THE MERCK VETERINARY MANUAL
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Overview of Respiratory Diseases of Horses

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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.

Secondary bacterial respiratory infections are primarily initiated by viral disease, because viral respiratory infections impair and/or destroy respiratory defense mechanisms (ie, influenza destroys the mucociliary apparatus, EHV destroys bronchial-associated lymphoid tissue). The most common organisms associated with pneumonia in horses are opportunistic bacteria originating from the resident microflora of the upper respiratory tract. Clinical evidence of a secondary bacterial infection includes mucopurulent nasal discharge, depression, persistent fever, abnormal lung sounds, hyperfibrinogenemia, and leukocytosis. Secondary bacterial disease may result in mucosal bacterial infections (rhinitis and tracheitis) or may produce more serious invasive disease such as pneumonia and pleuropneumonia. Streptococcus equi zooepidemicus is the most common opportunistic pathogen of the equine lung, although Actinobacillus equuli, Bordetella bronchiseptica, Escherichia coli, Pasteurella spp, and Pseudomonas aeruginosa are frequently isolated. S equi equi, the causative agent of strangles (see Strangles in Horses), is a primary bacterial pathogen of the upper respiratory tract and is capable of mucosal invasion without predisposing factors. Rhodococcus equi is a primary pathogen of the lower respiratory tract of foals and produces pulmonary consolidation and abscessation. R equi pneumonia has been reported in adult horses with a compromised immune system.

Noninfectious respiratory disease is a common, performance-limiting condition that affects adult horses of various ages. Inflammatory airway disease is characterized by excessive tracheal mucus, airway hyperreactivity, and poor exercise performance in young horses. The etiology is unclear, but viral respiratory infection (EHV-2), allergy, and environmental factors may play a role in the pathophysiology. Reactive airway disease (heaves) is triggered by exposure to organic dusts in older horses with a genetic predisposition to allergic airway disease. Small airways are obstructed by bronchoconstriction and excessive mucus production. The severity of clinical signs ranges from exercise intolerance to dyspnea at rest.

The respiratory system is one of the most accessible body systems to test diagnostically. Endoscopic examination allows direct visualization of the upper respiratory tract, guttural pouches, trachea, and mainstem bronchi. Indications for endoscopic examination include upper airway noise, inspiratory difficulty, poor exercise performance, and unilateral or bilateral nasal discharge. Radiographs of the skull are indicated to investigate facial deformity, abnormalities of the sinus (sinusitis, dental abnormalities, and sinus cyst), guttural pouch (empyema, tympany), and soft-tissue structures (epiglottis, soft palate). The most important techniques for evaluation of lower respiratory tract secretions are transtracheal wash and bronchoalveolar lavage. Transtracheal wash is indicated to obtain secretions for bacterial and fungal culture of the lower respiratory tract. Bronchoalveolar lavage is indicated for cytologic evaluation of the lower respiratory tract in animals with diffuse, noninfectious pulmonary disease. Nasal swab culture is inappropriate for investigation of pulmonary infectious disease but is indicated for horses with suspected strangles infection.

Thoracic radiography and ultrasonography are valuable for assessing lower respiratory tract disease. Thoracic radiography is used to identify abnormalities of the pulmonary parenchyma, mediastinum, and diaphragm. Pulmonary consolidation (pneumonia), peribronchial disease, pulmonary abscessation, interstitial disease, and mediastinal masses (neoplasia, abscess, granuloma) are most easily identified via thoracic radiography. Thoracic ultrasonography is the most appropriate technique to evaluate fluid in the pleural space, peripheral pulmonary consolidation, and peripheral pulmonary abscessation. Ultrasonographic examination can identify the volume, location, and character of pleural fluid or air within the pleural space (pneumothorax). Additionally, it can identify fibrin tags, gas echoes (anaerobic infection), masses, and loculated fluid pockets, and it allows the clinician to determine the most appropriate site for centesis and to formulate a prognosis.

Pleurocentesis is performed in animals with accumulation of fluid in the pleural space and should be conducted with ultrasonographic guidance. Lung biopsy and fine needle aspiration are invasive procedures and performed only after other diagnostic procedures have been exhausted. Pulmonary neoplasia, pulmonary fibrosis, and interstitial diseases may require lung biopsy to obtain a definitive diagnosis.

Vaccination does not always prevent respiratory infections in horses, but duration and severity is usually lessened in horses that have been vaccinated regularly, depending on factors such as the disease and specific vaccine. Vaccines are commercially available for equine influenza, viral rhinopneumonitis, equine viral arteritis, and strangles. The cost and hazards of each vaccination must be weighed against the probability of exposure and potential disease. Vaccination recommendations and schedules vary according to use of the horse and its potential for exposure to contagious animals. The American Association of Equine Practitioners (AAEP) Infectious Disease Committee has developed guidelines for all core and risk-based equine vaccinations; recommendations are posted on the AAEP Web site (www.aaep.org/vaccination_guidelines.htm).

Regardless of the type of respiratory disease, environmental factors and supportive care are important to aid recovery. A dust and ammonia-free stable environment prevents further damage to the mucociliary apparatus. Highly palatable feeds are indicated to prevent weight loss and debilitation during the treatment and recovery period. Adequate hydration will decrease the viscosity of respiratory secretions, facilitating their removal from the lower respiratory tract. A comfortable, dry, temperature-appropriate environment will allow the horse to rest and minimize the role of the respiratory tract in thermoregulation.

Last full review/revision January 2014 by Bonnie R. Rush, DVM, MS, DACVIM

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