Horsefly Bite Identification: Why It Hurts and Bleeds

The NHS description is short and matches what people report: horsefly bites are large, painful and may bleed, and they often become infected.

Penn State Extension explains the mechanism. Only female horse flies and deer flies bite, and their mouthparts cut the skin to produce a pool of blood rather than piercing a single vessel the way a mosquito's proboscis does. The fly then feeds from the pool. That is why the bite hurts at the moment it happens instead of being noticed later, and why it can keep bleeding after the fly has gone.

The practical consequence is that a horsefly bite is an open wound, not just a raised itchy bump, which is the reason the NHS singles it out as prone to infection.

Penn State Extension describes horse flies and deer flies together as stout-bodied flies with varying patterns and large eyes that are often brightly coloured. Horse flies belong to the genera Tabanus or Hybomitra; deer flies are smaller and mostly belong to the genus Chrysops.

The large, often iridescent eyes are the feature most people notice at close range, and they are distinctive enough to separate these flies from the house flies and other two-winged flies that do not bite.

Penn State also notes a detail worth knowing outdoors: dark-coloured animals attract more flies than light-coloured ones, and fly sheets with zebra patterns decrease fly attacks on horses. Photograph the fly on a pale surface and the app identifies it.

Penn State Extension states that horse and deer flies are daytime biters, and that they are most problematic during the warmer months when their aquatic breeding habitats support larval development. Females lay eggs on vegetation adjacent to wet habitats such as marshes, streams or ponds, and the larvae develop in the mud in those areas.

That is a useful piece of geography. If the bites happened on a walk beside a pond, a marsh or a slow stream on a warm, bright day, a horsefly fits. If they appeared overnight indoors, it does not — look at bed bug bites instead.

Penn State's control measures are written for horses, not people, but the principle transfers: shelters and shade away from wooded edges reduce contact, and repellents need frequent reapplication.

The NHS steps for insect bites apply. Wash the area with soap and water. Apply a cold compress for at least 20 minutes if it is swollen. Take pain relief if you need it. Use antihistamines or hydrocortisone cream for itching. Avoid scratching, because scratching is how a bite becomes infected — and the NHS specifically notes that horsefly bites often become infected.

The NHS says to see a pharmacist if the area shows signs of infection: heat, redness, pus, swelling and pain. The American Academy of Dermatology adds that a rash, fever, headache or body aches after a bug bite is a reason to see a board-certified dermatologist right away, as is a bite that looks infected.

Because the bite is an open wound, keep it clean and covered rather than exposed to dirt and water.

The NHS says to get urgent advice through NHS 111, or your local equivalent, if symptoms get worse, if the sting or bite is in the mouth, throat or eyes, if you feel dizzy, have a high temperature, were bitten or stung many times, or have had a severe allergic reaction before.

Call emergency services immediately for the anaphylaxis signs the NHS lists: swelling of the lips, mouth, throat or tongue; difficulty breathing or wheezing; throat tightness; skin, tongue or lips turning blue, grey or pale; confusion or fainting.

A horsefly bite that is getting redder, hotter and more painful over days rather than settling is following the infection course the NHS warns about, and needs a clinician rather than more cream. Nothing on this page is a diagnosis or a substitute for care.