Equine Respiratory Conditions: Diagnosis and Management

Equine respiratory diseases range from chronic inflammatory airway disorders to acute viral and bacterial infections that can threaten performance and life. This article provides horse owners and equine veterinarians with diagnostic approaches and management strategies for common respiratory conditions including equine asthma (heaves), influenza, strangles, pneumonia, and exercise-induced pulmonary hemorrhage. The guidance is based on peer-reviewed evidence and official veterinary resources from the American Association of Equine Practitioners (AAEP), Merck Veterinary Manual, American College of Veterinary Internal Medicine (ACVIM), and World Organisation for Animal Health (WOAH). Owners should use this information to recognize clinical signs, implement environmental controls, and determine when veterinary intervention is required.

At a Glance

Recognizing Clinical Signs of Respiratory Disease

Horse owners should monitor for changes in breathing pattern, nasal discharge, cough, and exercise tolerance. The Merck Veterinary Manual provides guidance on common respiratory signs that warrant veterinary attention. A normal adult horse at rest has a respiratory rate of 8 to 16 breaths per minute. Tachypnea (elevated respiratory rate) or dyspnea (labored breathing) indicates respiratory compromise.

Coughing in horses can result from environmental irritants, infections, or chronic airway inflammation. A dry, hacking cough that persists after exercise or during feeding suggests airway irritation. Nasal discharge may be serous (clear), mucoid (white or cloudy), or purulent (yellow-green). Bilateral purulent discharge often accompanies bacterial infections such as strangles. Unilateral discharge may indicate guttural pouch infection or dental disease.

Fever (temperature above 101.5°F or 38.6°C) is a hallmark of infectious respiratory disease. The AAEP advises owners to take rectal temperature daily during outbreaks and to isolate any horse with fever and respiratory signs. Lethargy, reduced appetite, and enlarged submandibular lymph nodes are additional indicators of systemic infection.

Exercise intolerance is a common presenting complaint in horses with chronic respiratory conditions. Owners may observe that a horse tires more quickly, coughs during or after work, or shows prolonged recovery after exertion. These signs warrant veterinary evaluation including endoscopy and bronchoalveolar lavage.

Equine Asthma (Heaves)

Equine asthma encompasses a spectrum of inflammatory airway diseases ranging from mild to severe. The condition was previously termed heaves, recurrent airway obstruction (RAO), or inflammatory airway disease (IAD). The ACVIM consensus statement supports the term equine asthma to reflect the similarities to human asthma. Severe equine asthma is characterized by bronchoconstriction, airway inflammation, and mucus accumulation triggered by inhaled allergens such as mold spores, dust, and endotoxins from hay and bedding.

Pathophysiology and Risk Factors

The inflammatory response in equine asthma involves pulmonary intravascular macrophages and neutrophils. A study published in Research in Veterinary Science examined serum cortisol concentrations in horses with heaves treated with fluticasone propionate over a one-year period. The research indicated that inhaled corticosteroids can be used long-term without significant adrenal suppression. The role of pulmonary intravascular macrophages in the development of heaves was described in a 2008 study, highlighting the importance of these cells in the pulmonary inflammatory cascade.

Risk factors for equine asthma include stabling on straw bedding, feeding dusty hay, poor ventilation, and confinement. Horses housed indoors for prolonged periods have higher exposure to airborne particulates. Pasture turnout significantly reduces exposure to stable dust and mold. The Merck Veterinary Manual notes that genetic predisposition may play a role, with certain breeds such as Thoroughbreds and Warmbloods showing increased susceptibility.

Diagnostic Approach

Diagnosis of equine asthma requires a combination of clinical examination, endoscopy, bronchoalveolar lavage (BAL) cytology, and thoracic radiography. A 2023 review in Animals titled "Decision Making in Severe Equine Asthma-Diagnosis and Monitoring" emphasized the importance of standardized diagnostic protocols. The review noted that BAL cytology remains the gold standard for confirming airway inflammation.

Endoscopy allows visualization of the trachea and bronchi. Horses with equine asthma often exhibit excessive mucus accumulation, tracheal collapse, and bronchial wall thickening. A scoring system for tracheal mucus (0 to 5 scale) helps quantify severity. BAL fluid analysis reveals increased neutrophil percentage (above 5% in mild asthma, above 25% in severe asthma). Eosinophils and mast cells may also be elevated.

Thoracic radiography is useful for ruling out other causes of respiratory signs such as pneumonia or neoplasia. In severe equine asthma, radiographs may show a prominent bronchial pattern, interstitial opacities, and hyperinflation of the lungs. A 2017 article in Equine Veterinary Journal titled "Science-in-brief: Equine asthma diagnosis: Beyond bronchoalveolar lavage cytology" discussed additional diagnostic tools including pulmonary function testing and airway hyperresponsiveness assessment.

