Chronic obstructive pulmonary disease

Mildly affected or young horses may show only an intermittent cough and exercise intolerance when worked. Moderately to severely affected horses have a raised respiratory rate and cough at rest, nasal discharge, excess mucus, poor performance and obviously increased breathing effort at rest, with a heave line along the flank. Severe cases lose weight and appetite and can reach respiratory distress. Bronchoalveolar lavage shows excess neutrophils.

In horses, chronic obstructive pulmonary disease is an older name, along with recurrent airway obstruction and heaves, for what is now called severe equine asthma. It is a hypersensitivity of the lower airways to inhaled mould, organic dust and endotoxin, chiefly from hay and straw. Exposure causes neutrophilic airway inflammation, airway hyper-responsiveness, bronchoconstriction and bronchospasm, and excess mucus, which together narrow the airways. Chronic disease remodels the airways, including increased smooth muscle mass; part of this remodelling reverses with corticosteroid treatment and removal from the trigger. The sustained effort of breathing out enlarges the abdominal muscles, forming the characteristic heave line.

Severe equine asthma affects mature to older horses, generally over seven years, with average onset at 9-12 years. It affects about 14-17% of horses in countries with cool northern climates, and both sexes. Heredity matters: offspring risk is about 10% when neither parent is affected and 44% when both are. The barn-associated form affects stabled, hay-fed horses and worsens in winter and spring; a pasture-associated form affects horses grazing in summer and early autumn, notably in the south-eastern United States.

Mild to moderate equine asthma (formerly inflammatory airway disease) is separated mainly by the absence of increased respiratory effort at rest. Bacterial pneumonia or other infectious respiratory disease should be excluded: history should cover past respiratory infection, and a transtracheal wash is used in horses showing signs of underlying infection.

Lower airways (bronchi and bronchioles), airway smooth muscle and mucus glands, lungs, abdominal respiratory muscles

Maximise pasture turnout for the barn form, use low-dust bedding such as chopped paper or cardboard, soak hay or feed complete pelleted rations, avoid round bales, and keep stables clean and well ventilated.

Chronic, needing lifelong management, with cycles of remission and flare. Many mild to moderate cases are controlled by environmental and dietary change alone; drugs bring rapid relief, but signs return if dust and mould exposure continues.