Aspiration pneumonia is a pulmonary infection characterized by inflammation and necrosis caused by 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.
Etiology:
Faulty administration of medicines is a common cause of aspiration pneumonia. Liquids administered by drench or dose syringe should not be given 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. Administration of liquids by nasal intubation is not without risk, and careful technique is especially necessary in debilitated animals.
Inhalation of irritant gases or smoke is an infrequent cause. Aspiration of vomitus or attempts by animals to eat or drink while partially choked can result in aspiration pneumonia as well. Disturbances of deglutition, as in anesthetized or comatose animals (eg, mature cattle under general anesthesia or cows in lateral recumbency), vagal paralysis, acute pharyngitis, abscesses or tumors of the pharyngeal region, esophageal diverticula, cleft palate, megaesophagus, or encephalitis, are frequent predisposing causes.
Cats are particularly susceptible to pneumonia caused by aspiration of tasteless products such as mineral oil. In sheep, poor dipping technique may cause aspiration of fluid. Calves and lambs may inhale inflammatory debris if affected with diphtheritic laryngitis. Inhalation of milk by pail-fed calves can cause an acute necrotizing pneumonia due to the diffuse distribution of foreign material. 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 following delayed treatment for milk fever is highly fatal. In dogs with myasthenia gravis, aspiration pneumonia is the leading cause of death.
Clinical Findings:
A clinical history suggesting recent foreign-body aspiration is of greatest diagnostic value. Horses may develop fevers of 104-105°F (40-40.5°C), which can drop back into the normal range in a few days. Pyrexia is also seen in cats, dogs, and less commonly in cattle. The patient presents with acute dyspnea, tachypnea, and tachycardia. Associated findings are cyanosis and bronchospasm. A sweetish, fetid breath characteristic of gangrene may be detected, the intensity of which increases with disease progression. This is often associated with a purulent nasal discharge that sometimes is tinged reddish brown or green. Occasionally, evidence of aspirated material (eg, oil droplets) can be seen in the nasal discharge or expectorated material. On auscultation, wheezing sounds, pleuritic friction rubs, and crackling sounds of subcutaneous emphysema may be heard. In cows that aspirate ruminal contents, toxemia is usually fatal within 1-2 days. Cattle and pigs recover more frequently than horses, but mortality is high in all species. Recovered animals often develop pulmonary abscesses. In outbreaks after dipping of sheep, losses occur from day 2 to day 7 and then decrease gradually.
Lesions:
The pneumonia is usually in the anteroventral parts of the lung; it may be unilateral or bilateral and centers 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 becoming soft or liquefied, reddish brown, and foul smelling. There usually is an acute fibrinous pleuritis, often with pleural exudate.
Prevention and Treatment:
Atropine sulfate helps to control salivation stimulated by general anesthetics (eg, thiobarbiturates). Use of an endotracheal tube with an inflatable cuff prevents fluid aspiration during surgery.
The animal should be kept quiet. A productive cough should not be suppressed. Broad-spectrum antibiotics should be used in animals known to have inhaled a foreign substance, whether it be a liquid or an irritant vapor, without waiting for signs of pneumonia to appear. Care and supportive treatment are the same as for infectious pneumonias. In small animals, oxygen therapy may be beneficial. Despite all treatments, prognosis is poor, and efforts must be directed at prevention.