A 2024 review in Animals titled "Equine Asthma Diagnostics: Review of Influencing Factors and Difficulties in Diagnosing Subclinical Disease" highlighted challenges in detecting mild or subclinical asthma. Horses with mild disease may have normal BAL cytology but show airway hyperresponsiveness on challenge testing. The review recommended that veterinarians consider multiple diagnostic modalities when clinical signs are equivocal.

Environmental Management

Environmental modification is the cornerstone of equine asthma management. The AAEP recommends the following measures:

• Replace dusty hay with steamed hay, soaked hay, or haylage. Steaming hay at 212°F (100°C) for 60 minutes reduces respirable dust and mold spore counts by over 90 percent.
• Use low-dust bedding such as shredded paper, wood pellets, or rubber mats. Avoid straw and shavings with high mold content.
• Improve stable ventilation by opening windows, doors, and ridge vents. Use fans to promote air movement without creating drafts.
• Turn horses out on pasture for as many hours as possible. Pasture turnout reduces exposure to stable dust and allows natural clearance of airway secretions.
• Remove horses from the stable during mucking out and hay distribution to avoid peak dust exposure.

Owners should monitor clinical response to environmental changes. Improvement in cough frequency, nasal discharge, and respiratory effort typically occurs within two to four weeks. If no improvement is observed, veterinary reassessment is warranted.

Pharmacologic Treatment

Pharmacologic therapy for equine asthma includes bronchodilators and corticosteroids. Inhaled medications are preferred over systemic drugs to minimize side effects. The Merck Veterinary Manual provides guidance on medication classes:

• Bronchodilators: Beta-2 agonists such as albuterol (salbutamol) provide rapid relief of bronchoconstriction. These are used as rescue therapy during acute exacerbations. Long-acting bronchodilators such as clenbuterol are available but require veterinary prescription.
• Corticosteroids: Inhaled fluticasone propionate or beclomethasone dipropionate reduce airway inflammation. A study in Research in Veterinary Science examined serum cortisol concentrations in horses with heaves treated with fluticasone propionate over a one-year period. The study found no significant adrenal suppression, supporting the safety of long-term inhaled corticosteroid use.
• Systemic corticosteroids: Dexamethasone or prednisolone may be used for severe cases that do not respond to inhaled therapy. Long-term systemic corticosteroid use carries risks including laminitis and immunosuppression.

A 2018 article in the Journal of Veterinary Internal Medicine titled "Equine asthma: Integrative biologic relevance of a recently proposed nomenclature" discussed the importance of accurate diagnosis before initiating treatment. The article noted that horses with mild asthma may respond to environmental changes alone, while severe cases require combined environmental and pharmacologic management.

Monitoring and Prognosis

Regular monitoring of clinical signs and pulmonary function is essential for managing equine asthma. Owners should keep a daily log of cough frequency, nasal discharge, respiratory rate at rest, and exercise tolerance. Veterinarians may repeat BAL cytology every three to six months to assess airway inflammation.

The prognosis for equine asthma depends on disease severity and owner compliance with environmental management. Mild to moderate asthma can be well-controlled with appropriate measures. Severe asthma may require lifelong medication and strict environmental control. A 2024 study in Animals titled "Equine Asthma Does Not Affect Circulating Myostatin Concentrations in Horses" found that asthma did not alter myostatin levels, suggesting that muscle wasting in severe cases is due to reduced exercise instead of direct metabolic effects.

Common Failure Patterns

• Inconsistent environmental management: Owners who revert to dusty hay or bedding after initial improvement often see relapse within days.
• Underdosing of inhaled medications: Improper technique with metered-dose inhalers or spacers reduces drug delivery to the lower airways.
• Delayed veterinary reassessment: Horses that fail to improve after four weeks of treatment require re-evaluation including repeat BAL and radiography.
• Misdiagnosis: Horses with exercise-induced pulmonary hemorrhage or bacterial pneumonia may present with similar signs but require different treatment.

Professional Escalation Criteria

Veterinary consultation is required when:

• A horse shows respiratory distress (nostril flaring, heave line, extended head and neck) at rest.
• Fever persists beyond 48 hours despite environmental changes.
• Nasal discharge becomes purulent or blood-tinged.
• Cough worsens or becomes productive.
• Exercise intolerance progresses despite treatment.
• A horse fails to improve after four weeks of environmental management and pharmacologic therapy.

Equine Influenza

Equine influenza is a highly contagious viral respiratory disease caused by influenza A virus subtypes H7N7 and H3N8. The World Organisation for Animal Health (WOAH) classifies equine influenza as a notifiable disease due to its potential for rapid spread and economic impact on the horse industry. Outbreaks can disrupt competitions, breeding operations, and transport.

