Respiratory System

The respiratory system begins at the nose and ends at the distal alveoli. It is comprised of the upper and lower airways. The upper airway includes the nose, sinuses, and pharynx. The nose provides olfaction and temperature regulation in hyperthermic patients. The nasal turbinates initially humidify and warm air, and filter particulate matter. The lower airways include the trachea, bronchi, bronchioles, and alveoli. The primary function of the respiratory system is to deliver oxygen to the lungs to be exchanged with carbon dioxide.

For discussion of aspiration pneumonia in small animals, see Respiratory Diseases of Small Animals.

Diaphragmatic hernia is protrusion of abdominal organs or tissue through a congenital opening or traumatic rupture in the diaphragm. This may cause respiratory signs or may be subclinical. Thoracic radiography is highly diagnostic, and treatment centers on stabilization and surgical repair.

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).