Airborne invasion

Coughing, sneezing or a cluster of skin bumps could indicate a summer allergy, as Richard Hepburn FRCVS explains

- Richard Hepburn FRCVS

THE summer months bring increased levels of specific allergens (a molecule capable of eliciting an allergic response), namely pollens, fungi and insects.

Pollen released from grasses and trees peaks from 5am to 10am. As intact pollen is too large to enter the small airways of the lung, it is deposited on the horse’s skin and in the upper respiratory tract, mouth and eyes. Rain and moist conditions can cause pollen to shatter, liberating much smaller particles that can enter the lung.

Fungi exist in a symbiotic relationship with many grasses, and within hay are already implicated in stable-associated asthma. These fungi are also present on different parts of the growing grass plant, and the release of their spores increases with humidity – leading to peak airborne fungal spore levels in the early morning. Levels also rise during warm and windy weather.

Fungal spores can travel a great distance; in summer, your horse is not only exposed to local fungi, as he is in winter in the stable, but to those from a wider area.

Consistently warm weather creates an increase in flying insects. As they feed on your horse, they produce chemicals in their saliva which act as allergens.

Insects vary in their chosen feeding location on the horse and preferred feeding time. Culicoides midges breed near standing water, but will travel up to half a mile to feed, following a carbon dioxide trail from exhaled breath. Most active at dawn and dusk, they are poor fliers and struggle with windy conditions, congregating near hedgerows and woodland. Different midge species feed on different parts of the horse; the more species present, the more widespread the lesions.