Spiders are often misunderstood creatures, and a common point of confusion is the distinction between “poisonous” and “venomous.” An animal is considered poisonous if it releases toxins when ingested or touched, while a venomous animal injects its toxins, typically through a bite or sting. Spiders, therefore, are venomous, as they use their fangs to inject venom. While nearly all spiders possess venom, only a small number of species in North America produce venom that is medically significant to humans.
Understanding Venomous Spiders
Spider venom, a complex mixture of chemical substances, is primarily used by spiders to subdue their prey, injected through their fangs. Most spiders do not have fangs long enough or venom potent enough to cause serious harm to humans.
Venoms fall into two main categories: neurotoxic and cytotoxic. Neurotoxic venoms affect the nervous system, causing symptoms like muscle cramps, rigidity, and nerve impulse interference. Cytotoxic, or necrotic, venoms cause localized tissue damage, leading to inflammation, lesions, and blistering. Most spider bites are harmless or result in only minor local reactions.
Commonly Encountered Dangerous Spiders
In North America, the two most commonly encountered spiders with medically significant venom are the black widow and the brown recluse. These spiders are timid and bite only when threatened, often when accidentally pressed against skin.
Black widow spiders (genus Latrodectus) are recognizable by their shiny black bodies and a distinct red or orange hourglass-shaped marking on the underside of their abdomen. Adult females are about 0.5 inches long. They build irregular, messy webs in dark, undisturbed areas like woodpiles, sheds, and basements. A black widow bite can cause immediate pain, redness, and swelling. Systemic symptoms include severe muscle cramps, particularly in the abdomen, nausea, vomiting, sweating, and headaches, developing within minutes to hours.
Brown recluse spiders (genus Loxosceles) are identified by a dark, violin-shaped marking on their back, with the “neck” pointing towards the abdomen. They are light brown to grayish-brown and measure about 0.25 to 0.5 inches in length. Brown recluses prefer secluded, dark places such as attics, closets, and storage areas. Bites are often initially painless, with symptoms developing several hours later. A bite can lead to a blister surrounded by a bruise or reddish skin, resembling a bull’s-eye, which can rupture and form a slow-healing ulcer.
Responding to a Suspected Bite
A spider bite often appears as a red, inflamed, and sometimes itchy or painful bump, similar to other insect bites. If a spider bite is suspected, immediate first aid includes cleaning the wound with mild soap and water. Applying a cool compress or ice pack to the bite area for 15 minutes each hour can reduce pain and swelling. Elevating the affected area and taking over-the-counter pain relievers can also provide relief.
Seek medical attention if symptoms worsen or if the bite is from a black widow or brown recluse spider. Signs warranting medical evaluation include severe pain, abdominal cramping, spreading redness or red streaks from the bite site, or an open sore. Systemic symptoms like fever, chills, body aches, nausea, vomiting, or difficulty breathing also indicate a need for professional medical care. Children, the elderly, and individuals with underlying health conditions should seek medical attention promptly if bitten by a suspected venomous spider.