Clinical Presentation and Transmission

The incubation period for equine influenza is one to three days. Clinical signs include acute onset of fever (102 to 106°F or 39 to 41°C), a harsh dry cough, serous nasal discharge that may become mucopurulent, lethargy, and enlarged submandibular lymph nodes. The cough can persist for two to three weeks after fever resolves. Secondary bacterial pneumonia is a common complication, particularly in young or immunocompromised horses.

Transmission occurs through direct contact with infected horses, aerosolized respiratory droplets, and contaminated equipment such as water buckets, feed tubs, and grooming tools. The virus can survive on surfaces for up to 48 hours. The AAEP recommends strict biosecurity measures during outbreaks, including isolation of affected horses, dedicated equipment, and disinfection of stalls and trailers.

Diagnostic Methods

Diagnosis of equine influenza is confirmed by PCR testing of nasopharyngeal swabs or nasal washes. PCR can detect viral RNA within 24 to 48 hours of sample collection. Serology using hemagglutination inhibition (HI) testing can confirm exposure but requires paired acute and convalescent samples taken two to three weeks apart.

A 2006 study in the Journal of Comparative Pathology titled "Investigation of Equine Influenza Cases Exhibiting Neurological Disease: Coincidence or Association?" examined the potential link between influenza infection and neurological signs. The study found that while rare, neurological complications can occur, warranting careful monitoring of affected horses.

Treatment and Supportive Care

There is no specific antiviral treatment for equine influenza. Management focuses on supportive care and prevention of secondary infections:

• Rest: Horses should be rested for one week for each day of fever, with a minimum of three weeks of rest. Premature return to work increases the risk of secondary pneumonia and prolonged recovery.
• Anti-inflammatory therapy: Non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine or phenylbutazone reduce fever and improve comfort. Veterinary guidance is required for dosing and duration.
• Antimicrobial therapy: Antibiotics are indicated only if secondary bacterial pneumonia is suspected. The Merck Veterinary Manual advises against routine antibiotic use in uncomplicated influenza cases.
• Hydration and nutrition: Ensure access to fresh water and palatable feed. Horses with severe nasal discharge may have reduced appetite.

Vaccination Protocols

Vaccination is the most effective strategy for preventing equine influenza. The AAEP recommends annual vaccination for all horses, with more frequent boosters for performance horses and those at high risk of exposure. A 2004 study in the Veterinary Journal titled "Efficacy and duration of immunity of a combined equine influenza and equine herpesvirus vaccine against challenge with an American-like equine influenza virus (A/equi-2/Kentucky/95)" demonstrated that combined vaccines provide protective immunity for at least six months.

Vaccination protocols should follow manufacturer guidelines and local regulations. The WOAH recommends that horses traveling internationally be vaccinated within six months of travel and that vaccination records be maintained. Owners should consult their veterinarian to develop a tailored vaccination schedule based on risk factors including age, use, and geographic location.

Biosecurity Measures

During an influenza outbreak, the following measures are critical:

• Isolate affected horses in a separate barn or paddock at least 30 feet from healthy horses.
• Use dedicated equipment for each horse or disinfect items between uses.
• Implement footbaths with disinfectant at stable entrances.
• Restrict movement of horses on and off the premises.
• Monitor all horses for fever and respiratory signs twice daily.
• Report suspected cases to the state veterinarian or WOAH as required.

Common Failure Patterns

• Inadequate rest: Returning horses to work before full recovery increases the risk of pneumonia and prolonged viral shedding.
• Delayed isolation: Horses are most contagious during the first 24 to 48 hours of fever. Delayed isolation allows spread to the entire herd.
• Incomplete vaccination: Horses that are not up to date on vaccination are at higher risk of severe disease.
• Poor biosecurity: Shared water sources, communal grooming tools, and contaminated trailers facilitate transmission.

Professional Escalation Criteria

Veterinary consultation is required when:

• Fever exceeds 105°F (40.5°C) or persists beyond five days.
• Cough becomes productive or is accompanied by purulent nasal discharge.
• A horse shows signs of pneumonia including tachypnea, crackles on auscultation, or lethargy.
• Neurological signs such as ataxia or weakness develop.
• Multiple horses on the premises become ill within a short period.

Strangles

Strangles is a highly contagious bacterial infection caused by Streptococcus equi subsp. equi. The disease is characterized by fever, purulent nasal discharge, and abscess formation in the submandibular and retropharyngeal lymph nodes. The Merck Veterinary Manual describes strangles as one of the most common infectious diseases of horses worldwide.

Clinical Signs and Transmission

The incubation period for strangles is three to 14 days. Clinical signs include acute fever (102 to 106°F or 39 to 41°C), depression, anorexia, and serous nasal discharge that becomes purulent within 24 to 48 hours. Lymph node enlargement is palpable in the submandibular and retropharyngeal regions. Abscesses may rupture externally, draining thick yellow pus, or internally into the guttural pouches, leading to chronic infection.

