How flies spread parasites and disease - Flies are the bane of many horses’ existence from spring through autumn. In Canada, horse-biting flies include gnats (no-see-ums), black flies, stable flies, deer flies, horse flies, and mosquitoes, all of which create painful sores. Non-biting flies include house flies, eye gnats, face flies, bot flies, and warble flies, and can be just as irritating. It’s not just buzzing and biting that’s worrisome; flies may transmit parasites and diseases, can cause a horse’s immune system to overreact, and some use horses as hosts, all of which impact horse health. Preventing horses from being bitten is key. Before diving into solutions, here’s a summary of the damage that flies can inflict on Canada’s horses.