The fear of spiders is common, but while most of the thousands of species worldwide are harmless, a few possess venom that poses a medical risk to humans. Bites from the majority of spiders result in only minor, localized irritation, similar to a mosquito bite or bee sting. In North America, two primary groups of spiders are recognized as medically significant because their venom can produce severe reactions requiring professional attention. Understanding these species and their effects is crucial for knowing how to respond safely.
Venomous Versus Poisonous
The common question of whether a spider is “poisonous” involves a technical distinction in biology. Spiders are technically classified as venomous organisms, not poisonous. Venom is a toxin that is actively injected into a victim, typically through a bite or sting, using a specialized delivery system. Spiders use their fangs to pierce the skin and inject their toxin. Poison, in contrast, is a toxin that must be passively absorbed, inhaled, or ingested to cause harm, such as consuming a poisonous mushroom. Because spider toxins are delivered via injection, the correct term for their defense mechanism is envenomation. Nearly all spiders possess venom, but only a small number have venom potent enough to affect humans.
Recognizing High-Risk Species
Only two genera of spiders in North America are of medical concern: the widow spiders (Latrodectus) and the recluse spiders (Loxosceles). Identifying these spiders is crucial, though a bite should not be diagnosed solely on appearance.
The widow spiders, including the Southern, Northern, and Western Black Widows, are known for their glossy black bodies and bulbous abdomens. Mature females are about one-half inch long, and their most recognizable feature is the bright red or orange hourglass marking found on the underside of the abdomen. This hourglass shape can sometimes be incomplete or appear as two separate spots. Widow spiders are found across the United States, typically preferring dark, undisturbed areas like woodpiles, sheds, and crawl spaces. Male widow spiders are smaller and rarely bite, possessing less potent venom, so they are generally not medically important.
The second group, the recluse spiders, is represented most famously by the Brown Recluse (Loxosceles reclusa). These spiders are light to dark brown and are characterized by a distinct violin or fiddle-shaped marking on the top of their cephalothorax. Unlike most spiders that have eight eyes, recluse spiders have only six eyes arranged in three pairs. The Brown Recluse is primarily native to the South Central Midwest of the United States, including states like Missouri, Kansas, Oklahoma, and Texas. Recluse spiders are shy and non-aggressive, hiding in dark, quiet places. Bites usually occur when a person accidentally presses the spider against their skin, such as when putting on clothing or shoes.
Effects of Spider Venom
The venom of medically significant spiders is broadly categorized into two types, each causing a distinct set of symptoms.
Widow spider venom is neurotoxic, meaning it attacks the nervous system. The resulting syndrome, called Latrodectism, is characterized by the massive, uncontrolled release of neurotransmitters. Symptoms typically begin within 20 minutes to an hour of the bite, starting with intense, localized pain that spreads to the abdomen, chest, or back. This systemic reaction causes severe muscle cramping, rigidity, and spasms, sometimes mimicking appendicitis. Other common symptoms include sweating, headache, nausea, and elevated blood pressure. While severe, the symptoms of a Black Widow bite are rarely fatal to healthy adults, often resolving over three to seven days.
Recluse spider venom is cytotoxic, meaning it causes localized tissue destruction. The resulting condition, called Loxoscelism, is caused by enzymes like Sphingomyelinase D, which breaks down cell membranes. Most Brown Recluse bites result in only a minor, localized reaction that heals quickly. In severe cases, the bite site may develop a characteristic “red, white, and blue” lesion. The red ring is inflammation, the white ring is restricted blood flow (ischemia), and the blue or purple center indicates developing necrosis. This tissue death can lead to a sunken, ulcerated wound that may take weeks or months to heal and cause scarring. In rare instances, systemic Loxoscelism can occur, especially in children, leading to complications such as fever, chills, and hemolytic anemia (destruction of red blood cells).
Immediate Care and Medical Intervention
If a spider bite is suspected, immediate care should focus on cleaning the wound and reducing local symptoms. Wash the bite area thoroughly with mild soap and water to prevent secondary bacterial infection. Applying a cold compress or ice pack for about 15 minutes every hour can help reduce pain and swelling. If the bite is on an extremity, keeping the area elevated can also help minimize swelling.
A physician should be consulted promptly if the bite is suspected to be from a widow or recluse spider, or if any severe symptoms develop. Emergency medical attention is necessary if a person experiences systemic symptoms, such as severe, spreading pain, intense muscle cramping, or difficulty breathing.
Medical treatment for Latrodectism often includes pain medication and muscle relaxants to manage cramping. An antivenom for Black Widow bites is available and effective, typically used for severe cases, especially in children or those with underlying health conditions. Treatment for Loxoscelism is mainly supportive, focusing on meticulous wound care to manage the necrotic ulcer and prevent secondary infection. Antibiotics may be prescribed if a bacterial infection develops.