Spiders are venomous, not poisonous. Poisonous implies a toxin released by swallowing or absorption, while venomous means they inject a toxin using specialized fangs (chelicerae). Although nearly all US spider species possess venom, only a small handful are medically significant to humans. The real threat comes from two primary groups whose bites can produce serious, though rarely fatal, effects.
Which Spiders Pose the Greatest Threat
The two groups responsible for almost all severe envenomations in the US are the black widow complex (Latrodectus spp.) and the brown recluse complex (Loxosceles spp.). The black widow is widely distributed and is recognized by the adult female’s shiny black body and the distinctive red or orange hourglass marking on the underside of her abdomen. These spiders build irregular, messy webs near the ground in secluded, dark areas like woodpiles, sheds, and crawl spaces. Their potent neurotoxic venom attacks the nervous system.
The brown recluse (Loxosceles reclusa) is primarily confined to the south-central Midwest, ranging from Nebraska to Ohio and south through Texas to Georgia. Identification is complicated because many harmless brown spiders are mistaken for the recluse. A true recluse possesses a dark, violin-shaped marking on the cephalothorax, with the “neck” pointing toward the abdomen. They are also unique for having six eyes arranged in three pairs rather than the usual eight. These shy, nocturnal spiders build a minimal web used only for shelter, preferring to hide in dark, undisturbed clutter like stored clothing or boxes.
Medical Consequences of Venom Exposure
The health risks differ significantly due to the chemical composition of the venoms. Black widow venom contains neurotoxins that act on the nervous system, causing a systemic reaction known as latrodectism. Symptoms usually begin within an hour and include immediate, sharp pain at the bite site. This is followed by severe muscle cramping and rigidity, often in the abdomen, chest, or back. Other effects can involve sweating, nausea, vomiting, headache, and elevated blood pressure.
The brown recluse delivers a cytotoxic venom that causes a localized reaction called loxoscelism, which destroys tissue. The bite is often painless initially, but the area may become red, swollen, and painful over the next 12 to 24 hours. In a small number of cases, the bite site develops a dark, sunken lesion that can progress into a necrotic ulcer as the tissue dies. Systemic loxoscelism, characterized by fever, chills, weakness, and joint pain, is rare but possible. While most recluse bites heal without intervention, some result in a slow-healing wound that may leave a scar.
Necessary Steps Following a Bite
If a spider bite is suspected, immediate action should focus on wound care and symptom management. The bite area should be thoroughly cleaned with mild soap and water to prevent secondary infection. Applying a cool compress or ice pack for 15 minutes each hour can help reduce local pain and swelling. Elevating the affected limb is also recommended, and over-the-counter pain relievers may be used for discomfort.
Seek medical attention immediately if you suspect a bite from a black widow or brown recluse, even if symptoms are mild. Prompt medical evaluation is necessary if systemic symptoms develop, such as severe cramping, difficulty breathing, or vomiting. Medical attention is also required if the wound shows signs of spreading infection, like increased redness, warmth, or a growing necrotic lesion. If safely possible, capturing the spider for positive identification assists medical professionals in determining appropriate treatment.