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Spider bites are uncommon, but they can be painful and sometimes dangerous. However, very few species in the United States can harm humans.

A spider will only bite in self-defense, for example, when they feel trapped or under threat. This could happen if a person puts their hand in a box where a spider is living or puts on a jacket that has a spider hiding inside.

Most spiders use venom to kill their prey. In this sense, most spiders are venomous.

However, in nearly all spiders, their venom is too weak to have a significant effect on a human. Also, the fangs of many species cannot pierce human skin. If a spider does bite, it will usually cause no more harm to a person than a moderate insect bite.

However, some species produce venom that is powerful enough to harm a person. The Centers for Disease Control and Prevention (CDC) list two types of spider as venomous in the U.S. — black widow spiders and brown recluse spiders.

This article will look at how to identify a spider bite, which species in the U.S. and nearby are dangerous, what to do if a venomous spider bites, and how to prevent bites.

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Spider bites are often hard to diagnose.

Spider bites do not produce many distinctive features by which people can identify them. They can look similar to many other bug bites. According to a 2011 study, they can also resemble a number of bacterial and other infections.

For this reason, a spider bite can be hard to diagnose, unless the person takes the spider that bit them to the doctor.

General symptoms of a spider bite may be:

  • swelling around the bite
  • itching or a rash
  • pain radiating from the bite
  • muscle pain or cramping
  • skin blisters that turn reddish purple
  • headache
  • nausea and vomiting
  • fever, chills, and sweating
  • difficulty breathing
  • increased blood pressure
  • anxiety or restlessness

People who experience these symptoms should see a doctor as soon as possible.

In time, a bite from a brown recluse spider can result in ulceration and tissue death. In rare cases, some types of spider bite can be life threatening.

If someone knows or believes a venomous spider has bitten them, they should see a doctor immediately if they experience any of the following:

  • severe pain
  • abdominal cramping
  • breathing problems

If someone is unsure if the spider that bit them is hazardous, they should contact their doctor to be sure. Proper diagnosis can lead to more effective treatment and shorter recovery time.

If possible, catch the spider or take a photo of it for identification.

Anaphylaxis

If a person has an allergy to spider venom, they may experience a severe reaction known as anaphylactic shock.

Symptoms of anaphylactic shock include:

  • severe itching, swelling, or redness
  • rash or hives
  • difficulty breathing or a wheezing cough
  • stomach cramps
  • rapid swelling of the tongue, lips, eyes, or throat
  • loss of consciousness

Anyone who starts to experience any of these symptoms needs immediate medical attention.

People who know they have an allergy may carry an auto injector with them. Anyone else who is nearby may need to help them use this device to deliver vital medication for their allergy.

The CDC consider two spiders to be dangerous in the U.S. These spiders are the brown recluse and the black widow.

Brown recluse

The brown recluse spider likes to hide in dry, dark corners, such as rock piles, woodpiles, and closets. It lives in the Midwest and southern States.

It is brown with a violin shaped pattern on its back.

A brown recluse can only bite under physical pressure, for example, if it is trapped against a person’s skin.

When they bite, the following may happen:

  1. The initial bite will be painless, but it will become increasingly painful over the next 2–8 hours.
  2. There may be two small puncture marks with swelling around them.
  3. A red ring may form around the pale center of the bite.
  4. A white blister usually forms, and a lesion or ulcer can develop. The ulcer will turn bluish violet, and the center will be hard and sunken. Some skin may slough.
  5. After this, the wound will usually heal, but this may take several weeks.
  6. Without treatment, tissue death (necrosis) may occur in some cases.

Other symptoms include:

  • headache
  • nausea
  • musculoskeletal pain
  • general feeling of being unwell

Children may have a more severe reaction throughout the body, including:

  • weakness
  • fever
  • joint pain
  • hemolytic anemia, where blood cells die faster than the body makes them
  • low platelet levels in the blood
  • organ failure
  • blood clots throughout the body
  • seizures

A bite from a brown recluse spider can be fatal.

No specific antidote is available to treat a brown recluse spider bite, but the lesion will need medical attention. Treatment options may include:

  • antibiotics to prevent an infection
  • surgery to remove dead tissue

The most severe effects of a brown recluse spider bite will not occur immediately. However, the sooner a person seeks help, the lower the chance that tissue damage will occur.

Learn more here about the brown recluse spider.

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Wearing pants when hiking may help prevent spider bites.

People who live in an area with potentially hazardous spiders should take precautions to avoid bites.

Example of some precautions to take are:

  • Always zip up tents when camping and wear pants when hiking in areas where spiders are common.
  • Turn shoes upside down and tap them on the ground before putting them on.
  • Always check before reaching into boxes or in areas that are not regularly disturbed.
  • Keep basements and yards free of clutter.
  • Wear gloves when gardening or getting wood from the woodpile.

Doctors often misdiagnose a spider bite. This may be because the person says it was a spider, but they do not know which type. Sometimes another condition can resemble a spider bite.

Other causes include:

Most spiders will not bite a person, and very few are dangerous.

Some spiders like to rest in dark or sheltered places, such as shoes. If a person puts their foot or hand in a place where a spider is living, some spiders can bite.

People who live or spend time in areas where there are brown recluse or black widow spiders should know how to recognize them.

They should also check items such as boxes and boots for spiders if they have not used them for a while. Other forms of protection include wearing gloves when gardening or working around a wood or rock pile.

Anyone who has concerns about a bite or other skin symptoms should see a doctor for a diagnosis.

A range of products is available for purchase over the counter or online, including antihistamines, ibuprofen, and antibiotic ointment.

Q:

I have seen some advice to capture the spider so a doctor can identify the bite. Is this a good idea?

A:

Yes, it will help the doctor determine if the spider is a venomous spider and what type, and this could affect the treatment plan. Catch the spider in a glass jar or take a picture of it.

Deborah Weatherspoon, PhD, RN, CRNA Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.