddish brown, and foul smelling. There usually is an acute fibrinous pleuritis, often with pleural exudate. Animals that survive develop chronic abscesses and fibrous adhesions between the visceral and parietal pleura.
Broad-spectrum antibiotics should be used in animals known to have inhaled a foreign substance without waiting for signs of pneumonia to appear; however, this rarely occurs in farm animals presented with severe clinical signs. A transtracheal wash can help identify the causative agent for which an antibiotic sensitivity can be obtained. Care and supportive treatment include NSAIDs such as flunixin meglumine. In small animals, oxygen therapy can be useful. Saline nebulization and coupage may assist with generating a productive cough to facilitate clearance of the aspirated material. Despite all treatments, prognosis is poor, and efforts must be directed at prevention.