A common condition seen in young and old horses alike is allergic Respiratory Disease. Unlike humans, horses that have allergies tend to show more flu-like symptoms and less sinus and nasal effects. Their lungs become inflamed and are more susceptible to virus and bacterial infections. The horse owner sees this as frequent (recurrent) "chest colds" that the horse never quite gets over. Other symptoms include coughing, excessive eye discharge and exercise intolerance (gets tired easily).
The causes of allergic Respiratory Disease are many but some general classes of allergens are more common. Several types of mould spores and weeds found in otherwise high quality hay or straw seem to bother many horses. These contaminants are seasonal, so for the horse at pasture the problem may only be seen in spring and summer. However, hay that is taken from local fields represents a year round source. Each year, the horse usually shows more symptoms as the body's sensitivity to these allergens gets stronger and stronger.
Treatment of this problem can be as simple as keeping your horse outside where air circulation is optional (fresh air versus stagnant dust filled air). If your horse must stay in the stall most or all of the time, try to store your hay in a separate place; maximize airflow in the barn. If it is cold, put a blanket on the horse and soak the hay fed to the horse in water first.
If the problem is not taken care of by the above steps, then your vet should get more involved in the process. Diagnostically, the vet can help by performing a simple procedure that looks at the fluid and cells in the horse's lungs to determine the severity of the allergy. Secondly there is a relatively new blood test that can specifically determine what "things" the horse is allergic to and allow for production of a "customised" treatment for that individual horse. Other general treatments include corticosteroids (cortisone) and bronchodilators (like those taken by human asthma patients). The important thing to realise is that recognising your horse has this problem is more than half the battle. Once that is done, proper treatment can dramatically improve the usefulness of your horse.
Taken From Equine World