Flies are the most common and troublesome external parasites of horses. During the warmer months of the year no region is free of these pests. Flies cause much irritation, and can also spread disease. Large numbers can result in excessive blood loss. Fly bites can also result in skin allergies due to hypersensitivity reactions. The annoyance from flies can disrupt grazing; horses may run frantically to get away from biting flies, or seek out shady areas to swish and stomp. Flies can also distract a horse when you are riding or working with him; he may pay more attention to flies than he does to you.