Overview of Respiratory Diseases of Horses

ByKara M. Lascola, DVM, MS, DACVIM-LA, Department of Clinical Sciences, College of Veterinary Medicine, Auburn University
Reviewed/Revised Jul 2023

Respiratory disease represents a notable cause of morbidity and loss of performance in horses of all ages, breeds, and disciplines. It includes noninfectious conditions (equine asthma, laryngeal hemiplegia) and infectious diseases (strangles, viral infections, bacterial pneumonia) of the upper and lower respiratory tract.

Certain disorders of the upper respiratory tract, such as laryngeal hemiplegia (recurrent laryngeal neuropathy), represent important causes of poor performance and abnormal respiratory noise in sport, work, and race horses. These are often dynamic conditions for which clinical signs may be absent at rest. For other upper respiratory tract disorders (ethmoid hematoma, guttural pouch mycosis, nasal polyps) signs of nasal discharge or abnormal respiratory noise secondary to airway obstruction are often more obvious at rest.

Viral respiratory infections are the most important causes of respiratory disease in horses worldwide. Those of importance include equine influenza virus (EIV), equine herpesvirus (EHV), and equine viral arteritis (EVA). The clinical manifestations of respiratory disease are similar among these viruses and include pyrexia, serous nasal discharge, submandibular lymphadenopathy, anorexia, and cough. EHV and EIV infection may also damage natural defense mechanisms of the respiratory tract, increasing the risk for secondary bacterial pneumonia. Among herpesvirus, EHV-1 and EHV-4 are the most important causes of respiratory disease. EHV-1 infection can also cause abortion and neurologic disease. EHV-5 is associated with equine multinodular pulmonary fibrosis. Equine viral arteritis produces abortion, vasculitis, and respiratory disease. Equine rhinitis virus and equine adenovirus are ubiquitous but of less clinical importance as viral respiratory pathogens, with infection producing subclinical or minimal clinical disease in immunocompetent animals. Hendra virus is a zoonotic disease of horses in Australia; it causes severe and rapidly fatal respiratory or neurologic disease in horses and humans.

Both primary and secondary bacterial respiratory infections are reported in the horse. Streptococcus equi equi, the causative agent of strangles, is a primary bacterial pathogen of the upper respiratory tract and is capable of mucosal invasion without predisposing factors. Clinical signs include fever, lymphadenopathy, and swollen lymph nodes. Strangles is highly infectious among horses and is reportable in the US and other countries. Rhodococcus equi is a primary pathogen of the lower respiratory tract of foals and produces chronic suppurative bronchopneumonia and pulmonary abscessation. Clinical signs are often mild until more advanced stages of disease. Common extrapulmonary manifestations include gastrointestinal disease, polysynovitis, uveitis, and septic synovitis or osteomyelitis. R equi infection has only sporadically been reported in adult horses.

Secondary bacterial infections of the lower respiratory tract occur when opportunistic bacteria, typically resident microflora of the oral cavity or upper respiratory tract, gain access to the lower airways and overwhelm the natural respiratory defense mechanisms, resulting in bronchopneumonia or pleuropneumonia. These clinical syndromes are often severe and may be life-threatening; signs include fever, depression, abnormal lung sounds, and sometimes nasal discharge.

Fungal respiratory disease is relatively rare but may present with clinical signs similar to those of bacterial respiratory infection of the upper or lower respiratory tract. Fungal infections should be considered in geographic areas with recognized prevalence.

Chronic, nonseptic inflammatory disease of the lower airways is a common condition that affects adult horses of various ages and is referred to as equine asthma. This allergen-mediated condition is characterized by airway inflammation, reactivity and remodeling in response to organic dusts or molds in the environment. Signs range from poor exercise tolerance to cough and dyspnea even at rest. Environmental modification is the mainstay of treatment.

Diagnosis and Clinical Findings of Respiratory Disease in Horses

The goals of diagnostic evaluation for respiratory disease in horses are to localize the problem (lower vs upper respiratory tract), to determine whether the condition is of infectious or noninfectious origin, and to evaluate the severity of disease. A diagnostic approach to evaluating a horse suspected to have respiratory disease should include a thorough history and complete physical examination that includes detailed evaluation of the upper and lower respiratory tract. Presenting complaints of respiratory disease will vary according to location of the problem; however, they often include nasal discharge, abnormal respiratory noise at rest or work, cough, reduced performance, and abnormalities in breathing effort or pattern. Additional information that is useful to obtain in the patient’s history includes duration of the problem, recent travel, contact with other horses, and a recent history of fever. In very young foals, respiratory disease may be more difficult to identify because it is often accompanied by other comorbidities.

