Black flies, also known as buffalo gnats or biting gnats, are small, humpbacked insects found near flowing water where their larvae develop. They are a common nuisance globally, particularly during the late spring and early summer months. This article examines the black fly’s feeding mechanism and its role as a vector to explain the health risks they present, including localized pain, systemic reactions, and potential disease transmission.
How Black Flies Attack
Only the female black fly bites, requiring a blood meal to produce eggs; males feed only on plant nectar. Females are “pool feeders,” a method distinct from the fine-needle approach of a mosquito. They use specialized, blade-like mouthparts to cut a jagged hole in the skin, which is often immediately painful.
This cutting action lacerates capillaries just below the surface, creating a small pool of blood from which the fly drinks. To facilitate the blood meal, the fly injects saliva containing an anticoagulant and an anesthetic. This keeps the blood flowing freely and causes a localized, inflammatory response to the foreign proteins. Black flies feed only during the day, with activity often peaking in the late morning and late afternoon.
Immediate Health Dangers and Local Reactions
The most common health risk is the localized reaction to the injected salivary components. The immune response to the anticoagulant and other toxins leads to swelling, redness, and intense itching at the bite site. The resulting weal can range from a small bump to a swollen area the size of a golf ball, and discomfort may persist for days or weeks.
In cases of multiple bites, a systemic, non-allergic condition known as “black fly fever” can develop. This reaction is characterized by symptoms such as headache, nausea, low-grade fever, and swollen lymph nodes. Hypersensitive individuals can experience a severe allergic reaction, which may include extreme swelling, cellulitis, or anaphylactic shock. Scratching the intensely itchy bite sites can break the skin and introduce bacteria, leading to a secondary bacterial infection.
Black Flies and Disease Transmission
Black flies are globally known as the primary vector for the parasitic worm Onchocerca volvulus, which causes Onchocerciasis, or River Blindness. This illness can lead to disfiguring skin conditions, chronic itching, and progressive visual impairment, resulting in permanent blindness. Transmission occurs when an infected fly deposits the worm’s larvae into the skin while feeding on a human.
The risk of contracting River Blindness is geographically specific. Over 99% of infected individuals reside in 31 countries in sub-Saharan Africa, with endemic areas also existing in Yemen and parts of Latin America. In North America and Europe, the black fly species that bite humans are not known to transmit Onchocerca volvulus or any other human pathogens. While black flies can transmit parasites to livestock and birds, the systemic risk of disease transmission to the average person in non-endemic regions is negligible.
Treating Bites and Preventing Exposure
Immediate care involves cleaning the wound thoroughly with mild soap and water to reduce the chance of secondary infection. Applying a cold compress or ice pack wrapped in a cloth for 10 to 15 minutes helps reduce swelling and irritation. Over-the-counter remedies like topical anti-itch creams containing hydrocortisone or oral antihistamines can manage the swelling and intense itching.
Preventing black fly bites is the most effective defense. These insects are visually attracted to dark colors, so wearing light-colored clothing is recommended. Maximizing skin coverage with long-sleeved shirts and long pants is effective, as the flies cannot bite through fabric. Using insect repellents containing active ingredients like DEET or Picaridin on exposed skin deters the flies from landing and biting.