Summer usually brings relief for horses suffering from allergic respiratory disease - the equine equivalent of asthma caused by dust and moulds in hay and straw bedding that is now called recurrent airway obstruction, or RAO.

But, for some, there is no such escape. Summer pasture associated obstructive pulmonary disease (SPAOPD) was first described in the UK in the mid-1990s and there is a suggestion that it is becoming more common.

While it is virtually identical to RAO seen in stabled horses in terms of what is happening in the lungs and clinical signs shown, the triggers of SPAOPD are different with grass and crop pollens often being blamed.

Hypersensitivity to these allergens causes mucus production and narrowing of the airways. Mildly affected horses often have an occasional dry cough and a degree of exercise intolerance.

More severe disease results in laboured breathing, with characteristic heaving of the chest and abdominal muscles, flaring of the nostrils, wheezing and slight watery nasal discharge.

As with RAO, age of onset is usually five years or older such that the majority of affected animals are young to middle aged adults.

There is a strong seasonal incidence, with a peak of cases in April and May. This has been linked with the flowering of oilseed rape. It is also interesting to speculate that increasing prevalence of the condition has coincided with a boom in oilseed rape production.

Diagnosing SPAOD is relatively straightforward. While the occurrence of RAO-like signs in the summer can be misleading, endoscopy and analysis of fluid collected from the airways will confirm the presence of allergic respiratory disease.

Treatment involves a two-pronged approach - just as for RAO - with environmental changes to avoid allergens accompanied by therapies to reduce airway inflammation and relieve respiratory distress.

Removing affected animals from the offending environment is often sufficient, but this may mean moving them indoors. Symptoms usually subside within 48 hours.

Other, often more mildly affected horses, will do well in paddocks away from crops and where the grass has been trimmed.

Although the exact conditions that trigger SPAOPD are not known, it is believed that heat and humidity contribute, most probably by increasing pollen counts and encouraging formation of fungal spores.

Some horses may be fine in cooler weather, so turning out overnight or at dawn/dusk can be effective.

Haymaking in adjoining fields, as well as combining of cereals, can be also trigger the condition. Affected horses can usually return to these paddocks once work has been completed without recurrence of symptoms.

Management of some cases can be complicated by the fact that they also have RAO and so are sensitive to dust and moulds when stabled. It is these horses that often require veterinary treatment.

The most effective anti-inflammatories are corticosteroids (dexamethasone or prednisolone) and are given by injection or in tablet form for a short period in order to get the condition under control.

There are concerns over side-effects of corticosteroids, notably laminitis, but these are usually outweighed by their benefits and normal lung function is restored within a couple of days.

Inhaled steroids - delivered using a mask and metered dose ('asthma') inhaler - put the drug where it's needed the most, with less risk of side effects, but are more labour-intensive.

They tend not to be used initially as severe airway obstruction prevents inhaled drug reaching the lower airways. They are, however, the treatment of choice when long-term therapy is required.

Bronchodilator drugs may also be of benefit as they relax constricted airways, relieve airway distress, and decrease mucus production.

The most commonly used bronchodilators are clenbuterol (Ventipulmin), which comes as orally administered powder or liquid, and albuterol-containing inhalers (Ventolin).

De-sensitisation of horses has also been attempted but with limited success. This involves identifying possible allergens by skin and blood tests and administering small doses subcutaneously over a six-month period.

While SPAOPD can be frustrating, it is effectively managed with correct treatment and should not detract from enjoyment or performance. And it is certainly less of a scourge than other summer afflictions, most notably sweet-itch and head shaking.