Not all flies bite, but those that do seek a blood meal, resulting in an immediate physical reaction. Unlike mosquitoes that use a fine, needle-like probe, common biting flies such as horse flies, deer flies, and stable flies employ aggressive mouthparts, causing a noticeable injury to the skin. Understanding the mechanism of the injury, the potential for secondary risks like infection, and the correct steps for immediate care can help manage the aftermath of a fly bite.
The Mechanics of the Bite and Immediate Symptoms
The sharp pain felt during a fly bite results from how the insect’s mouthparts interact with the skin. Many biting flies, particularly horse flies and deer flies, possess specialized mandibles and maxillae that function like small, serrated blades. These parts slice or tear the skin, creating an open wound from which the fly laps up the resulting pool of blood. This cutting method causes bites to bleed immediately, unlike a mosquito bite which typically leaves only a puncture mark.
Once the skin is cut, the fly injects saliva containing compounds that prevent blood clotting and dilate blood vessels, ensuring a steady flow for the fly’s meal. The human body’s immune system reacts quickly to these foreign proteins, leading to immediate symptoms. Within minutes, the bite site typically develops a raised, red bump or welt (a wheal), accompanied by intense itching and localized swelling.
The reaction is a localized inflammatory response designed to neutralize the foreign substances introduced by the fly. Although the initial sharp pain subsides quickly, the subsequent itching and swelling can persist for several hours or even a day. Some people may experience a more widespread rash or hives around the area, indicating a stronger sensitivity to the fly’s salivary secretions.
Potential for Disease and Secondary Infection
The primary concern following a fly bite is the risk of a secondary bacterial infection. The slicing action of the fly’s mouthparts creates a wound that is larger and slower to heal than a simple puncture, making it vulnerable to contamination. Scratching the intensely itchy bite can break the skin further, allowing bacteria naturally present on the skin, such as Staphylococcus species, to enter the open tissue. A developing secondary infection may be indicated by localized signs like increasing warmth, spreading redness, worsening pain, or the presence of pus at the site.
In North America and Europe, the transmission of serious vector-borne diseases from common biting flies is rare. Deer flies, for example, are known to be mechanical vectors for the bacterium that causes tularemia, sometimes called “rabbit fever,” particularly in the western United States. Black flies cause painful bites and significant local swelling, but are not known to transmit diseases to humans in the United States, though they transmit onchocerciasis (“river blindness”) in other global regions.
The overall risk of contracting a fly-borne illness is low, but attention to systemic symptoms is important. Signs like a persistent fever, chills, swollen lymph nodes, or a severe headache following a bite warrant medical evaluation.
Treating the Bite and Recognizing Emergency Signs
Immediate care focuses on cleaning the wound and managing the inflammatory reaction. Gently wash the affected area with soap and water to remove residual saliva or potential contaminants. Applying a cold compress or an ice pack wrapped in a cloth for ten to twenty minutes helps reduce both swelling and pain. Over-the-counter treatments are often effective to relieve intense itching and localized swelling.
Oral antihistamines mitigate the body’s allergic response to the saliva proteins, and topical hydrocortisone cream or calamine lotion can soothe the skin directly. The goal of these treatments is to minimize itching and prevent scratching, which is the greatest contributor to secondary infection.
Certain symptoms signal a severe allergic reaction, or anaphylaxis, which requires immediate emergency medical attention. These signs include difficulty breathing, wheezing, swelling of the face, lips, or throat, or a feeling of lightheadedness and dizziness. If these symptoms occur, professional medical help should be sought immediately.