Almost all spiders are poisonous. However, the fangs of most species are too short or too fragile to penetrate human skin. Although at least 60 species in the United States have been implicated in biting people, serious injury occurs mainly from only two types of spiders:
Brown spiders are present in the Midwest and South Central United States, not in the coastal and Canadian border states, except when imported on clothing or luggage. Widow spiders are present throughout the United States. Although some people consider tarantulas dangerous, their bites do not seriously harm people. Spider bites cause fewer than three deaths a year in the United States, usually in children.
The bite of a widow spider usually causes a sharp pain, somewhat like a pinprick, followed by a dull, sometimes numbing, pain in the area around the bite. Cramping pain and muscle stiffness, which may be severe, develop in the abdomen or the shoulders, back, and chest. Other symptoms may include nausea, vomiting, sweating, restlessness, anxiety, headache, drooping and swelling of the eyelids, rash and itching, severe breathing problems, increased saliva production, and weakness.
The bite of a brown spider may cause little or no immediate pain, but some pain develops in the area around the bite within about an hour. Pain may be severe and may affect the entire injured area, which may become red and bruised and may itch. The rest of the body may itch as well. A blister forms, surrounded by a bruised area or by a more distinct red area that resembles a bull's-eye. Then the blister enlarges, fills with blood, and ruptures, forming an open sore (ulcer) that may leave a large crater-like scar. Uncommonly, nausea and vomiting, aches, fatigue, chills, sweats, blood disorders, and kidney failure develop.
There is no way to identify a particular spider on the basis of its bite mark. Therefore, a specific diagnosis can be made only if the spider can be identified. Widow spiders are recognized by a red or orange hourglass-shaped marking on the abdomen. Brown spiders have a violin-shaped marking on their back. However, these identifying marks can be difficult to discern, and the spider is rarely retrieved intact, so the diagnosis is usually uncertain and based on symptoms. Many people mistake skin infections, some potentially serious (such as methicillin-resistant Staphylococcus aureus [MRSA] infections), or other disorders for spider bites.
First-aid measures for a spider bite include cleaning the wound, placing an ice cube on the bite to reduce pain, and, if the bite is on an extremity, elevating the wound site.
For a widow spider bite, muscle pain and spasms can be relieved with muscle relaxants and analgesics. If the pain and spasms are severe, calcium given by vein (intravenously) may be required. Hot baths may relieve mild pain. Antivenom is given for severe poisoning. Hospitalization is usually required for people younger than 16 or older than 60 and for people with high blood pressure, heart disease, or severe symptoms.
Most brown spider bites heal without complications. Sores should be cleaned daily with a povidone-iodine solution and soaked 3 times a day in sterile salt water (saline). Moderately to severely damaged wounds may require surgical procedures.
Last full review/revision February 2009 by Robert A. Barish, MD, MBA