Page 684 - The Veterinary Care of the Horse

and narrowing of the lower airways.

When hay is baled with a high moisture content (above 20%), the bales heat up and the
VetBooks.ir growth of moulds such as Aspergillus fumigatus, Faenia rectivirgula and Thermoactinomyces

vulgaris occurs. Inhalation of spores of these fungi triggers the airways to become
hypersensitive. It has now been shown that inhaled endotoxin from bacteria present in the

environment triggers airway inflammation in normal horses. This is also likely to be a
contributory factor in causing the disease. The sources of these bacteria have not yet been
identified but they are likely to be from faeces, the horse’s coat and the forage.

It has now been established that some horses have a genetic susceptibility to RAO and

are more easily sensitized than others.

Predisposing factors

• Repeated exposure to hay and straw dust containing moulds, forage mites, endotoxins

and inorganic material.
• Dusty feeds.

• Long hours in the stable.

• Poor stable hygiene.

• Inadequate ventilation.
• A respiratory virus, e.g. equine influenza, equine rhinovirus or equine herpes virus which

damage the epithelial surface of the respiratory tract. This adversely affects the clearance

of inhaled allergens and may alter the immune response of the horse.

Clinical signs

The disease usually develops over a period of time. Affected horses do not have a

temperature and they appear well in themselves. The first signs include:

• reduced exercise tolerance
• increased respiratory rate

• increased expiratory effort – the abdominal muscles are used to force the air from the

lungs; this results in a characteristic biphasic expiratory movement
• an occasional cough, usually at the start of exercise

• milky-white nasal discharge from both nostrils, especially first thing in the morning and

after exercise (Figure 15.14)