Aspiration pneumonia is caused by inhalation of foreign substances that causes inflammation and bacterial infection of the lungs. Affected animals typically show anorexia, pyrexia, and dyspnea, with increased or absent lower airway sounds on auscultation. Diagnostic tests include physical examination with auscultation, radiographic and ultrasonographic imaging, and cytological evaluation and pathogen testing ofrespiratory samples (eg, pleurocentesis or transtracheal wash specimens). Treatment includes broad-spectrum antimicrobials and supportive care, such as supplemental oxygen and fluid therapy, as needed. The prognosis is guarded to grave, depending on severity.
For aspiration pneumonia in small animals, see Respiratory Diseases of Small Animals.
Aspiration pneumonia is a lung disease characterized by inflammation and necrosis caused by inhalation of foreign material. The most common foreign material inhaled by large animals is a large volume of liquid, aspirated because of weakness from a different primary problem or iatrogenically during oral administration of fluids or other treatments. Aspiration can also occur during induction of general anesthesia if the airway is inadequately protected.
Etiology of Aspiration Pneumonia in Large Animals
Aspiration is often caused by improper technique in the oral administration of fluids or other treatments or to sudden movement of the animal because of ineffective restraint during treatment. Aspiration can also occur during induction of general anesthesia, if the airway is inadequately protected.
Inappropriate administration of therapeutic agents is a common cause of aspiration pneumonia in large animals, and more common than in dogs and cats. Liquids administered via nasogastric tube or oral dosing syringe should not be delivered faster than the animal can swallow. Oral fluids or treatments should be administered to ruminants via an oral esophageal tube that is passed into the rumen; the location should be checked via auscultation.
Administration of oral fluids and treatments to horses via nasogastric tube should be done only by a veterinarian or by a trained technician under the direct supervision of a veterinarian, and with the horse properly restrained. Negative pressure should be present while the nasogastric tube is being passed down the esophagus. If the tube is passed into the trachea, negative pressure will not develop, and the horse should cough. In addition, it is advisable to start oral administration with only water, to ensure that the nasogastric tube is in the stomach and not the lung. Horses typically cough if fluid is delivered into the lung.
An animal might regurgitate into a nasogastric or esophageal tube if the rumen or stomach is full. Fluids administered via nasogastric or esophageal tube ideally are given when the rumen is not full, to decrease the risk of regurgitation. Administration of fluids via nasogastric or esophageal tube is particularly dangerous when the patient’s tongue is extended out, when the head is held high, or when the patient is coughing or bellowing.
To minimize risk of regurgitation and aspiration, the animal's head should be held down so that if the animal regurgitates or coughs, fluid drains instead of being aspirated. A tube should not be placed in an animal that has its head down or that is breathing with its mouth open and the tongue extended. Administration of fluids should be stopped immediately if regurgitation occurs.
Sometimes fluids are needed when the rumen is full or impacted with fibrous feed. If the rumen is overfilled (as is common in cases of forestomach outflow obstructions, impactions, and vagal indigestion, as well as in cattle fed large amounts of concentrates to maximize growth for shows), it is best to wait 4–6 hours and reevaluate before passing a tube. Alternatively, IV fluid therapy may be administered if oral fluid administration is not an option.
In sheep, poor dipping technique with repeated immersion of the patient’s head can cause aspiration of fluid.
Calves and lambs can inhale inflammatory debris if affected with diphtheritic stomatitis or laryngitis. The muscles of deglutition can be affected in lambs with nutritional myopathy.
Pigs that are fed fine particulate food in dry environments can inhale feed granules.
Aspiration pneumonia in cattle after treatment for milk fever is usually fatal.
Cervids affected with chronic wasting disease can develop aspiration pneumonia due to CNS dysfunction.
Both ruminant and equine neonates can aspirate meconium in situations of high fetal stress or dystocia (see Dystocia in Horses and Management of Dystocia in Cattle). Hypoxia-induced encephalopathy, which is also associated with dystocia and fetal metabolic acidosis, can result in aspiration pneumonia from weakness or from inability to protect the airway during nursing.
Bacteria in aspirated material can initiate acute infection or secondary infection later in the course of aspiration pneumonia. Bacterial pneumonia commonly involves mixed aerobic-anaerobic infection.
