- Spiders are 8-legged (arthropod) members of the order Araneae.
- Spider bites result from a self-defense reaction when a spider feels threatened and typically arise due to inadvertent meeting with a spider (i.e., accidental).
- There are several medically important spiders designated so because of cytotoxic factors introduced by their bites; these can result in inflammation and necrosis of the skin. Some spider bites can result in systemic symptoms.
- Fatal spider bites are extremely rare; in the 20th century, there were fewer than 100 confirmed deaths due to spider bites.
- Spider bite is often a misdiagnosis (and is typically overdiagnosed). The most important consideration in the differential diagnosis of a spider bite is an infectious abscess.
Most spider bites commonly result in mild, local inflammation but can result in localized tissue necrosis in severe cases. Systemic symptoms and fatal reactions are rare.
Loxoscelism is the condition resulting from the bite of a recluse spider (genus Loxosceles). Toxins of Loxosceles spiders are largely cytotoxic (dermonecrotic). Local reactions are most common, but generalized reactions can occur. Ninety percent of bites are not severe, and special treatment is not required. Lesions developing a central bullae or pustule, marked erythema, pain, or ischemia may require more aggressive therapy. Inflammation typically spreads in a dependent manner, representing gravitational spread of the toxins through tissues. Some of these bites may progress to become large necrotic ulcers that may take weeks to months to heal and may have subcutaneous involvement. No treatment has been convincingly demonstrated to improve outcome. Systemic loxoscelism occurs within 24 to 96 hours of the spider bite and may result in fever, hemolysis, scarlatiniform eruption, and a sepsis-like clinical picture.
Examples of Loxosceles spiders include the brown recluse spider (Loxosceles reclusa). Many other spider bites involve cytotoxic factors or inflammatory mediators resulting in inflammation and necrosis of skin. They are summarized in Table 1.
Table 1: Spiders and their toxins
|Common name||Characteristic physical features of the spider||Toxins contained in the spider bite||Notes|
|Black widow spider (Lacrodectus mactans)||Red hourglass on ventral abdomen||Alpha-latrotoxin||May result in colicky abdominal pain mimicking acute abdomen|
|Brown recluse (Loxosceles reclusa)||Inverted violin-shaped mark on dorsal cephalothorax||Sphingomyelinase D, hyaluronidase, lipase, esterase, alkaline phosphatase, 5’-nucleotide phosphohydrolase||Marked necrosis and gangrene|
|Wolf spider (Lycosidae)||Histamine|
|Hobo spider (such as Tegeneria agrestis)||Funnel web spider||Unknown||Herringbone pattern on abdomen
Bites may cause severe dermonecrotic reaction and may result in prolonged headaches
|Tarantula (Theraphosidae family)||Coarse hairs||No known toxin
Hairs are ejected from the spider as part of their defense reaction. This may result in a granulomatous contact urticaria or foreign body granuloma.
|If the hairs enter the eye, it causes ophthalmia nodosa (a form of chorioretinitis).|
|Sac spider (Chiracanthium)||Lipase||Spider forms a spin sac instead of a web|
|Green lynx spider (Peucetia viridans)||Bright green with red macules||Unknown|
|Jumping spider (Phidippus)||Hyaluronidase||Most common biting spider in US|
Brown recluse spiders are limited to certain geographic regions; in the USA, they are found in the Midwest, South, and Southeast (specifically: southeastern Nebraska, Kansas, Oklahoma, Texas, Louisiana, Arkansas, Missouri, Kentucky, Tennessee Mississippi, Alabama, northern Georgia, and southern portions of Ohio, Indiana, Illinois, and Iowa). Brown recluse spiders are rarely, if ever, found outside of this region. The diagnosis of brown recluse spider bite should not be made outside the endemic area unless the spider has been identified by an expert.
Other spiders can produce bites with severe systemic reactions. Bites of the Lactrodectus mactans, or the black widow spider, cause minimal local cutaneous reaction, but its venom contains components that are neurotoxic. Muscle spasm, including rigors of the abdominal muscles, suggesting an acute abdominal emergency, may occur.