Causes of Respiratory Disease in Animals

Congenital anomalies of the respiratory tract in animals are rare but do occur. Examples include the following:

• cysts in the sinuses and turbinates

• tracheal hypoplasia

• nasopharyngeal turbinates

• accessory lungs

Common causes of upper respiratory tract malfunction in animals include rhinitis (which results in fluid exudate containing neutrophils and macrophages) and erosion or ulceration of the nasal mucosa.

Rhinitis can be caused by viral, bacterial, fungal, or parasitic agents, as well as by hypersensitivity reactions, such as localized allergies and anaphylaxis (see The Biology of the Immune System). In addition, rhinitis in dogs and cats is often idiopathic. Atrophy of the turbinates (eg, in atrophic rhinitis in pigs) removes a major filtration function and exposes the lungs to much larger loads of dust and microorganisms.

Sinusitis can be a complication of upper respiratory infections, tooth root infection, or dehorning.

Laryngitis, tracheitis, and bronchitis (inflammation of the larynx, trachea, and a bronchus or bronchi, respectively) result in coughing and possibly inspiratory or expiratory dyspnea. Coughing can be nonproductive, if the irritation is caused by mucosal erosion; or it can be productive, if the irritation is caused by copious exudate in the major airways. Severe pulmonary edema and emphysema lead to extreme respiratory insufficiency.

A common cause of respiratory diseases is pneumonitis (inflammation or infection of the pulmonary parenchyma). Pneumonitis caused by an infection is known as pneumonia. Many systems are used to classify the various types of pneumonia. One useful system classifies them by distribution of lesions in the lungs.

• Focal pneumonia has one or more discrete foci in a random pattern (eg, abscesses due to emboli from other sites, tuberculosis, or actinomycosis).

• Lobular pneumonia accentuates the anatomical pattern of lobules (eg, bronchopneumonia caused by Pasteurella multocida in goats).

• Lobar pneumonia affects large areas of lobes and is often severe (eg, fibrinous pneumonic pasteurellosis in cattle).

• Diffuse or interstitial pneumonia often involves the entire lung (eg, maedi in sheep or hypersensitivity reactions).

A particular pneumonia can be further described according to its appearance or cause (eg, gangrenous, parasitic [verminous], aspiration).

Respiratory infection in an animal can result from one or more of the following factors:

• overwhelmed innate defense mechanisms

• stress (transport, illness)

• a highly virulent infectious agent

• a large inoculum

• immunocompromise

Bacterial proliferation in the pulmonary parenchyma is usually caused by a breakdown of the host's defenses as a result of stress (eg, stress brought on by transportation or concurrent illness) or cellular insult (eg, viral infection, toxicosis). Bacteria are breathed into the lung in large numbers and can overwhelm an animal's normal defense mechanisms, localize, multiply, and initiate inflammation. Bacterial pneumonia often originates at the bronchioalveolar junction. Resident microbiota along the airways of dogs and cats are a common cause of pneumonia; hematogenous spread is less likely.

Stress is also often a precursor of viral respiratory infections, particularly in groups of animals that have been recently congregated and stressed by travel, handling, and mixing. Some respiratory viral infections can cause temporary dysfunction of phagocytic mechanisms of the alveolar macrophages. This usually occurs several days after viral exposure. Inhaled bacteria proliferate and pneumonia ensues, often with an overwhelming infection and massive exudation into the alveoli.

Pneumonia can also be caused by direct infection with viruses, bacteria, and fungi, as well as by toxins arriving hematogenously, by inhalation, or by aspiration of food or gastric contents.