Meconium aspiration in neonates might result in secondary bacterial infections; however, most tissue damage results from direct contact with meconium, which creates chemical pneumonitis.
Clinical Findings of Aspiration Pneumonia in Large Animals
The severity of the inflammatory response and key signs of aspiration pneumonia depend on the type and volume of material aspirated and the distribution of aspirated material in the lungs.
Clinical signs of aspiration pneumonia in large animals can be consistent with cranioventral bronchopneumonia, such as absent respiratory sounds when auscultation is performed over the consolidated lung regions, increased bronchovesicular sounds in less affected portions of lung, cough, fetid and/or mucopurulent nasal discharge, fever, dehydration, and anorexia. In severe cases when large volumes of liquid have been aspirated, death can occur swiftly.
Animals with aspiration pneumonia separate from the rest of the group and show pyrexia (40–40.5°C [104–105°F]) early in the course of disease; however, fever might not be detected later in the course of disease, especially in patients with severe dehydration.
Animals with aspiration pneumonia often show visible signs of pain, such as an arched back (see ), and reluctance to move. They might also show anorexia, mental dullness, and dark red mucous membranes suggestive of endotoxemia. In addition, their respiratory rate might be elevated (> 40–60 breaths/min), with a shallow abdominal component or extended head and neck and fulminant respiratory distress.
Courtesy of Dr. Philip R. Scott.
Respiratory disease is often associated with purulent nasal discharge that sometimes is tinged reddish brown or green, with a fetid smell. Milk yield in lactating animals is greatly decreased, sometimes to zero.
Lesions of Aspiration Pneumonia in Large Animals
Aspiration pneumonia usually occurs in the cranioventral lobes of the lung. It can be unilateral when caused by lateral recumbency, or bilateral and centered on airways. Aspiration in a standing patient often involves the right cranial and right middle lung lobes, because they are the first bronchial branches off the trachea.
In early stages of aspiration pneumonia, 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 consolidated and soft or liquefied, reddish brown, and foul smelling. There usually is acute fibrinous pleuritis, often with pleural exudate (see ).
Courtesy of Dr. Philip R. Scott.
Tissues removed during necropsy examination do not deflate and do not float in formalin. Patients that survive aspiration pneumonia develop chronic abscesses and fibrous adhesions between the visceral and parietal pleura (see ). These patients typically have lower respiratory capabilities and, therefore, decreased heat abatement ability and athletic capacity.
Courtesy of Dr. Philip R. Scott.
Diagnosis of Aspiration Pneumonia in Large Animals
Thoracic auscultation
Diagnostic imaging (ultrasonography, radiography, CT)
Cytological evaluation of trans- or endotracheal wash specimens
Bacteriological culture and antimicrobial susceptibility testing
Ancillary diagnostic tests (CBC, serum biochemical analysis, and arterial blood gas analysis)
A history suggesting recent (1–2 days earlier) aspiration of foreign material is the clearest indicator of aspiration pneumonia. Cough, dyspnea, tachypnea, and exercise intolerance are typical of aspiration pneumonia with fibrinous pleuritis.
Thoracic auscultation reveals decreased lung sounds over affected consolidated lung regions, with increased breath sounds over normal lung. Lung sounds are especially decreased in cases with accumulation of pleural fluid. Pleuritic friction rubs might not be audible on auscultation.
In cases of suspected aspiration pneumonia, imaging of the affected pulmonary field can be accomplished using a 5-MHz linear reproductive ultrasonography probe. Typical lesions include consolidated or hepatized lung parenchyma (primarily in the cranioventral lung fields) and hyperechoic pleural fluid.
Thoracic radiographs in cases of aspiration pneumonia generally show a bronchoalveolar pattern in gravity-dependent ventral lung lobes (right cranial lobe and middle and left cranial lobes); however, pleural fluid can distort images. Pulmonary tissue damage resulting from aspiration pneumonia might not be evident on plain radiographs for one to several days after acute aspiration.
In mild to moderate cases of aspiration pneumonia, lesions might not be immediately visible with radiography or ultrasonography; however, a lack of visible lesions should not stop aggressive treatment of suspect cases.
CT may be considered for some patients (eg, small animals, exotics, foals).
Definitive diagnostic tests might also include cytological testing of pleurocentesis and of trans- or endotracheal wash specimens, as well as aerobic and anaerobic bacterial culture and susceptibility testing.
