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Almost all of the 30,000 species of spiders are venomous. However, the fangs of most species are too short or too fragile to penetrate the skin. Serious systemic reactions most frequently occur with bites from brown spiders (eg, violin, fiddleback, brown recluse—Loxosceles sp) and widow spiders (black widow—Latrodectus sp). Brown spiders are present in the Midwest and south central US, not in the coastal and Canadian border states, except when imported through clothing or luggage. Widow spiders are present throughout the US. Several venomous species (eg, Pamphobeteus, Cupiennius, Phoneutria) are not native to the US but may be imported on produce or other materials or through commercial trade in spiders as novelty pets. Spider bites cause < 3 deaths/yr in the US, usually in children.
Only a few spider venoms have been studied in detail. Of greatest significance are those having necrotizing venom components (in brown and some house spiders) and neurotoxic venom components (in widow spiders). The most toxic component of widow spider venom seems to be a peptide that affects neuromuscular transmission. The specific fraction of brown spider venom that causes the characteristic necrotic lesion has not been isolated.
Symptoms and Signs
Brown spider bites are most common in the US. Some bites are painless initially, but pain, which can be severe and involve the entire extremity, develops within 30 to 60 min in all cases. The bite area becomes erythematous and ecchymotic and may be pruritic. Generalized pruritus may also be present. A central bleb forms at the bite site, often surrounded by an irregular ecchymotic area (bull's eye lesion). The lesion may mimic pyoderma gangrenosum. The central bleb becomes larger, fills with blood, ruptures, and leaves an ulcer; a black eschar forms over the ulcer and eventually sloughs.
Most bites leave minimal residual scarring but some can leave a large tissue defect, which may involve muscle. Loxoscelism, a venom-induced systemic syndrome, may not be detected until 24 to 72 h after the bite and is uncommon. Systemic effects (eg, fever, chills, nausea and vomiting, arthralgias, myalgias, generalized rash, seizures, hypotension, disseminated intravascular coagulation, thrombocytopenia, hemolysis, renal failure) are responsible for all reported fatalities.
Widow spider bites usually cause an immediate, sharp, stinging sensation. Within 1 h after envenomation, there may be progression to persistent local pain, diaphoresis, erythema, and piloerection at the bite site. The pain may be described as dull and numbing and may be disproportionate to the clinical signs. Latrodectism, a systemic syndrome caused by neurotoxic venom components, manifests as restlessness, anxiety, sweating, headache, dizziness, nausea and vomiting, hypertension, salivation, weakness, diffuse erythematous rash, pruritus, ptosis, eyelid and extremity edema, respiratory distress, increased skin temperature over the affected area, and cramping pain and muscular rigidity in the abdomen, shoulders, chest, and back. Abdominal pain may be severe and mimic acute surgical abdomen, rabies, or tetanus. Latrodectism is very uncommon and most commonly develops in patients at age extremes and those with other medical conditions. Death is extremely rare. Symptoms lessen over 1 to 3 days, but residual spasms, paresthesias, agitation, and weakness may persist for weeks to months.
Tarantula bites are extremely rare and nonvenomous, but agitation of the spider may cause it to throw needle-like hairs. The hairs act as foreign bodies in skin or eyes and can trigger mast cell degranulation and an anaphylactoid reaction (eg, urticaria, angioedema, bronchospasm, hypotension) in sensitized people, usually pet owners who handle the spider daily.
Diagnosis
Spider bites are often falsely suspected by patients. Diagnosis is typically suspected based on history and physical signs, but confirmation is rare because it requires witnessed biting, identification of the spider (the spider is rarely recovered intact), and exclusion of other causes. In nonendemic areas, a brown spider bite should not be diagnosed without identifying the spider. Many patients incorrectly attribute much more common methicillin-resistant Staphylococcus aureus (MRSA) skin infections to brown recluse spiders bites. Such infections should be excluded, as should other conditions that mimic spider bites (see Table 3: Bites and Stings: Disorders That Mimic Spider Bites ). Severe cases of latrodectism should be distinguished from acute abdomen, rabies, or tetanus.
Spiders are identified by location and markings. Widow spiders live outdoors in protected spaces (eg, rock piles, firewood cords, hay bales, outhouses) and have a red or orange hourglass marking on the ventral abdomen. Brown spiders live indoors in protected spaces (eg, in clothing, behind furniture, under baseboards) and have a fiddle- or violin-like marking on the dorsal cephalothorax, ranging from the eyes to the abdomen. This marking may be difficult to recognize even in the intact spider.
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Table 3
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| Disorders That Mimic Spider Bites |
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Category
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Examples
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Insect bites
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Ant bites
Bedbug bites
Flea bites
Fly bites
Reduviid (eg, assassin, wheel, kissing) bug bites
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Other arachnid bites
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Mite bites
Tick bites
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Skin disorders
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Erythema chronicum migrans
Erythema nodosum
Leukocytoclastic vasculitis
Sporotrichosis
Toxic epidermal necrolysis
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Infections
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Chronic herpes simplex
Cutaneous anthrax
Disseminated gonococcal infection
Methicillin-resistant Staphylococcus aureus
Septic emboli in endocarditis or IV drug use
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Trauma
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Self-inflicted injuries
Subcutaneous drug injection
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Treatment
Treatment common to all spider bites includes wound cleaning, ice to reduce pain, extremity elevation, tetanus prophylaxis, and observation. Most local reactions respond to these measures alone.
For brown spider bites, limiting intervention to standard wound care and measures that minimize infection risk is usually most prudent:
No intervention has been proved to reduce morbidity or improve outcome after a brown spider bite. Commonly touted or poorly studied treatment options are controversial or potentially harmful. Dapsone (eg, 100 mg po once/day until inflammation subsides) is often considered for ulcers > 2 cm, but its benefit is unproved. Benefit is variable, and dose-related hemolysis almost always develops; agranulocytosis, aplastic anemia, and methemoglobinemia have been documented. Local injection of corticosteroids into necrotic lesions has no value.
Latrodectism is initially treated supportively. Myalgias and muscle spasms due to widow spider bites respond poorly to muscle relaxants and opioid analgesics. A 10% Ca gluconate bolus given slowly IV in increments of 2 to 3 mL as needed may relieve pain briefly but requires continuous cardiac monitoring. Patients < 16 yr or > 60 yr, those with hypertension, and those with symptoms of severe envenomation should be hospitalized. Equine-derived antivenom is available for patients with severe latrodectism. It may be considered early in the course if symptoms are severe but can be effective up to 36 h after the bite. Clinical response can be dramatic. The dose for children and adults is 1 vial (6000 units) IV in 10 to 50 mL of normal saline usually over 3 to 15 min. The manufacturer recommends skin testing before administering the antivenom; however, skin testing does not always predict adverse reactions such as acute anaphylaxis.
Last full review/revision February 2009 by Robert A. Barish, MD, MBA
Content last modified February 2009
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