Transmission occurs through direct contact with infected horses or contaminated equipment. The bacteria can survive in the environment for up to four weeks. The AAEP emphasizes that recovered horses can carry S. equi in the guttural pouches for months to years, serving as a source of infection for naive horses.

Diagnostic Methods

Diagnosis is confirmed by bacterial culture or PCR of nasal swabs, guttural pouch lavage, or abscess drainage. PCR is more sensitive than culture and can detect S. equi DNA even in samples with low bacterial loads. Serology using an ELISA for antibodies to S. equi M protein can identify carriers but is not useful for acute diagnosis.

Veterinarians should perform guttural pouch endoscopy in recovered horses to identify carriers. The ACVIM recommends testing all horses that have been exposed to strangles before they are introduced to new populations.

Treatment and Management

Treatment of strangles depends on the stage of disease:

• Early stage (fever without abscessation): Supportive care including NSAIDs for fever and pain. Antibiotics may be used in the early stage to prevent abscess formation, but this is controversial because antibiotics can delay immune response and increase the risk of chronic carriage.
• Abscess stage: Hot packing or poulticing of enlarged lymph nodes promotes maturation and drainage. Once abscesses rupture, gentle flushing with dilute povidone-iodine solution aids healing. Antibiotics are generally avoided during abscess formation because they can interfere with abscess maturation.
• Complicated cases: Horses with internal abscesses, guttural pouch empyema, or metastatic abscessation (bastard strangles) require aggressive antibiotic therapy based on culture and sensitivity. The Merck Veterinary Manual advises that these cases be managed by a veterinarian with hospitalization if needed.

Biosecurity and Control

Control of strangles requires strict biosecurity:

• Isolate affected horses for a minimum of four weeks after clinical signs resolve.
• Test recovered horses for guttural pouch carriage before releasing isolation.
• Disinfect stalls, equipment, and trailers with accelerated hydrogen peroxide or bleach solutions.
• Implement quarantine for new arrivals for at least two weeks.
• Vaccination with an intranasal modified-live vaccine or intramuscular extract vaccine may reduce severity but does not prevent infection. The AAEP recommends vaccination for horses at high risk of exposure.

Common Failure Patterns

• Premature release from isolation: Horses that are still shedding S. equi can infect naive horses.
• Inadequate carrier detection: Failure to test recovered horses for guttural pouch carriage leads to ongoing transmission.
• Overuse of antibiotics: Early antibiotic use can suppress abscess formation and increase carrier risk.
• Poor environmental disinfection: S. equi can survive on surfaces for weeks if not properly disinfected.

Professional Escalation Criteria

Veterinary consultation is required when:

• Abscesses do not rupture within seven to 10 days.
• A horse shows signs of respiratory distress due to lymph node enlargement.
• Fever persists beyond 10 days.
• Neurological signs develop (suggesting guttural pouch infection or metastatic abscessation).
• Multiple horses on the premises become infected despite biosecurity measures.

Pneumonia

Pneumonia in horses can be caused by bacterial, viral, or fungal pathogens. Bacterial pneumonia is most common in foals (Rhodococcus equi) and adult horses with compromised immune systems. The Merck Veterinary Manual notes that aspiration pneumonia can occur in horses with dysphagia or esophageal obstruction.

Clinical Signs and Diagnosis

Clinical signs of pneumonia include fever, tachypnea, cough, nasal discharge (often purulent), lethargy, and reduced appetite. On auscultation, crackles, wheezes, or areas of decreased lung sounds may be heard. Thoracic radiography reveals alveolar or interstitial patterns, consolidation, or abscesses. Ultrasonography can identify pleural effusion or peripheral lung abscesses.

Definitive diagnosis requires transtracheal wash (TTW) or bronchoalveolar lavage (BAL) for bacterial culture and cytology. Bloodwork may show leukocytosis with a left shift, hyperfibrinogenemia, and elevated serum amyloid A.

Treatment

Treatment of bacterial pneumonia requires antimicrobial therapy based on culture and sensitivity. Common antibiotics include ceftiofur, gentamicin, and trimethoprim-sulfonamide combinations. The Merck Veterinary Manual advises that treatment duration is typically two to four weeks, with repeat radiography to confirm resolution.

Supportive care includes NSAIDs for fever and pain, bronchodilators for airway inflammation, and fluid therapy for dehydrated horses. Foals with Rhodococcus equi pneumonia require macrolide antibiotics (azithromycin or clarithromycin) combined with rifampin for eight to 12 weeks.

Prognosis

The prognosis for pneumonia depends on the causative agent, severity, and timeliness of treatment. Uncomplicated bacterial pneumonia in adult horses has a good prognosis with appropriate therapy. Foals with Rhodococcus equi pneumonia have a guarded prognosis, with mortality rates of 20 to 30 percent. Aspiration pneumonia carries a poor prognosis due to the severity of lung damage.

