Respiratory System - Merck Veterinary Manual

Respiratory System Introduction

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.
Aspiration Pneumonia in Large Animals

For discussion of aspiration pneumonia in small animals, see Respiratory Diseases of Small Animals.
- Aspiration Pneumonia in Large Animals
Diaphragmatic Hernia

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.
- Diaphragmatic Hernia in Animals
Fungal Pneumonia

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.
- Fungal Pneumonia in Animals
Laryngeal Disorders

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.
Lungworm Infection

Lungworm infection, also known as verminous bronchitis or verminous pneumonia, is an inflammatory disease of the lower respiratory tract caused by a variety of nematodes. Coughing and dyspnea are the most common clinical signs, which can be exacerbated by concomitant bacterial or viral infections. The Baermann technique is used to detect first-stage larvae in fecal samples. In some countries, an ELISA test is available to detect antibodies against Dictyocaulus viviparus; however, interpretation is hampered by persistent titers. Treatment and control of bovine lungworm is achieved primarily via strategic use of anthelmintics, although an orally administered vaccine is available in some countries.
- Lungworm Infection in Animals
Pharyngitis

Pharyngitis is an inflammatory condition of the oro- or nasopharynx. It develops secondary to viral or bacterial infections of the upper respiratory tract. Clinical signs of pharyngitis include upper respiratory tract noise, nasal discharge, coughing, and occasionally dysphagia. The diagnosis of pharyngitis is confirmed with upper airway endoscopy. Treatment of pharyngitis includes administration of antimicrobials and anti-inflammatories and restriction of exercise.
- Overview of Pharyngitis in Animals
- Pharyngeal Trauma in Animals
Pulmonary Emphysema

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.
- Pulmonary Emphysema in Animals
Respiratory Diseases of Cattle

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

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 (All.see page 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

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, classical swine fever, the pneumonic forms of pseudorabies, porcine circovirus-associated disease, and porcine reproductive and respiratory syndrome (PRRS). The causal viruses may persist in a herd, but outbreaks of overt disease tend to be self-limiting.
Respiratory Diseases of Sheep and Goats

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 of Small Animals

A varying flora of indigenous commensal organisms (including Pasteurella multocida, Bordetella bronchiseptica, streptococci, staphylococci, pseudomonads, and coliform bacteria) normally reside in the nasal passages, nasopharynx, and upper trachea, and at least intermittently in the lungs of dogs and cats, without causing clinical signs. Opportunistic infections by these bacteria may occur when respiratory defense mechanisms are compromised by infection with a primary pathogen (eg, distemper, parainfluenza virus, or canine type 2 adenovirus in dogs, and rhinotracheitis virus or calicivirus in cats), other insults (eg, inhalation of smoke or noxious gases), or diseases such as congestive heart failure and pulmonary neoplasia.