Horse fly and deer fly bites hurt immediately and noticeably more than mosquito bites because these insects don’t pierce your skin with a needle-like probe. Instead, they slash it open. Female horse flies and deer flies have two pairs of cutting blades in their mouthparts that lacerate the skin, creating a small wound that bleeds freely. The fly then laps up the pooling blood with a sponge-like mouthpart while injecting saliva that contains anticoagulant compounds to keep the blood flowing. Only females bite; males have weaker mouthparts and feed on nectar instead.
What the Bite Looks and Feels Like
You’ll know when you’ve been bitten. The cutting action produces a sharp, burning pain at the moment of the bite, not the subtle sting of a mosquito. Within minutes, the area typically swells into a raised, red welt that can be larger than a mosquito bite because of the actual tissue damage involved. Some people develop hives (raised, itchy patches) or firm papules localized around the bite site. A small amount of bleeding from the wound is normal and expected given how the fly feeds.
Horse fly bites tend to be slightly more painful and produce larger welts than deer fly bites simply because horse flies are bigger insects, sometimes over an inch long. Deer flies are smaller, about the size of a housefly, and often target the head and neck. Both leave a similar type of wound.
For most people, the pain and swelling peak within the first hour and gradually fade over a few days. If your symptoms persist longer than that, it’s worth getting the bite checked.
Treating a Bite at Home
Start by cleaning the bite with soap and water. Because the wound is an actual cut rather than a pinprick, keeping it clean matters more than with typical insect bites. Apply a cold compress or ice wrapped in a cloth to reduce swelling and numb the pain. Over-the-counter hydrocortisone cream or calamine lotion can help with itching, and an oral antihistamine can tamp down the local allergic response that causes the welt to swell.
Resist the urge to scratch. The open wound from a tabanid bite is already more vulnerable to bacteria than a mosquito bite, and breaking the skin further raises your infection risk. Keep the area clean and loosely covered if it’s in a spot that rubs against clothing.
Signs of Infection
The laceration-style wound these flies create makes secondary bacterial infection (cellulitis) a real concern. Watch for these signs in the days after a bite:
- Expanding redness or red streaking spreading outward from the bite
- Increasing warmth in the skin around the wound
- Pus or drainage from the bite site
- A fluid-filled blister forming around the bite
- Fever, chills, or fatigue
- Swollen lymph nodes near the bite area
Any of these symptoms signal a skin infection that needs antibiotics. A bite that simply stays red and itchy for a day or two is normal. A bite that gets progressively worse, especially with warmth, streaking, or fever, is not.
Allergic Reactions
The anticoagulant saliva that horse flies and deer flies inject can trigger allergic reactions beyond the normal local swelling. Most reactions stay mild: a larger-than-expected welt, itching that spreads a few inches from the bite, or hives near the site. These respond well to antihistamines.
Rarely, bites can cause anaphylaxis. This is a whole-body allergic emergency that can develop within minutes of being bitten. Symptoms include generalized itching over large areas of the body, widespread hives, swelling of the face or throat, throat tightness, shortness of breath, nausea, vomiting, heavy sweating, and loss of consciousness. Anaphylaxis requires immediate treatment with epinephrine. If you’ve had a severe reaction to a horse fly or deer fly bite before, carrying an epinephrine auto-injector during outdoor activities in fly season is a reasonable precaution to discuss with your doctor.
Disease Risk: Tularemia
Deer flies in particular can transmit a bacterial infection called tularemia, sometimes known as “deer fly fever” or “rabbit fever.” The bacterium is highly infectious, meaning it takes very few organisms to cause illness. Deer fly bites are one of the most common ways humans pick up this pathogen, alongside tick bites and handling infected animals like rabbits.
Tularemia typically causes a skin ulcer at the bite site along with swollen lymph nodes, fever, and fatigue. It’s treatable with antibiotics but can become serious if ignored. If you develop an ulcer or persistent swelling at a deer fly bite site, especially if accompanied by fever, mention the bite to your healthcare provider so they can test for it.
Why Repellents Don’t Work Well
Here’s the frustrating reality: standard insect repellents, including DEET and picaridin, don’t reliably deter horse flies or deer flies. According to University of New Hampshire Extension entomologists, no repellents really work well on these groups of flies. The flies are aggressive, persistent feeders that rely heavily on visual cues (movement, dark colors, contrast) to find hosts, so chemical barriers on your skin have limited effect.
Your best protection is physical. Wear light-colored, long-sleeved clothing when you’re in areas with heavy fly activity, particularly near water, marshes, and wooded edges during summer months. Horse flies and deer flies are most active on warm, sunny, calm days and are especially drawn to dark colors and movement. A hat helps against deer flies, which prefer to circle and land on the head. Some people find success with sticky patches worn on hats that trap deer flies before they bite.
Avoiding peak activity times helps too. These flies are daytime feeders, most active in the late morning and afternoon hours. They’re far less active on windy days, overcast days, and at night.