Respiratory System

The respiratory system performs several functions. Most importantly, it delivers oxygen to the cardiovascular system for distribution to the body and it removes carbon dioxide. Gas transfer occurs in the alveoli of the lungs, where the air-blood barrier is a thin, permeable membrane. Failure or major dysfunction of gas transfer due to disease processes that compromise this membrane or its air or blood supply have serious effects. In addition to gas exchange, the respiratory system performs numerous other functions, including maintaining acid-base balance, acting as a blood reservoir, filtering and probably destroying emboli, metabolizing some bioactive substances (eg, serotonin, prostaglandins, corticosteroids, and leukotrienes), and activating some substances (eg, angiotensin). The respiratory system also protects its own delicate airways by warming and humidifying inhaled air and by filtering out particulate material. The upper airways also provide for the sense of smell (olfaction) and play a role in temperature regulation in panting animals.

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.

A break in the continuity of the diaphragm allows protrusion of abdominal viscera into the thorax.

Fungal infections occur more commonly in dogs than cats and even less frequently in large animals. Clinical signs include chronic cough, acute respiratory distress, and lethargy. Diagnosis can be made by serologic testing or detection of fungal antigens in urine (for Histoplasma and Blastomyces). Treatment includes systemic antifungal medication, oxygen, and sometimes anti-inflammatory medications.

Laryngeal disorders are characterized by stertorous breathing and can often lead to death if untreated. The mainstay of treatment is steroids or NSAIDs. Presumptive diagnosis is largely based on clinical signs.

An infection of the lower respiratory tract, usually resulting in bronchitis or pneumonia, can be caused by any of several parasitic nematodes, including Dictyocaulus viviparus in cattle, llamas, and alpacas; D filaria in goats, sheep, llamas, and alpacas; D eckerti in deer; D arnfieldi in donkeys and horses; Protostrongylus rufescens and Muellerius capillaris in sheep and goats; Metastrongylus apri, M pudendotectus, and M salmi in pigs; Oslerus osleri, Crenosoma vulpis, and Eucoleus aerophilus in dogs; and Aelurostrongylus abstrusus and E aerophilus in cats. Other lungworm infections occur but are less common.

Pharyngitis is an inflammatory condition of the walls of the oro- or nasopharynx. Pharyngitis may develop secondary to viral or bacterial infections of the upper respiratory tract, eg, strangles in horses and distemper in dogs.

Pulmonary emphysema is one of the two conditions under the umbrella term chronic obstructive pulmonary disease (COPD), with the other being chronic bronchitis. Pulmonary emphysema is identified by marked overdistention of alveoli along with destruction of supporting alveolar and interstitial structures. Computed tomography is currently the best way to diagnose the disease, and treatment includes a combination of bronchodilators and anti-inflammatory medications. Because it causes permanent damage, prevention is key.

Respiratory disease is among the most economically important diseases of cattle in production on a worldwide basis.

Viral respiratory infections are common in horses; the most notable are equine herpesvirus infection, equine influenza, and equine viral arteritis. The clinical manifestations are similar and include pyrexia, serous nasal discharge, submandibular lymphadenopathy, anorexia, and cough. In addition to respiratory disease, equine herpesvirus type 1 (EHV-1) can cause abortion and neurologic disease, and equine herpesvirus type 5 (EHV-5) is a newly recognized cause of multinodular pulmonary fibrosis. Equine viral arteritis produces respiratory disease, vasculitis, and abortion. Equine herpesvirus type 2 (EHV-2), equine rhinitis virus, and reovirus are ubiquitous viral respiratory pathogens, and infection results in minimal clinical disease. Adenovirus pneumonia is most often seen in association with severe combined immunodeficiency in Arabian foals. Hendra virus (see Hendra Virus Infection) is a zoonotic disease of horses identified in Australia; it is rapidly fatal in horses, and close contact is necessary for disease transmission.

Respiratory diseases of pigs can be classified into two broad categories based on the extent and duration of overt disease: those that affect large numbers of pigs and may be serious but of limited duration, and those that persist in a large number of pigs for indefinite periods. Diseases in the first category can be costly, but the losses are limited rather than ongoing. They include swine influenza (see Swine Influenza), classical swine fever (see Classical Swine Fever), the pneumonic forms of pseudorabies (see Pseudorabies), porcine circovirus-associated disease (see Porcine Circovirus Diseases), and porcine reproductive and respiratory syndrome (see Porcine Reproductive and Respiratory Syndrome). The causal viruses may persist in a herd, but outbreaks of overt disease tend to be self-limiting.

The sheep nose bot fly, Oestrus ovis, is a cosmopolitan parasite that, in its larval stages, inhabits the nasal passages and sinuses of sheep and goats. Its geographic distribution is worldwide.

Respiratory diseases are common in dogs and cats. Although clinical signs such as coughing and dyspnea are commonly referable to primary problems of the respiratory tract, they may also occur secondary to disorders of other organ systems (eg, congestive heart failure).