Horsefly bites can be painful, but they are not usually dangerous. Covering up with clothing is one way to prevent bites, but horseflies can often bite through fabric. Cleaning the area and applying a cool compress may help ease the pain.

Avoiding horseflies outdoors in summer can be difficult, and horsefly bites can be more painful than bites from other bugs. This is because they have saw-like mouthpieces they use for biting. Also, they do not inject any type of anesthetic when they bite.

While painful, a horsefly bite will rarely lead to severe complications. In rare cases, however, a person may have a severe allergic reaction to a horsefly bite and need medical attention.

This article describes the most effective way to treat horsefly bites, how to identify them, and ways to prevent the bites.

Horsefly bites are painful compared with other insect bites. They can also be slow to heal. This is because of the method the flies use to bite.

The following are features of a horsefly bite:

  • The bite will be very painful because the horsefly does not release any anesthetic when it bites.
  • The fly makes a cut rather than a small puncture hole with mouthparts like a serrated saw.
  • The fly uses an anticoagulant to stop blood from clotting as it consumes it.
  • The fly anchors to the skin while drinking the blood with the help of small hooks along its mouthparts.
  • After a bite, the area around will be raised, and welts will develop.

The pain and welts can help identify horsefly bites.

The following may also occur:

  • a large, red, raised rash at the site or over a wider area
  • dizziness
  • weakness
  • wheezing
  • swelling and puffiness elsewhere in the body

Alone, a red, raised bump on the skin is not usually a sign of concern but if any of the above symptoms develop or if breathing becomes difficult, a person should seek medical attention as soon as possible. That person could be having a severe allergic reaction.

The bite can take time to heal, and an infection may develop. If pus or other signs of infection appear, the person should seek medical advice.

What do horseflies look like?

Horseflies have the following appearance:

  • They are large, often 2/3-inch in length.
  • They are dark in color and have striped chests and black bellies.
  • They have large, compound eyes.

Horsefly bites can be very painful but are not usually harmful to humans. However, there have been some cases of horseflies passing on tularemia — also known as rabbit fever — to humans.

Tularemia is a bacterial infection with a range of possible signs and symptoms. It can be life threatening but is treatable with antibiotics.

However, a person should seek medical help if:

  • there are signs of an allergic reaction or anaphylaxis
  • symptoms are severe and persistent
  • an infection develops at the site of the wound and discharges pus

A horsefly bite will cause:

  • pain
  • swelling in the area
  • lighter skin will become red, and darker skin tones may become darker
  • in some cases, dizziness, wheezing, and puffiness around the eye and lips due to an allergic reaction

Some people may have a more severe allergic reaction, or anaphylaxis, including:

  • tongue and throat swelling
  • swollen face, lips, hands, or feet away from the site of the bite
  • nausea, vomiting, or diarrhea
  • feeling very unwell
  • difficulty swallowing
  • dizziness
  • wheezing and breathing problems
  • puffiness around the eye and lips

This is an emergency, and the person will need urgent medical attention.

Anaphylaxis is a severe allergic reaction that can be life threatening. The symptoms develop suddenly and include:

  • hives
  • swelling of the face or mouth
  • wheezing
  • fast, shallow breathing
  • a fast heart rate
  • clammy skin
  • anxiety or confusion
  • dizziness
  • vomiting
  • blue or white lips
  • fainting or loss of consciousness

If someone has these symptoms:

  1. Check whether they are carrying an epinephrine pen. If they are, follow the instructions on the side of the pen to use it.
  2. Dial 911 or the number of the nearest emergency department.
  3. Lay the person down from a standing position. If they have vomited, turn them onto their side.
  4. Stay with them until the emergency services arrive.

Some people may need more than one epinephrine injection. If the symptoms do not improve in 5–15 minutes, or they come back, use a second pen if the person has one.

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Treating a horsefly bite is similar to treating other types of insect bites.

Tips for treating the symptoms of a horsefly bite are:

  • cleaning the area with soap and plain warm water using a clean cloth or cotton wool
  • apply a cold compress or ice pack wrapped in a cloth to ease the pain and reduce swelling
  • use acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs) to ease pain
  • taking oral antihistamines to reduce itching

A person should avoid scratching the wound, as this will likely worsen it and increase the risk of infection. Vinegar and baking soda are unlikely to help.

If a horsefly bite does become infected, the person should speak with a doctor. They may prescribe antibiotics.

Horseflies do not leave behind any mouthpart or stinger when they bite.

A horsefly is a flying insect often found in rural farmland areas. Many species lay their eggs on plants near water or in damp soil.

Worldwide, there are over 4,500 species of horsefly, some of which are called clegs.

Female horseflies bite large animals to maintain the life cycle. They need blood to provide proteins that their eggs need to develop after fertilization. This is similar to processes followed by other insects, such as mosquitos.

Horseflies mainly take blood from cows and horses, but they can bite humans, too.

Female horseflies need up to 0.5 milliliters (ml) of blood for reproduction, which is a large amount considering their size. They can take up an estimated 200 milligrams (mg) of blood in a few minutes.

The horsefly is specially adapted to consume as much blood as possible. It releases substances that stop blood clotting where the skin breaks. Historically, some forms of medication have used horseflies for their anti-clotting characteristics.

Males do not need blood and do not have the appropriate mouthparts. Horseflies have large eyes, and, on a male horsefly the eyes will meet in the middle of the head.

Horseflies can be difficult to avoid in the summer, as their habitats are widespread.

There are some practical steps, however, that can help reduce the risk of horsefly bites:

  • Keep the skin covered with shoes, long pants, and long-sleeved tops.
  • Wear clothes that are light in color.
  • Wear loose clothing, as horseflies can bite through fabric.
  • Avoid perfumed cosmetic products, as these may attract flies.
  • Stay away from water in summer, where horseflies breed.
  • Using insect repellent may deter flies but probably not prevent horsefly bites, though it is helpful against mosquitos that may be present in the same areas. The most effective repellents have 50% DEET.
  • Hang fly tape around buildings where animals live, such as a barn.

Here are some questions people often ask about horsefly bites.

What happens when a horsefly bites you?

Female horseflies bite when they need blood to develop their eggs. They use saw-like mouthpieces to cut through the skin. The bite can be very painful.

What does a horsefly bite look like?

When a horsefly bites a person, there will likely be pain and swelling in the area of the bite. If swelling occurs in other parts of the body or a person starts to have difficulty breathing, this may be a sign of an allergic reaction that needs emergency medical help.

What can you do for a horsefly bite?

A person should wash the area thoroughly with soap and water and avoid scratching it. Applying ice or a cold pack wrapped in a cloth can help ease pain and swelling. Over-the-counter (OTC) pain relievers can also help with pain, and oral antihistamines may reduce itching.

Horseflies are large flies that usually bite cows and horses but can also bite humans. They have a very painful bite, as their mouthpieces cut into the skin. Only the females bite, and they do this to obtain blood to help develop their fertilized eggs.

As well as a painful bite, there may be large welts and swelling around the area. A horsefly bite is not usually dangerous, but some people can have an extreme allergic reaction or anaphylaxis. Anyone with symptoms of swelling or difficulty breathing after a bite needs emergency medical help.

A horsefly bite can take time to heal. While healing, it is essential not to scratch it, as it may become infected. Home and OTC remedies can help manage pain and itching.