In the horse environment the presence of numerous insects can bother your horses.

Insects attack especially the head of the horse, generally the eyes contour where they accumulate. They are mainly flying insects, that we usually call “fly” that cause problems to the horses and the riders. They are ectoparasites (external parasites). 
Unfortunately, the main problem is not the bite but the transmission of vector-borne diseases. 
They settle on the horses skin biting and injecting an enzyma that prevent the wound to close. Insects settle on the horses skin biting and injecting an enzyma that prevent the wound to close, they have an irritating consequence that can be either physical or psychological. On the psychological side, the constant attacks conduct to a moral weariness of the horse and a tiredness due to the movements done to chase them away. This may conduct to umpredictable and violent reactions.
These insects can be dangerous either or the horse than the rider. Indeed, in Ivory Coast for example, whenever you are washing your horse, the tsetse fly can approach and try to bite the horse and in the same occasion bite the rider aswell, giving him the sleeping sickness that might be mortal. (Alexis’s experience)

Many different solutions exist to protect your horse for the insects: insect repellent that can be vaporised directly on the horse, some insect repellent shampoo to apply while washing the horse body. 
At the same time, we can find some equipments (cup, masks..) that can be used while riding or not.
Moreover, try not to ride the horse during sunset, time of the day were the insects multiplicate due to the humidity.