Many thousands of spider species are found worldwide, and nearly all possess venom glands connected to fangs in the paired jaw-like structures known as chelicerae. Fortunately, only a very few spider species have fangs long and tough enough to pierce human skin. In the United States, these spiders include Latrodectus (widow spider) and Loxosceles (brown spider) species, tarantulas (a common name given to several large spider species), and a few others.

Patient complaints of “spider bites” occur much more commonly than do actual spider bites. Unexplained skin lesions, especially those with a necrotic component, are often ascribed to spiders, especially the brown recluse spider. Health care providers should consider alternative etiologies in the absence of a convincing clinical history and presentation. Many alleged “spider bites” are actually infections, with community-acquired methicillin-resistant Staphylococcus aureus (MRSA) being a common etiology.

Latrodectus species (black widow spiders) are ubiquitous in the continental United States, and the female can cause serious envenomations with rare fatalities. Black widows construct their chaotic webs in dark places, often near human habitation in garages, wood piles, outdoor toilets, and patio furniture. The spider has a body size of ~10-15 mm and is characteristically shiny black with a red to orange-red hourglass shape on the ventral abdomen. The brown widow spider (L. geometricus) has recently been introduced into southern California and has spread along the Gulf of Mexico coast from Florida to Texas. This spider has variegated tan, brown, and black markings, also with a reddish hourglass on the abdomen, and envenomations result in the same clinical effects as from black widows.

Loxosceles reclusa (the brown recluse spider) is found only in the central and southeastern United States (e.g., Missouri, Kansas, Arkansas, Tennessee). Rare individual specimens have been found in other areas, but they represent stowaways on shipments from endemic areas. Other Loxosceles species may be found in the desert southwest, although they tend to cause less serious envenomations. The spider's nocturnal hunting habits and reclusive temperament result in infrequent contact with humans, and bites are generally defensive in nature. The spider is 6-20 mm in length and light to dark brown in color, with a characteristic violin- or fiddle-shaped marking on the dorsum of the cephalothorax.

Tarantulas rarely cause significant envenomation but can produce a painful bite because of their large size. Tarantulas also bear urticating hairs that they can flick at predators and that cause intense mucosal irritation. People who keep pet tarantulas have developed ocular inflammation (ophthalmia nodosa) when these hairs embed in their corneas, usually while they are cleaning their spiders' cages.

Spiders use their hollow fangs (chelicerae) to inject their venoms, which contain various protein and polypeptide toxins that appear to be designed to induce rapid paralysis of their insect victims and aid in digestion.

1. Latrodectus (widow) spider venom contains alpha-latrotoxin, which causes opening of nonspecific cation channels, leading to an increased influx of calcium and indiscriminate release of acetylcholine (at the motor endplate) and norepinephrine.
2. Loxosceles (brown spider) venom contains a variety of digestive enzymes and sphingomyelinase D, which is cytotoxic and chemotactically attracts white blood cells to the bite site and also has a role in producing systemic symptoms such as hemolysis.

Spider venoms are generally extremely potent toxins (far more potent than most snake venoms), but the delivered dose is extremely small. The size of the victim may be an important variable.

Manifestations of envenomation are quite different depending on the spider genus.

1. Latrodectus (widow spider) bites may produce local signs ranging from mild erythema to a target lesion a few centimeters in size, with a central puncture site, inner blanching, and an outer erythematous ring.
   1. The bite is often initially felt as an acute stinging, but may go unnoticed. The site almost always becomes painful within 30-120 minutes. By 3-4 hours, painful cramping and muscle fasciculations may occur in an involved extremity. This cramping progresses centripetally toward the chest, back, or abdomen and can produce board-like rigidity, weakness, dyspnea, headache, and paresthesias. Widow spider envenomation may mimic myocardial infarction or an acute surgical abdomen. Symptoms can wax and wane and often persist for 12-72 hours.
   2. Additional common symptoms may include hypertension, regional diaphoresis, restlessness, nausea, vomiting, and tachycardia.
   3. Other, less common symptoms include leukocytosis, fever, delirium, and arrhythmias. Rarely, hypertensive crisis or respiratory arrest may occur after severe envenomation, mainly in very young or very old victims.
2. Loxosceles bites are best known for causing slowly healing skin ulcers, a syndrome often called “cutaneous loxoscelism” or “necrotic arachnidism.”
   1. Envenomation usually produces a painful burning sensation at the bite site within 10 minutes but can be delayed. Over the next 1-12 hours, a “bull's eye” lesion forms, consisting of a blanched ring enclosed by a ring of ecchymosis. The entire lesion can range from 1 to 5 cm in diameter. Over the next 24-72 hours, an indolent necrotic ulcer develops that may take several weeks to heal. However, in most cases, necrosis is limited and healing occurs rapidly.
   2. Systemic illness may occur in the first 24-48 hours and does not necessarily correlate with the severity of the ulcer. Systemic manifestations include fever, chills, malaise, nausea, and myalgias. Rarely, intravascular hemolysis and disseminated intravascular coagulopathy may occur.
3. Other spiders. Bites from most other spider species are of minimal clinical consequence. Bites from a few species can cause mild-to-moderate systemic symptoms (myalgias, arthralgias, headache, nausea, vomiting). As with many arthropod bites, a self-limited local inflammatory reaction may occur, and any break in the skin may become secondarily infected. In addition to Loxosceles spiders, a few other species have been reported to cause necrotic skin ulcers (eg, Phidippus spp and Tegenaria agrestis), but these associations are questionable.

