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Basics ⬇

Outline

BASICS

Definition!!navigator!!

An inflammatory condition of the lower airways characterized by bronchospasm, excess mucus production, and airway remodeling leading to reversible airway obstruction.

Pathophysiology!!navigator!!

  • Stabling and feeding dusty hay leads to inflammation and obstruction of the lower airways in susceptible horses
  • Hypoxemia is common
  • A hypersensitivity reaction to thermophilic molds and Actinomyces antigens is suspected
  • The role of endotoxins and nonspecific environmental dust particles on the induction and maintenance of heaves is currently ill defined

Systems Affected!!navigator!!

Respiratory

Genetics!!navigator!!

  • A genetic susceptibility has been demonstrated in selected breeds
  • Multiple genes may be involved but differ between affected families
  • Genes and environment interactions contribute to the expression of the disease

Incidence/Prevalence, Geographic Distribution!!navigator!!

10–20% of horses in countries with temperate and cold climates, where horses are stabled for prolonged periods

Signalment!!navigator!!

  • No clear sex or breed predilections
  • Incidence increases with age; uncommon in horses <7 years

Signs!!navigator!!

  • During clinical exacerbation, respiratory distress at rest and biphasic expiratory efforts define the syndrome. Increased respiratory rate and flared nostrils are also present. Emaciation and hypertrophy of the external abdominal oblique muscles (“heaves line”) may develop
  • Signs may be limited to exercise intolerance and occasional cough when horses are in remission. Coughing generally increases as the disease progresses, becoming paroxysmal
  • Clinical signs often wax and wane. Clinical signs can worsen in hot and humid summer days

Causes!!navigator!!

Inhalation of dust particles from moldy hay and straw induces clinical exacerbation in susceptible horses.

Risk Factors!!navigator!!

  • Moldy hay and straw
  • Prolonged stabling

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Pharyngitis and mild dysphagia can cause chronic cough—lung auscultation and endoscopy should identify the upper airways as the site of the problem
  • Inflammatory airway disease—no history of episodes of labored breathing
  • Viral and bacterial airway infections—can be differentiated from heaves on the basis of febrile episodes, other signs of infection, and duration of clinical signs
  • Summer pasture-associated equine asthma—occurs when horses are pastured rather than stabled and fed hay
  • Lungworm infection—larvae in feces or tracheal secretions

CBC/Biochemistry/Urinalysis!!navigator!!

Usually within normal ranges. Blood neutrophils and total proteins can increase during disease exacerbation.

Other Laboratory Tests!!navigator!!

  • Generally >25% neutrophils in BALF cytology. Can be normal when horses are asymptomatic. Presence of bacterial and fungal elements in BALF cytology are common but indicate impaired mucociliary clearance rather than an ongoing intrapulmonary septic process
  • Cytology of tracheal aspirates also reveals neutrophilia, but this is less specific than BALF. Bacterial culture of tracheal secretions may yield bacterial growth, but without other signs of infection. This may represent colonization of the lower airways because of impaired mucociliary clearance
  • Arterial blood gases—PaO2 values are <80 mmHg and can be as low as 40 mmHg during exacerbation; PaCO2 values may be elevated
  • Histologic scores performed on endobronchial biopsies correlate with the degree of airway obstruction
  • Transcutaneous lung biopsies for diagnosis of heaves is not recommended

Imaging!!navigator!!

  • During exacerbation, endoscopy reveals copious mucopurulent exudate
  • Thoracic radiography can reveal an increased bronchointerstitial pattern. Bronchiectasis may also be present
  • Thoracic ultrasonography is usually unremarkable
  • Endobronchial ultrasonography reveals thickening of the bronchial wall

Other Diagnostic Procedures!!navigator!!

  • The diagnosis is established on the basis of signalment, history, and clinical findings combined with the exclusion of other common diseases affecting the respiratory tract, and response to therapy
  • Thoracic auscultation—wheezes and expiratory crackles are common findings. Bronchovesicular lung sounds can be decreased during severe episodes. Auscultation can be normal during remission
  • Thoracic percussion may reveal hyperresonance of the ventral and caudal borders of the lung fields because of air trapping
  • Lung function measurements reveal increased resistance and elastance—to date, few techniques are adapted to clinical settings

Pathologic Findings!!navigator!!

  • Chronic active bronchiolitis, with intraluminal accumulation of mucus and neutrophils, and epithelial hyperplasia with goblet cell metaplasia. Histomorphometry reveals deposition of extracellular matrix and increased airway smooth muscle mass in the airway wall
  • Interstitial emphysema, mostly in the cranial regions, and patchy areas of alveolar hyperinflation may be seen

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

Control airway inflammation and decrease airway obstruction.

Appropriate Health Care!!navigator!!

In- or outpatient medical management.

Nursing Care!!navigator!!

  • Incurable, but the clinical signs and airway inflammation are reversible with proper control of environmental dust, which is best achieved by keeping horses outdoors, preferably on pasture or replacing hay with cubed or pelleted food, or haylage, and use of shredded paper or good quality wood shavings instead of straw. Pasteurized (steam) hay improves the clinical signs in some horses, but inflammation may be incompletely controlled
  • Airway remodeling is only partially reversible
  • Respiratory signs recur within days to weeks of re-exposure to dusty hay and bedding
  • Without drug therapy, a few weeks to months of environmental dust control may be required before affected horses become free of respiratory signs
  • In horses with profound hypoxemia (PaO2 <60 mmHg), inhaled oxygen supplementation via a nasopharyngeal tube improve breathing, but is rarely required

Activity!!navigator!!