Ancillary diagnostic tests could include CBC, serum biochemical analysis, and arterial blood gas analysis. In hospitalized cases, serial arterial blood gas analysis is indicated to monitor cardiopulmonary function.
In cattle that become recumbent because of hypocalcemia and subsequently aspirate, endotoxemia is usually fatal within 1–2 days. In ruminants or horses that aspirate oral fluids or treatments, clinical signs of aspiration pneumonia can take longer to become apparent, especially if only a small amount of liquid was aspirated. However, if aspiration is witnessed during oral administration of treatments or induction of general anesthesia, the patient should be treated as aggressively as possible with antimicrobials and supportive care for the best prognosis.
Pearls & Pitfalls
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Treatment and Prevention of Aspiration Pneumonia in Large Animals
Broad-spectrum antimicrobials and supportive care
Prevention of iatrogenic aspiration
Avoiding prolonged weakness and recumbency
The main focus of treatment for aspiration pneumonia is to treat early and aggressively with broad-spectrum antimicrobials that include anaerobic activity. For food animals and food-producing animals, drug choices include penicillin or oxytetracycline. Applicable regulations and withdrawal times should be observed.
More options, including metronidazole and chloramphenicol, are available for horses. In the US, it is illegal to administer chloramphenicol to food animals and food-producing animals.
Antimicrobial choice can be directed further by culture and susceptibility results from testing trans- or endotracheal wash samples.
Antimicrobials should be administered for at least 10–14 days and are commonly administered for 21–28 days.
Supportive care is important to maintain hydration and includes nutritional support. Oral or IV fluids might be indicated. Some patients require oxygen supplementation, placement of a chest tube, or more invasive measures.
Anti-inflammatory therapy is indicated for pain and to combat toxemia. NSAIDs used in horses and cattle with bacterial pneumonia include flunixin meglumine and phenylbutazone. In horses, flunixin meglumine is preferable over phenylbutazone. Note that regulatory restrictions and withdrawal times might apply to use in food animals and food-producing animals.
Corticosteroids used in large animals include dexamethasone for cattle and horses and prednisolone for horses.
Humidified nasal oxygen might be needed in severe cases of aspiration pneumonia or in neonates, and it is indicated when hypoxemia is diagnosed via arterial blood gas analysis.
Animals with aspiration pneumonia have difficulty with heat abatement when ambient temperatures are high, so well-ventilated or air-conditioned housing might be needed.
Athletic animals should be rested and not returned to training or competition without veterinary examination.
Aspiration pneumonia in large animals is prevented primarily by the prevention of primary diseases that could cause weakness or recumbency. In cattle, such diseases include traumatic injury, toxemia (eg, mastitis, metritis), and metabolic deficiencies (eg, deficiencies of calcium, potassium, magnesium, or phosphorus) that result in downer cow syndrome.
Neonates with hypoxia-induced encephalopathy or meconium aspiration are at high risk of aspiration pneumonia.
Usually, aspiration pneumonia is iatrogenic in origin and can be prevented by properly training personnel who administer oral fluids and treatments, as well as ensuring that the patient is adequately restrained for treatment.
The prognosis for large animals with aspiration pneumonia is guarded to grave, depending on how quickly treatment is instituted and how extensive the tissue damage is.
Key Points
Aspiration pneumonia in large animals is often caused iatrogenically, such as during administration of oral fluid therapy or induction of general anesthesia.
Large animals with generalized weakness and recumbency for any reason can also be affected, secondary to the primary disease process.
Diagnosis is based on history and signalment, as well as physical examination, diagnostic imaging, and cytological examination of samples submitted for pathogen identification.
Treatment includes broad-spectrum antimicrobials with antianaerobic activity (guided by susceptibility testing) and supportive care.
The prognosis is guarded to grave.
For More Information
Peek SF, Ollivett TL, Divers TJ. Respiratory diseases. In: Peek SF, Divers TJ, Rebhun's Diseases of Dairy Cattle. 3rd ed. Elsevier; 2018:94-167.
Wilkins PA, Lascola KM, Woolums AR, et al. Diseases of the respiratory system. In: Smith BP, Van Metre DC, Pusterla N, eds. Large Animal Internal Medicine. 6th ed. Mosby; 2021:515-701.e42.
Also see pet owner content regarding aspiration pneumonia in horses.
