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What to do about a horsefly bite

Horsefly bites are painful. Some people develop serious allergies to horsefly bites, but this is rare. Avoiding horseflies can be difficult outdoors in summer, but dealing with their bites is usually simple.

Because of the way they do their damage, horsefly bites can be more trouble than bites from other bugs, however.

What are horseflies?

A horsefly sits on an index finger
Horseflies are prevalent in hot weather and near water.

Horseflies are a flying insect found most in rural, farmland areas, where they feed on large mammals. They can be found across North America.

They can also be found in urban areas near their breeding sites where there is water, such as a lake.

Scientists call horseflies blood-feeding arthropods. The scientific name for their family of species is Tabanus.

Horseflies come in numerous species, including flies known as clegs. In the United Kingdom, for example, there are 30 species of horsefly.

Horseflies must bite large animals including horses, cattle, dogs, and humans as part of their lifecycle.

Only female horseflies bite. Male horseflies do not have the mouthparts that females use to bite animals. Females do this to drink the blood that they need to produce their eggs.

Female horseflies need a large amount of blood for reproduction up to 0.5 milliliters. Scientists estimate that they can take up as much as 200 milligrams of blood in a few minutes.

This process is similar to that of mosquitoes and other blood-sucking arthropods such as ticks.

The horsefly is specially adapted to get as much blood as possible. It releases substances that stop the blood clotting at the point where they wound the host mammal. Traditional Chinese medicine has even used horseflies for these anticlotting materials.

Horseflies are more of a problem when the weather is hot and sunny, and when the wind is low. They are found during the day in the middle of the summer months. They can be more of a pest during thundery hot weather, according to wildlife conservationists.

What do horseflies look like?

Horseflies:

  • Are large
  • Dark-colored, with striped chests and black bellies
  • Have large compound eyes
  • Are strong fliers
  • Can pick out their hosts from a distance

Identifying a bite

A brown horse in a field with lots of flies on its head
Horseflies may spread swamp fever to horses.

Compared with other insect bites, horsefly bites are particularly painful and slow to heal. This is because of the way the flies bite.

The bite from a horsefly:

  • Is a cut type of wound, rather than a small puncture hole
  • The mouthparts of the horsefly use a scissor-like action to create a wound in our skin
  • The fly mops up the bleed after cutting through the skin
  • The fly anchors to the skin while drinking the blood, with the help of small hooks along its mouthparts

The bite in the skin itself is:

  • Painful and may be itchy
  • Usually red and surrounded by a raised area of skin, called a weal or hive

The pain, redness, and weal help to tell horsefly bites from other bites.

Healing tends to take longer than with other types of insect bite. The wounds are also more prone to getting infected.

If a bite wound gets infected, people should bathe it with clean, warm water using a clean cloth or cotton wool. It is also good to speak to a healthcare professional. A doctor may prescribe antibiotic treatment.

Horseflies are not usually very harmful to humans. They do not transmit any disease through us, although they do spread swamp fever in horses. This disease can be fatal for the mammals.

Symptoms of serious horsefly reactions

Serious allergy to a horsefly bite is not common, but it may be signaled by extra symptoms:

  • Feeling dizzy and weak
  • Wheezing
  • Temporarily swollen skin, for example around the eye and lips

More severe allergy is rare but is an emergency. People should call for an ambulance for any signs of anaphylaxis, which include:

  • Swelling, itching, or a rash
  • Face, lips, hands, and feet are most likely to swell
  • Tongue and throat swelling are dangerous symptoms
  • Nausea, vomiting, or diarrhea
  • Feeling very unwell
  • Difficulty swallowing or breathing

People who show a severe allergy to horsefly have always been bitten previously. Their immune system then adapts for a future response but is oversensitive to the further biting.

People who suffer severe allergies may need to carry an emergency epinephrine injection for the treatment of future bite reactions.

Treatment

An older man who looks to be in discomfort scratches his neck with his left hand.
Bites on humans should not be scratched but treated with warm water.

The advice for dealing with a horsefly bite is similar to advice for other insect bites. There is no special treatment for horsefly bites, except to be wary of infection. Horsefly bites are more likely to become infected than other insect bites because of the way that horseflies damage the skin.

People should watch for the skin becoming redder, or pus or other discharge oozing from the bite wound. Infection can also be signaled by increased pain and swelling.

If a horsefly bite does become infected, people should take the advice of a doctor. They may prescribe antibiotics.

For any horsefly bite, home care includes:

  • Not scratching the wound as this is likely to make it worse and risk infection
  • Cleaning the bitten skin with plain warm water using a clean cloth or cotton wool
  • Easing the pain and reducing swelling with a cold compress or ice pack held over the bite for 10 minutes
  • Not using any other remedies aside from plain water

Vinegar or bicarbonate of soda are unlikely to help. The horsefly will also not leave behind any mouthpart or stinger when they bite people.

When to call a doctor

Medical attention is not needed unless the wound gets infected. In the rare cases of a severe allergy soon after a bite, people should call for an ambulance.

How to prevent a horsefly bite

Horseflies are difficult to avoid in the summer because their habitats are widespread. There are some practical steps that can be taken to reduce the risk of bites, however:

  • Keeping skin covered with long-sleeved clothing that is light in color
  • Avoiding walking in long grass, and wearing shoes and long pants
  • Avoiding perfumed cosmetic products these may attract flies
  • Staying away from water in summer, where horseflies breed
  • Using insect repellent the most effective ones have 50 percent DEET diethyltoluamide