Common Failure Patterns

• Delayed diagnosis: Horses with mild pneumonia may be misdiagnosed as having equine asthma or viral infection.
• Inadequate antibiotic duration: Short courses of antibiotics lead to relapse and antimicrobial resistance.
• Poor supportive care: Dehydration and inadequate nutrition impair immune response.
• Failure to identify underlying cause: Horses with recurrent pneumonia require investigation for guttural pouch infection, esophageal dysfunction, or immunodeficiency.

Professional Escalation Criteria

Veterinary consultation is required when:

• A horse shows respiratory distress or cyanosis.
• Fever persists beyond 72 hours of antibiotic therapy.
• Radiographic lesions worsen or fail to improve after two weeks.
• A foal develops pneumonia (requires immediate veterinary attention).
• A horse has a history of dysphagia or choke.

Exercise-Induced Pulmonary Hemorrhage

Exercise-induced pulmonary hemorrhage (EIPH) is the presence of blood in the lower airways after intense exercise. The condition is common in racehorses and other performance horses. The Merck Veterinary Manual notes that EIPH occurs in up to 90 percent of Thoroughbred racehorses.

Pathophysiology

EIPH results from high pulmonary capillary pressure during maximal exercise, leading to stress failure of the pulmonary capillaries. The bleeding originates from the dorsocaudal lung lobes. Repeated episodes can cause pulmonary fibrosis and reduced lung function.

Clinical Signs and Diagnosis

The most obvious sign of EIPH is epistaxis (visible bleeding from the nostrils) after exercise. However, most cases are subclinical and detected only by endoscopy. Horses with EIPH may show poor performance, coughing after exercise, or prolonged recovery.

Diagnosis is confirmed by endoscopic examination of the trachea within 30 to 90 minutes after exercise. The presence of blood in the trachea is graded on a scale of 0 to 4. Bronchoalveolar lavage can detect hemosiderophages (macrophages containing iron from degraded red blood cells), indicating previous bleeding episodes.

Management

Management of EIPH focuses on reducing pulmonary capillary pressure and minimizing bleeding:

• Furosemide (Lasix) is a loop diuretic that reduces pulmonary artery pressure and is used in many racing jurisdictions. The AAEP notes that furosemide is administered intravenously four hours before racing.
• Nasal dilator strips reduce upper airway resistance and may decrease bleeding severity.
• Environmental management includes avoiding dusty environments and ensuring adequate warm-up and cool-down periods.
• Rest is recommended for horses with severe EIPH (grade 3 or 4). The duration of rest depends on the severity and recurrence rate.

Prognosis

The prognosis for EIPH is variable. Many horses continue to race with mild to moderate EIPH without significant decline in performance. Horses with recurrent severe EIPH may have a guarded prognosis for continued athletic use. Pulmonary fibrosis secondary to chronic EIPH can lead to irreversible lung damage.

Common Failure Patterns

• Failure to diagnose subclinical EIPH: Horses with poor performance may be misdiagnosed with equine asthma or musculoskeletal issues.
• Inadequate rest: Returning to intense exercise too soon after a bleeding episode increases the risk of recurrence.
• Overreliance on furosemide: Furosemide reduces but does not eliminate bleeding. Environmental and training modifications are also important.

Professional Escalation Criteria

Veterinary consultation is required when:

• Epistaxis is profuse or persists beyond 30 minutes after exercise.
• A horse shows respiratory distress after exercise.
• EIPH is recurrent despite management measures.
• A horse has a sudden decline in performance with no other identifiable cause.

Diagnostic Methods Overview

Accurate diagnosis of equine respiratory conditions requires a systematic approach. The following table summarizes common diagnostic methods and their applications.

Records and Measurements

Horse owners should maintain accurate records to support diagnosis and management of respiratory conditions. The following measurements and observations should be documented:

• Daily respiratory rate at rest (normal: 8 to 16 breaths per minute)
• Rectal temperature (normal: 99 to 101.5°F or 37.2 to 38.6°C)
• Cough frequency and character (dry, productive, honking)
• Nasal discharge volume, color, and consistency
• Exercise tolerance and recovery time
• Feed and water intake
• Body weight and body condition score
• Vaccination history with dates and product names
• Medication administration including drug, dose, route, and frequency
• Environmental changes (bedding type, hay source, turnout hours)

Veterinarians should maintain records of diagnostic procedures, laboratory results, and treatment plans. Serial BAL cytology results and radiographs allow objective assessment of disease progression or resolution.

Welfare and Safety Context

Respiratory disease significantly impacts equine welfare. Horses with chronic respiratory conditions experience reduced quality of life due to coughing, dyspnea, and exercise intolerance. The World Organisation for Animal Health (WOAH) recognizes respiratory diseases as a priority for animal health surveillance and control.

