In the summer, a horse can be very bothered by flies and gnats. A fly rug can then be a godsend! Fly rugs are made of a firm mesh fabric so the fly cannot irritate the skin of the horse and the rug breathes enough during the summer months. Because the fly rug moves over the horse's body it makes it difficult for the fly to land at all. Fly rugs with zebra print have an additional disorienting effect and are therefore more fly repellent than rugs without zebra print (source).
How do you recognize if the horse is suffering from flies?
A horse that is bothered by flies can behave restless by, for example, shaking its head a lot, wagging its tail or stamping or kicking its feet.