Adjust exercise level according to the degree of respiratory dysfunction.

Diet!!navigator!!

  • Cubed or pelleted food, haylage/silage, or pasteurized hay is preferred if pasture not available
  • Soaked hay may be an adequate alternative for some horses, but is usually less effective

Client Education!!navigator!!

  • A dust-free environment is paramount for the long-term management
  • Never feed moldy hay to horses

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Corticosteroids

  • Allow effective control of the clinical signs and airway obstruction, but the airway neutrophilia persists if horses are stabled and fed hay. The effects are also short lived after treatment cessation in absence of proper dust control
  • When systemically administered, expect a delay of 3–7 days between initiation of therapy and maximal clinical response
  • For severe attacks, dexamethasone (initial dose of 0.05 mg/kg PO, IV, IM, until control of clinical signs and then decrease and administer on alternate days for 10–20 days) is most effective
  • A single dose of triamcinolone acetonide (20–40 mg IM/500 kg) also improves clinical signs and airway obstruction for 3–5 weeks, but is less desirable because of increased risk of side effects
  • Oral prednisolone (1–4 mg/kg) may also be used, but its efficacy is less than with dexamethasone, without demonstrated better toxicity profile. Prednisone is poorly absorbed orally in horses and should not be used
  • Inhaled corticosteroids allow maximal concentration of drugs at the effector sites and minimize side effects. Nebulization chambers or masks have been designed for use of MDIs in horses
  • Inhaled beclomethasone diproprionate (beclometasone diproprionate) (2000–3500 µg/500 kg every 12–24 h) and fluticasone propionate (2000–3000 µg/500 kg every 12–24 h) are efficacious and well tolerated but have few residual effects. A delay in response of 7 days or longer should be expected

Bronchodilators

  • Bronchodilators are symptomatic treatments used to relieve lower airway obstruction caused by the airway smooth muscle contraction. They may also improve delivery of inhaled corticosteroids to the distal airways
  • Long-term administration should be combined with environmental dust control or corticosteroids, because inflammation could progress despite the improvement of clinical signs
  • Clenbuterol (0.8–3.2 µg/kg every 12 h) has bronchodilator effects and increases mucociliary transport. Clinical efficacy is inconsistent if exposure to dust is maintained. Decreased efficacy is expected after 2 weeks of treatments due to tachyphylaxis
  • Ipratropium bromide (0.4–1 µg/kg every 6 h), albuterol (salbutamol) (1–2 µg/kg), salmeterol (0.5 µg/kg every 8 h), and pirbuterol (1.3 µg/kg every 6 h) are bronchodilators that can be used by inhalation with MDIs
  • Atropine and Buscopan (hyoscine butylbromide) administered systemically have been used to assess the reversibility of clinical signs. However, they have limited therapeutic use because their effects are transient. Atropine may also be associated with severe side effects
  • Aminophylline has limited efficacy in heaves

Expectorant, Mucolytic, and Mucokinetic Agents

Evidence of efficacy for these agents in improving clinical signs is sparse.

Contraindications!!navigator!!

Corticosteroid administration in the face of sepsis or laminitis, or in horses prone to laminitis.

Alternative Drugs!!navigator!!

  • Inhaled sodium cromoglycate (cromoglicic acid) (200 mg every 12–24 h) in some horses in clinical remission prevents the appearance of clinical signs
  • Nedocromil sodium (24–60 mg every 12 h), another cromone, may be used by inhalation

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Sequential PaO2 determination to monitor response to therapy in horses with severely compromised respiratory function.

Prevention/Avoidance!!navigator!!

Hay substitutes and dust-free environment.

Possible Complications!!navigator!!

  • Heaves is a wasting disease that may rarely lead to death in severe cases if not treated
  • Bronchiectasis and right heart failure rare complications in severe cases
  • Risk of botulism with haylage and silage in unvaccinated horses

Expected Course and Prognosis!!navigator!!

The clinical signs at rest are reversible with prolonged environmental control and therapy. Some degrees of exercise intolerance may persist.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Age-Related Factors!!navigator!!

  • Rarely seen in horses <7 years
  • Severity may increase with age

Pregnancy/Fertility/Breeding!!navigator!!

  • Anecdotal reports suggest clinical signs may improve in some mares during pregnancy
  • Fetal growth retardation may occur in mares with severely compromised respiratory function

Synonyms!!navigator!!

  • Severe equine asthma
  • RAO
  • Improper terminologies—equine chronic obstructive pulmonary disease, chronic bronchitis, chronic bronchiolitis, broken wind, pulmonary emphysema

Abbreviations!!navigator!!

  • BALF = bronchoalveolar lavage fluid
  • MDI = metered dose inhaler
  • PaCO2 = partial pressure of carbon dioxide in arteral blood
  • PaO2 = partial pressure of oxygen in arteral blood
  • RAO = recurrent airway obstruction

Suggested Reading ⬆ ⬇

Leclère M, Lavoie-Lamoureux A, Lavoie JP. Heaves, an asthma-like disease of horses. Respirology 2011;16:1027–1046.

Ramseyer A, Gaillard C, Burger D, et al. Effects of genetic and environmental factors on chronic lower airway disease in horses. J Vet Intern Med 2007;21:149–156.

Author(s) ⬆

Author: Jean-Pierre Lavoie

Consulting Editors: Mathilde Leclère and Daniel Jean