Horseflies are the large, dark flies that appear near water, farmland, woodland and horses from late May to September. Unlike a mosquito, which pierces the skin, a horsefly cuts it with scissor like mouthparts and feeds on the blood that pools in the wound. That is why the bite hurts immediately, bleeds a little, and reacts far more than a midge or mosquito bite. Every summer they bring a steady run of patients into the clinic, and a fair proportion of those bites are infected by the time they arrive.
What a horsefly bite looks like
- A raised, red, hot lump, usually 1 to 5 cm across but sometimes much larger, appearing within minutes of the bite
- Often a small cut, blister or dot of blood at the centre where the skin was broken
- Sometimes a wider pale or pink area of swelling around it, particularly on the lower leg, forearm or face
- Occasionally a cluster of bites, because the fly bites more than once if it is disturbed
Compared with other insect bites, a horsefly bite is bigger, more painful and slower to fade. A mosquito bite itches and shrinks within a day or two; a horsefly bite throbs, stays raised for days and often develops a hard, tender area underneath.
Symptoms
Most people get a sharp pain at the moment of the bite, followed within an hour by itching, burning, swelling and tenderness. The lump may weep clear fluid or form a small blister. On the lower leg, where circulation is slower, the swelling can be dramatic and the skin around it can feel tight. These are all a normal reaction to the fly's saliva, which contains chemicals that stop blood clotting and provoke a strong local immune response.
A smaller number of people have a larger allergic reaction: hives spreading away from the bite, swelling of a whole limb, wheeze, dizziness or swelling of the lips, tongue or throat. Any breathing or swallowing difficulty after a bite is anaphylaxis until proven otherwise and needs 999, not a GP.
How long a horsefly bite lasts
Pain is usually worst in the first 24 to 48 hours. Swelling and itching settle over three to seven days, and the lump itself may take two weeks to flatten completely. A faint mark or a small area of darker skin can persist for a few weeks, especially on the legs. A bite that is getting bigger, hotter or more painful after the second day is not following this course and should be looked at.
Treating a horsefly bite at home
- Clean it straight away with soap and water or an antiseptic wipe. Because the skin is cut rather than pierced, bacteria get in easily.
- Cold compress for 10 minutes at a time, several times a day, to reduce swelling and pain.
- Antihistamine tablets (cetirizine or loratadine from a pharmacy, taken as directed on the packet) for the itching and swelling. Chlorphenamine works too but causes drowsiness.
- Hydrocortisone 1% cream from a pharmacy, applied thinly twice a day for up to a week, calms the local reaction. Not on broken skin or the face without advice.
- Paracetamol or ibuprofen for the pain.
- Do not scratch, and keep nails short. Scratching is the commonest route to infection. Cover the bite with a plaster if you cannot leave it alone.
- Raise the limb if it is a leg bite, which reduces the swelling that makes these bites so uncomfortable.
Tea tree, vinegar, toothpaste and bicarbonate pastes are popular online. None has evidence behind it, and anything that irritates the skin raises the infection risk.
- the redness is spreading outwards, especially in streaks, or the area is hot, shiny and increasingly painful after 48 hours
- there is pus, a yellow crust, or the bite smells
- you have a fever, chills, or feel unwell
- the swelling is large and tight, or a whole hand, foot or forearm is swollen
- you have diabetes, poor circulation, lymphoedema or a weakened immune system, where any skin infection moves faster
- a child has a bite on the face or near the eye that is swelling
Call 999 for any difficulty breathing or swallowing, swelling of the lips or tongue, or faintness after a bite.
Infected or badly swollen bite? Be seen today
A GP can assess the bite, prescribe antibiotics or a stronger steroid cream if needed, and rule out cellulitis. In clinic £65, or by video £45 if a photo and a conversation are enough.
Book a same day GP appointmentInfected horsefly bites
Horsefly bites are the insect bite most likely to become infected, for three reasons: the skin is cut open, the bite itches intensely so people scratch, and the swelling slows the local circulation. The usual infection is cellulitis, a bacterial infection of the skin and the tissue beneath it. It shows as spreading redness that is warm, firm and tender, sometimes with red streaks running towards the groin or armpit, swollen glands, and a fever. It needs a course of oral antibiotics, started promptly, and occasionally hospital treatment if it is spreading fast or the person is unwell.