Owners have a responsibility to provide an environment that minimizes respiratory disease risk. This includes adequate ventilation, low-dust bedding and feed, and access to pasture turnout. Failure to address environmental risk factors constitutes a welfare concern.

Biosecurity measures protect both individual horses and the broader equine population. The AAEP and WOAH provide guidelines for disease prevention and outbreak control. Owners should implement biosecurity protocols for all horses entering the premises, including quarantine, vaccination, and testing.

Practical Decision Framework for Selecting Diagnostic Tests in Equine Respiratory Cases

Selecting the appropriate diagnostic test for a horse with respiratory signs requires a systematic approach that balances clinical presentation, test characteristics, cost, and turnaround time. Horse owners and veterinarians can use the following decision framework to guide test selection and interpretation. This framework is based on evidence from the Merck Veterinary Manual, AAEP guidelines, and peer-reviewed literature on equine respiratory diagnostics.

Step 1: Categorize the Clinical Presentation

Begin by classifying the horse's signs into one of three presentation categories:

Acute febrile respiratory disease: Sudden onset of fever (above 101.5°F or 38.6°C), cough, nasal discharge, and lethargy. This presentation is most consistent with viral infections such as equine influenza or equine herpesvirus. The Merck Veterinary Manual notes that acute febrile respiratory disease in groups of horses should raise immediate suspicion for influenza.

Chronic cough with exercise intolerance: Persistent cough lasting more than two weeks, nasal discharge, and reduced performance without significant fever. This presentation suggests equine asthma (mild to severe) or exercise-induced pulmonary hemorrhage (EIPH). A 2024 review in Animals titled "Equine Asthma Diagnostics: Review of Influencing Factors and Difficulties in Diagnosing Subclinical Disease" highlighted that horses with mild asthma may show only subtle performance deficits.

Purulent nasal discharge with lymphadenopathy: Thick yellow-green nasal discharge, enlarged submandibular or retropharyngeal lymph nodes, and fever. This presentation is classic for strangles caused by Streptococcus equi subsp. equi. The AAEP emphasizes that strangles should be considered whenever a horse presents with fever and lymph node abscessation.

Step 2: Select Initial Diagnostic Tests Based on Category

For acute febrile respiratory disease:

• Collect a nasopharyngeal swab or nasal wash for PCR testing. PCR can detect influenza virus and equine herpesvirus within 24 to 48 hours. The Merck Veterinary Manual recommends PCR as the first-line test for viral respiratory disease due to its high sensitivity and rapid turnaround.
• If PCR is unavailable, submit paired serum samples for serology (hemagglutination inhibition for influenza, complement fixation for herpesvirus). Acute samples should be collected within the first three days of illness, and convalescent samples two to three weeks later.
• Perform a complete blood count and fibrinogen measurement to assess inflammatory response and rule out secondary bacterial infection.

For chronic cough with exercise intolerance:

• Schedule endoscopy with bronchoalveolar lavage (BAL) as the primary diagnostic procedure. A 2023 review in Animals titled "Decision Making in Severe Equine Asthma-Diagnosis and Monitoring" confirmed that BAL cytology remains the gold standard for diagnosing equine asthma. BAL fluid should be analyzed for total cell count, differential cell count, and presence of hemosiderophages.
• Perform endoscopy immediately after exercise (within 30 to 90 minutes) if EIPH is suspected. The presence of blood in the trachea is graded on a 0 to 4 scale.
• Obtain thoracic radiographs to evaluate for pneumonia, lung abscesses, or neoplasia. Radiography is particularly important if the horse has a history of fever or if BAL cytology shows evidence of sepsis.

For purulent nasal discharge with lymphadenopathy:

• Collect a nasal swab or guttural pouch lavage for PCR testing for Streptococcus equi subsp. equi. PCR is more sensitive than bacterial culture and can detect the organism even in samples with low bacterial loads.
• If abscesses have ruptured, collect pus from the draining tract for bacterial culture and antimicrobial sensitivity testing.
• Perform guttural pouch endoscopy in recovered horses to identify carriers. The ACVIM recommends testing all horses that have been exposed to strangles before they are introduced to new populations.

Step 3: Interpret Results and Determine Next Steps

Positive PCR for influenza or herpesvirus: Confirm diagnosis and implement isolation protocols. The AAEP recommends isolating affected horses for at least 14 days after clinical signs resolve. Monitor for secondary bacterial pneumonia, which occurs in 10 to 20 percent of cases. If fever persists beyond five days or if cough becomes productive, perform thoracic radiography and consider transtracheal wash for bacterial culture.

Negative PCR for influenza or herpesvirus with persistent fever: Consider other causes including equine herpesvirus myeloencephalopathy, bacterial pneumonia, or non-respiratory infections. Repeat PCR testing 48 to 72 hours later if clinical signs are strongly suggestive of viral infection, as viral shedding may be intermittent.

