Overview. Black flies, also known as buffalo gnats, are small, biting flies that irritate people and animals that live, work, or play near running rivers and streams. In North America, black flies do not spread disease, but their bites can cause swelling, bleeding, pain and itching.

Do black flies serve any purpose?

Black flies drink blood and spread disease such as river blindness -- creating misery with their presence. A new study, however, demonstrates that the pesky insects can be useful. Black flies drink blood and spread disease such as river blindness-creating misery with their presence.

What does Black fly cause?

On rare occasions, black flies have been known to cause exsanguination (death due to blood loss) from extreme rates of biting. Saliva injected by biting black flies can cause a condition known as "toxic shock" in livestock and poultry, which may result in death.