Horses suffer from an allergy to a particle which is often present on both hay and straw, namely fungal spores. These spores are small and light enough to be caried down into the deepest parts of the lungs, where they can come into direct contact with sensitive lung tissue.

The horse's lungs contain thousands of tiny airways. A fungal spore gaining entry into an alveolus of an allergic horse will trigger off a local reaction. The airways will constrict, mucus will be released and that tiny section of the lungs is no longer fully functional. The more spores which are breathed in, the more tiny segments of lungs stop functioning.

Colin Vagal

MRCVS

Hay and straw becomes infected with fungi which release spores into the crevices between the individual stems of roughage. These in turn are shaken into the air by the horse, moving around on the straw or by the hay net. This invisible smog of fungal spores is then breathed in by the horse.

COPD, which bear many resemblances to human asthma, is caused by an allergic reaction in the lungs to inhaled spores released from mouldy hay and straw. It has been estimated that a horse eating badly contaminated hay may inhale as many as ten billion of these spores at each breath.

Bruce Goran

BSc, BVM&S, MRCVS