BAL cytology showing increased neutrophils (above 5 percent): Diagnose equine asthma. The severity of asthma correlates with neutrophil percentage: mild asthma (5 to 25 percent neutrophils), moderate asthma (25 to 50 percent), and severe asthma (above 50 percent). A 2017 article in Equine Veterinary Journal titled "Science-in-brief: Equine asthma diagnosis: Beyond bronchoalveolar lavage cytology" noted that some horses with mild asthma may have normal BAL cytology but show airway hyperresponsiveness on challenge testing. If clinical signs persist despite normal BAL cytology, consider pulmonary function testing or bronchial challenge.

BAL cytology showing hemosiderophages: Confirm EIPH. Grade the severity based on the number of hemosiderophages and the presence of fresh blood. Horses with grade 3 or 4 EIPH should be rested for at least two to four weeks before returning to intense exercise.

Positive PCR for Streptococcus equi: Confirm strangles. Isolate the horse and implement strict biosecurity measures. The AAEP recommends that affected horses remain in isolation for a minimum of four weeks after clinical signs resolve. Test recovered horses for guttural pouch carriage before releasing isolation.

Step 4: Monitor Response to Treatment

After initiating treatment based on diagnostic results, monitor the horse's response using objective criteria:

• Respiratory rate at rest: Should return to normal (8 to 16 breaths per minute) within 48 to 72 hours of appropriate treatment.
• Cough frequency: Should decrease by at least 50 percent within one week of environmental changes or pharmacologic therapy for equine asthma.
• Fever: Should resolve within 48 hours of starting antimicrobial therapy for bacterial infections.
• Nasal discharge: Should change from purulent to serous and then resolve within five to seven days.
• Exercise tolerance: Should improve within two to four weeks of treatment for equine asthma or EIPH.

If the horse fails to show improvement within these timeframes, reassess the diagnosis and consider additional testing. A 2018 article in the Journal of Veterinary Internal Medicine titled "Equine asthma: Integrative biologic relevance of a recently proposed nomenclature" emphasized that horses with severe asthma may require combined environmental and pharmacologic management for several weeks before clinical improvement is observed.

Record System for Diagnostic Decision Making

Maintain a standardized record for each respiratory case that includes:

• Date of onset of clinical signs
• Presenting category (acute febrile, chronic cough, purulent discharge)
• Diagnostic tests performed with dates and results
• Treatment administered including drug, dose, route, and duration
• Clinical response at 48 hours, one week, and two weeks
• Any complications or adverse reactions
• Final diagnosis and follow-up plan

This record system allows for objective assessment of diagnostic accuracy and treatment efficacy. The Merck Veterinary Manual recommends that veterinarians maintain detailed medical records for all respiratory cases to facilitate ongoing management and to identify patterns that may indicate environmental or management problems.

Common Failure Patterns in Diagnostic Decision Making

• Testing too early or too late: PCR testing for influenza is most sensitive within the first 48 hours of clinical signs. Testing after five days may yield false-negative results. Conversely, serology requires paired samples taken two to three weeks apart and is not useful for acute diagnosis.
• Inadequate sample collection: Nasopharyngeal swabs must be inserted deeply enough to contact the respiratory epithelium. Superficial swabs may miss viral shedding. BAL requires proper technique to avoid contamination with upper airway secretions.
• Overreliance on a single test: No single diagnostic test has 100 percent sensitivity and specificity. A 2024 review in Animals titled "Equine Asthma Diagnostics: Review of Influencing Factors and Difficulties in Diagnosing Subclinical Disease" recommended that veterinarians use multiple diagnostic modalities when clinical signs are equivocal.
• Failure to test for coinfections: Horses with strangles may also have equine influenza or equine herpesvirus. Testing for multiple pathogens simultaneously is recommended during outbreaks.
• Delayed testing for carriers: Horses that have recovered from strangles should be tested for guttural pouch carriage before being released from isolation. Failure to identify carriers leads to ongoing transmission within the herd.

Professional Escalation Criteria for Diagnostic Uncertainty

Consult a veterinary specialist (board-certified internist) when:

• Diagnostic test results are discordant with clinical signs (e.g., negative PCR for influenza in a horse with classic clinical signs and known exposure).
• BAL cytology shows atypical cell populations suggesting neoplasia or fungal infection.
• A horse fails to respond to treatment despite a confirmed diagnosis.
• Multiple horses on the premises develop respiratory disease with an unknown etiology.
• Advanced diagnostic procedures such as pulmonary function testing, computed tomography, or lung biopsy are required.

Frequently Asked Questions

What is the difference between equine asthma and heaves?