A localised abscess (a collection of pus under the skin) can also form; this may need draining as well as antibiotics. Impetigo, with honey coloured crusting, is more common in children.
What a GP does: examines the bite and marks the edge of the redness so progress can be checked, decides whether antibiotics are needed and which, checks for signs the infection is spreading, and arranges a review in 48 hours if there is any doubt. If you are given antibiotics, finish the course and keep the limb raised.
Is it a horsefly bite or something else?
| Bite | How it differs |
|---|---|
| Mosquito | Smaller, itchier, little pain at the time, settles in a day or two |
| Midge | Many small bites in clusters, intensely itchy, usually at dusk |
| Tick | Painless; the tick is often still attached. A spreading bullseye rash days later needs a GP (Lyme disease) |
| Wasp or bee sting | Immediate sharp pain and a pale raised weal; bees leave a sting behind |
| Flea | Small red spots in lines or groups, usually around the ankles |
| Spider | Rare in the UK; two puncture marks, usually mild |
Preventing horsefly bites
Horseflies are drawn to movement, dark colours, warmth and the carbon dioxide you breathe out, and they are most active on hot, still, humid days. Light coloured, loose, long sleeved clothing is the single most effective measure. DEET based repellents help but are less reliable against horseflies than against mosquitoes, so cover up as well. Avoid standing water, stables and long grass in the middle of the day, and keep moving: horseflies are slow and clumsy and give up on a target that walks away.
When to see a doctor
Book an appointment the same day for any of the signs in the box above. If the bite is simply painful and swollen but not spreading, there is no fever and the skin is not broken down, home treatment for a week is reasonable. It is also worth being seen if a bite is not improving at all after a week, or if you get large reactions to every bite, because a stronger antihistamine or a short course of steroid tablets can be prescribed for the next one.
Questions people ask
What does a horsefly bite look like?
A raised, red, hot lump, usually 1 to 5 cm across, often with a small cut, blister or spot of blood at the centre where the fly broke the skin. It appears within minutes, is painful rather than just itchy, and can swell considerably, particularly on the lower legs. Several bites close together are common if the fly was disturbed and bit again.
How long does a horsefly bite take to go down?
Pain peaks in the first one to two days; swelling and itching usually settle over three to seven days, and the lump may take up to two weeks to flatten fully. A bite that is getting larger, hotter or more painful after day two is likely to be infected and should be checked.
How do you know if a horsefly bite is infected?
Redness spreading beyond the original lump, heat, increasing pain after the first 48 hours, pus or yellow crusting, red streaks running up the limb, swollen glands or a fever. Any of these means cellulitis is likely and you should see a GP the same day for antibiotics.
Do horsefly bites need antibiotics?
Not usually. An uninfected bite needs cleaning, a cold compress, antihistamines and hydrocortisone cream. Antibiotics are needed when the bite becomes infected, which horsefly bites do more often than other insect bites because the skin is cut rather than pierced. A GP decides based on the spread of redness, pain, heat and whether you have a fever.
Why do horsefly bites hurt so much?
Because the fly cuts the skin with blade like mouthparts rather than piercing it, and its saliva contains anticoagulants and other chemicals that trigger a strong local immune reaction. The result is immediate pain, a lot of swelling and a lump that persists for days.
Can a horsefly bite make you feel unwell?
A normal bite should not. Feeling feverish, shivery or generally unwell after a bite suggests infection, or less commonly a wider allergic reaction, and needs assessment the same day. Difficulty breathing, swelling of the face, lips or tongue, or faintness after a bite is an emergency: call 999.
Can I get a horsefly bite checked by video?
Yes, for many bites a clear photograph and a conversation are enough for a GP to decide whether antibiotics are needed and to prescribe them. If the GP needs to feel the area, check for an abscess or you are unwell, you will be asked to come in. OneMedicine offers both, same day.
Same day GP appointment for an insect bite
In clinic at Kings Heath, Birmingham, or by video. Assessment, prescription where needed, and a clear plan. Book online with OneMedicine, CQC registered.
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Specialist interest: Cardiovascular Medicine · OneMedicine, CQC registered clinic