Of the more than 30,000 recognized species of spiders, only ~100 defend themselves aggressively and have fangs sufficiently long to penetrate human skin. The venom that some spiders use to immobilize and digest their prey can cause necrosis of skin and systemic toxicity. Whereas the bites of most spiders are painful but not harmful, envenomations by recluse or fiddleback spiders (Loxosceles species) and widow spiders (Latrodectus species) may be life-threatening. Identification of the offending spider is important because specific treatments exist for bites of widow spiders and because injuries attributed to spiders are frequently due to other causes. Except in cases where the patient actually observes a spider immediately associated with the bite or fleeing from the site, lesions reported as spider-bite reactions are most often due to other injuries or to infections with bacteria such as methicillin-resistant Staphylococcus aureus (MRSA).
Recluse Spider Bites and Necrotic Arachnidism
Brown recluse spiders live mainly in the south-central United States and have close relatives in Central and South America, Africa, and the Middle East. Bites by brown recluse spiders usually cause only minor injuries, with edema and erythema. Envenomation, however, occasionally causes severe necrosis of skin and subcutaneous tissue and more rarely causes systemic hemolysis. These spiders are not aggressive toward humans and will bite only if threatened or pressed against the skin. They hide under rocks and logs or in caves and animal burrows. They invade homes and seek dark and undisturbed hiding spots in closets, in folds of clothing, or under furniture and rubbish in storage rooms, garages, and attics. Despite their impressive abundance in some homes, these spiders rarely bite humans. Bites tend to occur while the victim is dressing and are sustained primarily on the hands, arms, neck, and lower abdomen.
Initially, the bite is painless or may produce a stinging sensation. Within the next few hours, the site becomes painful and pruritic, with central induration surrounded by a pale ischemic zone that itself is encircled by a zone of erythema. In most cases, the lesion resolves without treatment in just a few days. In severe cases, the erythema spreads, and the center of the lesion becomes hemorrhagic or necrotic with an overlying bulla. A black eschar forms and sloughs several weeks later, leaving an ulcer that eventually may create a depressed scar. Healing usually takes place in ≤6 months but may take as long as 3 years if adipose tissue is involved. Local complications include injury to nerves and secondary bacterial infection. Fever, chills, weakness, headache, nausea, vomiting, myalgia, arthralgia, maculopapular rash, and leukocytosis may develop ≤72 h after the bite. Reports of deaths attributed to bites of North American brown recluse spiders have not been verified.
TREATMENT Recluse Spider Bites
Initial management includes RICE (rest, ice, compression, elevation). Analgesics, antihistamines, antibiotics, and tetanus prophylaxis should be administered if indicated. Early debridement or surgical excision of the wound without closure delays healing. Routine use of antibiotics or dapsone is unnecessary. Patients should be monitored closely for signs of hemolysis, renal failure, and other systemic complications.
The black widow spider, common in the southeastern United States, measures ≤1 cm in body length and 5 cm in leg span and is shiny black with a red hourglass marking on the ventral abdomen. Other dangerous Latrodectus species occur elsewhere in temperate and subtropical parts of the world. The bites of the female widow spiders are notorious for their potent neurotoxins.
Widow spiders spin their webs under stones, logs, plants, or rock piles and in dark spaces in barns, garages, and outhouses. Bites are most common in the summer and early autumn and occur when a web is disturbed or a spider is trapped or provoked. The initial bite is perceived as a sharp pinprick or may go unnoticed. Fang-puncture marks are uncommon. The venom that is injected does not produce local necrosis, and some persons experience no other symptoms. α-Latrotoxin, the most active component of the venom, binds irreversibly to presynaptic nerve terminals and causes release and eventual depletion of acetylcholine, norepinephrine, and other neurotransmitters from those terminals. Painful cramps may spread within 60 min from the bite site to large muscles of the extremities and trunk. Extreme rigidity of the abdominal muscles and excruciating pain may suggest peritonitis, but the abdomen is not tender on palpation and surgery is not warranted. The pain begins to subside during the first 12 h but may recur during several days or weeks before resolving spontaneously. A wide range of other neurologic sequelae may include salivation, diaphoresis, vomiting, hypertension, tachycardia, labored breathing, anxiety, headache, weakness, fasciculations, paresthesia, hyperreflexia, urinary retention, uterine contractions, and premature labor. Rhabdomyolysis and renal failure have been reported, and respiratory arrest, cerebral hemorrhage, or cardiac failure may end fatally, especially in very young, elderly, or debilitated persons.