Initial physical examination is best conducted with the patient at rest; however, it may include evaluation of the patient while exercising when there is a primary complaint of poor performance or upper respiratory noise. The goals of examination are to rule out primary problems involving other body systems and to localize the problem to the upper or lower respiratory tract. Characterization of respiratory efforts and patterns is important, as is thorough auscultation of the lung fields on both sides of the chest. In horses that are not demonstrating overt signs of increased respiratory effort at rest, a rebreathing examination can be useful to identify and better characterize adventitial lung sounds (wheezes, crackles, pleural friction rubs). The goal of the rebreathing examination is to increase the rate and depth of respiration by having the horse “rebreathe” exhaled CO2. This improves the veterinarian’s ability to auscult breath sounds in the peripheral airways.

Additional diagnostic testing is essential to identify the etiologic agent of infectious conditions and for refining treatment decisions. Important diagnostic tests include endoscopy (upper and lower airway), respiratory tract fluid analysis, and imaging of the respiratory tract (radiography, ultrasonography).

Endoscopy is an essential tool for direct examination of the upper respiratory tract including the nasal passages, ethmoid turbinates, opening of maxillary sinuses, guttural pouches, pharynx, larynx, and trachea. Endoscopy at rest often provides a definitive diagnosis; however, dynamic respiratory endoscopy allows for evaluation of the horse during exercise on a treadmill or when ridden (overground telemetric endoscopy) and is essential for evaluating dynamic function of the upper respiratory tract. Longer endoscopes can be used to visualize the mainstem bronchi. Indications for endoscopic examination include upper airway noise, inspiratory difficulty, poor exercise performance, and unilateral or bilateral nasal discharge.

Imaging is essential for diagnosis of upper respiratory conditions in particular. Radiographs of the skull are indicated to investigate facial deformity, sinus abnormalities (sinusitis, dental abnormalities, and sinus cyst), guttural pouch conditions (empyema, tympany), and soft tissue structures (epiglottis). CT is especially valuable to more comprehensively and specifically define abnormalities of the skull and upper airway.  In very young foals, CT scan of the thorax can be useful in characterizing pulmonary disease. 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; thoracic ultrasonography allows the veterinarian to determine the most appropriate site for centesis and to formulate a prognosis.

Nasopharyngeal swab, wash culture or PCR array are indicated tests for upper respiratory conditions such as suspected strangles or viral infection, but are inappropriate for investigation of pulmonary infectious disease. A guttural pouch wash can also be used in horses with suspected strangles. The most important techniques for evaluation of lower respiratory tract secretions are tracheobronchial aspiration (TBA) and bronchoalveolar lavage (BAL). TBA is indicated to obtain secretions for bacterial and fungal culture of the lower respiratory tract in horses with bronchopneumonia. BAL is indicated for cytologic evaluation of the lower respiratory tract in animals with diffuse, noninfectious pulmonary disease such as equine asthma or exercise-induced pulmonary hemorrhage (EIPH). Pleurocentesis is performed in animals with accumulation of fluid in the pleural space due to septic or neoplastic disease and should be conducted with ultrasonographic guidance. If large amounts of fluid are present, removal of the fluid via pleurocentesis can greatly improve the horse’s ability to ventilate. Pulmonary neoplasia, pulmonary fibrosis, and interstitial diseases may require lung biopsy to obtain a definitive diagnosis; this is performed only after other diagnostic procedures have been exhausted.

Prevention and Treatment of Respiratory Disease in Horses

Vaccination remains the cornerstone of prevention of many infectious respiratory pathogens in horses and should be a part of all preventive care and biosecurity plans. Additionally, although it does not always prevent respiratory infections in horses, regular vaccination can lessen the duration and severity of some diseases. 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 infectious animals. The American Association of Equine Practitioners (AAEP) Infectious Disease Committee has developed guidelines for all core and risk-based equine vaccinations.

Treatment for horses with respiratory disease varies according to the nature of the disease. 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.

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