Through natural processes, possibly aided by appropriate therapy, exudate can be removed from the lungs, and mucosal lesions of the air passages can heal. However, serious sequelae can persist. Bronchiectasis is a chronic lesion. Ulceration of bronchioles caused by viral agents can lead to organized plugs of connective tissue in small bronchioles; these lesions, called bronchiolitis obliterans, can cause permanent obstruction, atelectasis, and severe respiratory insufficiency. Constriction of bronchi and bronchioles in chronic allergic bronchitis and bronchiolitis results in similar clinical signs; however, administration of bronchodilators rapidly relieves airway obstruction in cases of allergic bronchitis (eg, asthma in cats, heaves in horses). Some chronic pneumonias (eg, maedi in sheep) are characterized by firm diffuse lesions caused by hyperplasia of lymphoid follicles, hyperplasia of smooth muscle around bronchioles, diffuse fibrosis, and diffuse lymphocytic infiltration.

Most infectious pneumonias develop in the cranioventral portions of the lungs. However, infectious agents, as well as malignant tumor cells, can invade the lungs via the bloodstream.

Fluid or air within the pleural space (ie, empyema, hydrothorax, chylothorax, or pneumothorax) can also seriously impair respiratory function. Pleural space disease, whether or acute or chronic, causes patients to develop a short choppy breathing pattern with tachypnea and increased effort, often co-occurring with paradoxical movement of the chest and abdomen with each breath.

Pulmonary thrombosis leads to acute, often fulminant, respiratory failure as a result of a lack of pulmonary arterial blood flow to ventilated regions of the lung.

Toxic injury, as with acute bovine pulmonary emphysema and edema (ABPEE), causes edema, emphysema, and necrosis of alveolar epithelium, followed by compensatory hyperplasia of these cells; the effects on gas exchange result in severe hypoxia and dyspnea.

Pulmonary edema, the abnormal accumulation of fluid in the interstitial tissue and alveoli, can occur in animals in conjunction with circulatory disorders, particularly left-sided heart failure or increased capillary permeability, and occasionally in anaphylactic and allergic reactions. Head trauma, choking, chewing an electrical cord, and generalized seizures can cause noncardiogenic pulmonary edema. Animals with pulmonary edema stand rather than lie down, lie only in sternal recumbency, or might assume a sitting position (ie, orthopnea). Auscultation of the chest might reveal crackles or rales.

Diseases of the pleural space in animals include the following:

• pleuritis

• empyema

• hemothorax

• hydrothorax

• chylothorax

• pneumothorax

Pleuritis (pleurisy) is inflammation of the pleural lining. It can be caused by any pathogen that enters the pleural cavity, and it can be an extension of bronchopneumonia and chronic effusion. Clinical signs of pleuritis include rapid, shallow breathing; fever; and thoracic pain. Auscultation of the chest might reveal friction sounds ("rubbing") and dull lung sounds.

Empyema, or pyothorax (purulent exudate in the pleural cavity), is caused by pyogenic bacteria or fungi that reach the thoracic cavity via the blood or by extension of pneumonia, traumatic reticulitis, migrating foreign body, or penetrating chest wound. Clinical signs can include cough, fever, pain, tachypnea, and orthopnea.

Hemothorax (accumulation of blood in the pleural cavity) is usually due to thoracic trauma, coagulopathy, or thoracic neoplasia (eg, hemangiosarcoma).

Hydrothorax (accumulation of transudate in the pleural cavity) is usually due to interference with venous blood flow or lymph drainage, or it can be secondary to decreased colloid osmotic pressure from hypoalbuminemia.

Chylothorax (accumulation of chyle in the pleural cavity) is relatively rare and occurs most often in cats; however, it can also occur in dogs. Chylothorax can be caused by thoracic duct rupture; however, it is often idiopathic. It can also be secondary to left-sided congestive heart failure.

Pneumothorax (air in the pleural cavity) can be of traumatic or spontaneous origin. Air can enter the pleural cavity through penetrating wounds of the thoracic wall or by extension from pulmonary emphysema or ruptured bullae. The lung collapses if a large volume of air enters the pleural cavity. Bilateral pneumothorax can develop if the mediastinum is weak or incomplete. Inspiratory dyspnea or rapid, shallow breathing is evident.

• Also see pet owner content regarding lung and airway disorders in dogs, cats, and horses.