Horses can have allergies. Acknowledging that possibility, are the chickens in direct contact with the horses? Do the chickens create lots of dust and debris in the air in the vicinity of the horses? If so, separating the chickens and horses may be an appropriate approach. In addition, before you get rid of the chickens, have your veterinarian check the tear ducts in your horse's eyes. Sometimes the ducts become clogged and the draining is impaired; hence, the question about the contact between the two species.
