Since there are many types of flies that can bite horses, I can only give you general recommendations: continue frequent (per label directions) application of repellants such as SWAT, make sure to frequently remove any moist bedding, manure, and heaps of plant material that flies can lay eggs in, stable him during the day if possible with a strong fan near the stall to push out the more weak flying insects, use a face fly mask, and call the vet out to make sure that he does not have some other cause for dermatitis on the face and chest (which may or may not be a fungus, it could be a bacterial infection, or a contact reaction to the fly sprays or benzidine/shampoos). Additionally, if the horse is very itchy, the vet may recommend a steroid injection for short term relief.
Kimberly Coyner, DVM
Diplomate American College of Veterinary Dermatology
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