Most commonly is based on the characteristic clinical presentation. Bite marks of all spiders but the tarantulas are usually too small to be easily visualized, and victims may not recall feeling the bite or seeing the spider. Spiders (especially the brown recluse) have bad reputations that far exceed their actual danger to humans, and patients may ascribe a wide variety of skin lesions and other problems to spider bites. Many other arthropods and insects also produce small puncture wounds, pain, itching, redness, swelling, and even necrotic ulcers. Arthropods that seek blood meals from mammals are more likely to bite humans than are spiders. Several other medical conditions can cause necrotic skin ulcers, including bacterial, viral, and fungal infections and vascular, dermatologic, and even factitious disorders. Thus, any prospective diagnosis of “brown recluse spider bite” requires careful scrutiny. Unless the patient gives a reliable eyewitness history, brings the offending animal for identification (not just any spider found around the home), or exhibits systemic manifestations clearly demonstrating spider envenomation, the evidence is circumstantial at best.

1. Specific levels. Serum toxin detection is used experimentally but is not commercially available.
2. Other useful laboratory studies
   1. Latrodectus. Electrolytes, calcium, glucose, CPK, and ECG (in cases with chest pain).
   2. Loxosceles. CBC, BUN, and creatinine. If hemolysis is suspected, serum LDH, haptoglobin and urine dipstick for occult blood (positive with free hemoglobin) may be useful; repeat daily for 1-2 days.

1. Emergency and supportive measures
   1. General
      1. Cleanse the wound and apply cool compresses or intermittent ice packs. Treat infection if it occurs.
      2. Give tetanus prophylaxis if indicated.
   2. Latrodectus envenomation
      1. Monitor victims for at least 6-8 hours. Because symptoms typically wax and wane, patients may appear to benefit from any therapy offered.
      2. Maintain an open airway and assist ventilation if necessary (see airway) and treat severe hypertension if it occurs.
   3. Loxosceles envenomation
      1. Consider admitting patients with systemic symptoms, to monitor for hemolysis, renal failure, and other complications.
      2. The usual approach to wound care in cases of necrotic arachnidism is watchful waiting. The majority of these lesions will heal with minimal intervention over the course of a few weeks. Standard wound care measures are indicated, and secondary infections should be treated with antibiotics if they occur. Surgical debridement and skin grafting may be indicated for large and/or very slowly healing wounds; however, prophylactic early surgical excision of the bite site is not recommended.
2. Specific drugs and antidotes
   1. Latrodectus
      1. Most patients will benefit from opioid analgesics and often are admitted for 24-48 hours for pain control in serious cases.
      2. Muscle cramping has been treated with intravenous calcium (see calcium or skeletal muscle relaxants such as methocarbamol. However, these therapies are often ineffective when used alone.
      3. Antivenom (see Antivenom) is rapidly effective but infrequently used because symptomatic therapy is often adequate and because of the small risk of anaphylaxis. It is indicated for seriously ill, elderly, or pediatric patients who do not respond to conventional therapy for hypertension, muscle cramping, or respiratory distress and for pregnant victims threatening premature labor. Widow spider antivenom is more routinely used in some other countries, including Australia and Mexico. The perceived risk of anaphylaxis may be overestimated in the United States. A F(ab')₂ fragment antivenom, which may pose an even lower risk of anaphylaxis, has been tested in clinical trials but is not currently commercially available.
   2. Loxosceles. Therapy for necrotic arachnidism has been difficult to evaluate because of the inherent difficulty of accurate diagnosis.
      1. Dapsone has shown some promise in reducing the severity of necrotic ulcers in anecdotal case reports but has not been effective in controlled animal models.
      2. Steroids usually are not recommended.
      3. There is no commercially available antivenom in the United States.
      4. Hyperbaric oxygen has been proposed for significant necrotic ulcers, but results from animal studies are equivocal, and insufficient data exist to recommend its use.
3. Decontamination. These measures are not applicable. There is no proven value in early excision of Loxosceles bite sites to prevent necrotic ulcer formation.
4. Enhanced elimination. These procedures are not applicable.