Most spiders don't bite humans and only do so if they're provoked. When spiders bite, the vast majority of bites only cause a mild reaction in people, such as minor swelling, inflammation, or itching.
However, there are a few poisonous spiders in North America that can be dangerous. They are mostly found in the southern regions of the United States, but black widow spiders can also be found in southern regions of Canada. The venom that these spiders use to kill their prey is also harmful to humans. Being bitten by a venomous spider may cause severe local and systemic reactions and can sometimes be fatal.
Many spiders bite, but very few are a risk to humans. The following North American spiders are poisonous:
- black widow spiders (found in the US and southern Canada)
- brown recluse spiders (found in southern US)
- tarantulas (found in southern US)
The venom of black widow spiders is a neurotoxin (poisonous to the nervous system). Bites from adult females are the most toxic. The adult female is jet black with two reddish markings (often in the shape of an hourglass) on the underside of her body. These spiders are not very big – their body size is about 1 cm and their leg span about 5 cm. Bites occur mostly in the summer and early autumn, usually if the spider is provoked or if her web is disturbed. These spiders build their webs under logs, plants, or rocks, or in dark places in garages and outhouses.
The venom of brown recluse spiders causes necrosis (damage and breakdown) of skin and tissue. Their venom kills cells around the site of the bite. These spiders are brown and have a dark violin pattern on their back. They are small – their body can be up to 1.5 cm and their leg span up to 4 cm. Brown recluse spiders usually only bite when provoked. They hide under rocks and logs and come out at night to hunt. They often enter homes and hide in dark, quiet places.
Tarantulas have hairs on their body and hind legs that can pierce human skin and inject a toxin. They do not usually bite unless threatened. Tarantulas are large, hairy spiders with a leg span up to 25 cm. Some are native to the US, but other species are imported as pets.
Symptoms and Complications
Most spider bites cause minor swelling, inflammation, and itching. However, bites from poisonous spiders can cause more severe reactions.
Black widow bites affect the nervous system. The bite from a black widow may go unnoticed or may feel like a pinprick. Two small, red spots show where the fangs pierced the skin. Some people will not suffer any ill effects. However, others may suffer severe pain and cramps that spread from the site of the bite to the large muscles of the arms, legs, and abdomen. This usually occurs in 30 to 60 minutes after the bite.
The toxin can also cause salivation, high blood pressure, constriction of blood vessels, fast heart rate, pain, numbness, tingling, headache, anxiety, sweating, nausea, and vomiting, among other symptoms. The venom can cause whole-body reactions and can even be fatal, especially in very young and old people.
Brown recluse spider bites affect the skin and surrounding tissue. The bite of a brown recluse spider is often painless, but it may sting. Within a few hours, it becomes painful and itchy. The bite area is first surrounded by a bluish-white ring. In most people, the bite will heal in 2 to 3 days. However, in others, the toxin will cause swelling, tissue death, and destruction of red blood cells. A scab will form over the site and may leave a scar. The venom may also affect the eyes, causing swelling or drooping of the eyelid and other effects.
The toxin can also cause other symptoms within 72 hours of the bite including fever, chills, nausea, vomiting, headache, and weakness. Bites can cause whole-body reactions and can even be fatal. Rare complications of brown recluse spider bites include kidney failure and hemolytic anemia, which is anemia due to the destruction of red blood cells.
Tarantula hairs contain a toxin that can cause a serious skin rash, an allergic reaction, and possibly even anaphylactic shock. A threatened tarantula spider will eject up to thousands of small hairs from its hind legs that can pierce human skin. This produces itchy bumps on the skin that can last for weeks. People should wear gloves or wash their hands after handling pet tarantulas because the hairs are easily transferred to the eyes and can cause very serious effects. Tarantulas rarely bite – the bites are usually harmless but occasionally cause pain and swelling.
Making the Diagnosis
If a spider bites you, try to catch it and take it with you to the doctor. The diagnosis and necessary treatment depend on the type of spider. A spider bite often has a white ring around it.
Treatment and Prevention
Treatment for spider bites varies according to the type of spider. For mild bites, pain medications, antibiotics, and antihistamines may be given. Antivenin* is available for black widow and brown recluse spider bites but should be given as soon as possible after the bite.
Black widow spider bites: Clean the bite and apply an ice pack to it. Get a tetanus shot. Black widow bites can cause severe muscle cramps that may require intravenous medications to relax muscles. High blood pressure may also require medication. You should seek medical attention immediately.
Brown recluse spider bites: If you are bitten by a brown recluse spider, you should clean the wound, apply a sterile dressing and cold compress, and elevate the site. Get a tetanus shot. You may need hospitalization if complications occur. For example, hemolysis (the breakdown of red blood cells) may require kidney dialysis, and bites that become large areas of dying tissue may need surgery and skin grafting.
Tarantula hairs or bites: The bite should be cleaned and elevated. A tetanus shot is recommended. If your skin is exposed to the hairs, treatment with antihistamines and glucocorticoids can be helpful.
To prevent spider bites, be careful when working outside around woodpiles or in dark places in barns and garages, and use gloves for protection. Shake out unworn shoes before wearing and be careful in basements and crawlspaces. Many people get bitten while using an outhouse. If you come across a spider, don't disturb it. Few spiders are aggressive, and most will only bite when disturbed or provoked.
*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.