Equine asthma is the modern term for what was previously called heaves or recurrent airway obstruction (RAO). The ACVIM consensus statement adopted the term equine asthma to reflect the inflammatory nature of the disease and its similarity to human asthma. Heaves specifically referred to severe asthma with visible respiratory effort and a heave line along the flank. Mild equine asthma (formerly inflammatory airway disease) may not cause visible respiratory effort but still affects performance.

How is equine asthma diagnosed?

Diagnosis of equine asthma involves clinical examination, endoscopy to assess tracheal mucus, bronchoalveolar lavage (BAL) for cytology, and thoracic radiography to rule out other conditions. BAL fluid showing increased neutrophil percentage (above 5 percent for mild asthma, above 25 percent for severe asthma) confirms airway inflammation. A 2023 review in Animals titled "Decision Making in Severe Equine Asthma-Diagnosis and Monitoring" emphasized the importance of standardized diagnostic protocols.

Can equine influenza be prevented with vaccination?

Yes, vaccination is the most effective prevention strategy for equine influenza. The AAEP recommends annual vaccination for all horses, with more frequent boosters for performance horses. A 2004 study in the Veterinary Journal demonstrated that combined influenza and herpesvirus vaccines provide protective immunity for at least six months. Vaccination reduces disease severity and viral shedding but does not guarantee complete protection.

What should I do if my horse has strangles?

If you suspect strangles, isolate the horse immediately and contact your veterinarian. Do not share equipment or water sources with other horses. The veterinarian will confirm the diagnosis with PCR or culture and advise on treatment. Most horses recover with supportive care including hot packing of abscesses and drainage. Antibiotics are used only in specific circumstances as determined by the veterinarian.

How long should a horse with influenza be rested?

A horse with influenza should be rested for one week for each day of fever, with a minimum of three weeks of rest. Premature return to work increases the risk of secondary bacterial pneumonia and prolonged recovery. The cough may persist for two to three weeks after fever resolves. The Merck Veterinary Manual advises that horses be fully recovered before resuming exercise.

What causes exercise-induced pulmonary hemorrhage?

Exercise-induced pulmonary hemorrhage (EIPH) results from high pulmonary capillary pressure during maximal exercise, causing stress failure of the pulmonary capillaries. The condition is common in racehorses and other performance horses. Management includes furosemide administration (where permitted), nasal dilator strips, and environmental modifications. The Merck Veterinary Manual notes that EIPH occurs in up to 90 percent of Thoroughbred racehorses.

How can I reduce my horse's risk of respiratory disease?

Reduce respiratory disease risk by providing good ventilation in stables, using low-dust bedding and feed, turning horses out on pasture as much as possible, implementing biosecurity protocols for new arrivals, maintaining current vaccinations, and monitoring horses daily for signs of respiratory illness. The AAEP provides guidelines for environmental management and disease prevention.

When should I call a veterinarian for a horse with respiratory signs?

Call a veterinarian if your horse shows respiratory distress at rest, fever above 101.5°F (38.6°C) that persists beyond 48 hours, purulent nasal discharge, cough that worsens or becomes productive, exercise intolerance that progresses, or signs of pneumonia such as tachypnea and lethargy. Foals with any respiratory signs require immediate veterinary attention.

Related Veterinary Guides

• Swine Respiratory Disease Observation And Diagnostics
• Senior Horse Care
• Miniature Horse Care
• Diagnostic Medical Sonography Programs
• Broiler Respiratory Health Observation And Testing

References and Further Reading

• aaep.org
• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Decision Making in Severe Equine Asthma-Diagnosis and Monitoring.. Animals : an open access journal from MDPI, 2023.
• Science-in-brief: Equine asthma diagnosis: Beyond bronchoalveolar lavage cytology.. Equine veterinary journal, 2017.
• Equine asthma: Integrative biologic relevance of a recently proposed nomenclature.. Journal of veterinary internal medicine, 2018.
• Equine Asthma Does Not Affect Circulating Myostatin Concentrations in Horses.. Animals : an open access journal from MDPI, 2024.
• Antigen array for serological diagnosis and novel allergen identification in severe equine asthma.. Scientific reports, 2019.
• Equine Asthma Diagnostics: Review of Influencing Factors and Difficulties in Diagnosing Subclinical Disease.. Animals : an open access journal from MDPI, 2024.
• The role of pulmonary intravascular macrophages in the development of heaves in horses. 2008.
• Serum cortisol concentration in horses with heaves treated with fluticasone proprionate over a 1 year period.. Research in Veterinary Science, 2015.
• Levamisole phosphate as a treatment for heaves [Respiratory problems in horses]. 1978.
• Efficacy and duration of immunity of a combined equine influenza and equine herpesvirus vaccine against challenge with an American-like equine influenza virus (A/equi-2/Kentucky/95). Veterinary Journal, 2004.
• Investigation of Equine Influenza Cases Exhibiting Neurological Disease: Coincidence or Association?. Journal of Comparative Pathology, 2006.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.