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Approach to the patient with a suspected spider bite

Approach to the patient with a suspected spider bite

Richard S Vetter, MS
David L Swanson, MD
Section Editors
Daniel F Danzl, MD
Stephen J Traub, MD
Deputy Editor
James F Wiley, II, MD, MPH


Last literature review version 19.3: Fri Sep 30 00:00:00 GMT 2011 | This topic last updated: Tue Mar 16 00:00:00 GMT 2010 (More)

INTRODUCTION — Spider bites are rare medical events. Of the thousands of spider species that exist around the world, only a handful cause problems in humans [1] . There are a variety of more common disorders that can mimic a spider bite, some of which represent a far greater threat to the patient if not recognized and treated appropriately.

Thus, accurate diagnosis is the initial goal of the clinician evaluating a patient with a lesion that might represent a spider bite. Discerning among the various conditions in the differential diagnosis of a spider bite requires familiarity with these disorders, as well as a rudimentary understanding of the distribution and behavior of medically important spiders.

The spiders of medical importance, the clinical manifestations, and diagnosis and differential diagnosis of spider bites will be reviewed here. The treatment of spider bites is discussed separately. (See "Bites of recluse spiders" and "Bites of widow spiders".)

MEDICALLY IMPORTANT SPIDERS — Spiders are arachnids (a group of arthropods), which have four pairs of legs, similar to scorpions, mites, and ticks (figure 1). They use sharp fangs at the end of their chelicerae to bite prey (typically insects, other arthropods, or small vertebrates) and inject paralyzing venom.

Most spiders pose no threat to humans. The venom of most spiders has little or no effect on mammalian tissues [2] . In addition, only a few species have cheliceral muscles powerful enough to penetrate human skin, and most of these spiders bite humans only in rare and extreme circumstances (eg, as they are being fatally crushed between skin and some object).

The spiders most likely to inflict medically significant bites in humans include widow and false black widow spiders, recluse spiders, Australian funnel web spiders, and Phoneutria spiders. Each of these spiders are described briefly below, and their appearance and geographical distribution are summarized in the table (table 1).

Widow spiders — Latrodectus, or widow spiders (found worldwide), include the Southern black widow (picture 1) and Western black widow in the United States, and the Australian redback spider. Although less toxic than the black widows, the brown widow (picture 2) is causing increased interest because of its novelty as a non-native spider quickly spreading throughout the Southern US. Widow bites cause unremarkable local lesions that are sometimes accompanied by a characteristic systemic reaction with prominent, proximally-spreading pain and localized diaphoresis surrounding the site of the bite. Antivenoms are available for several species. (See "Bites of widow spiders".)

False black widow spiders — Steatoda, or false black widow spiders (found worldwide) are less often implicated in human bites, and cause less severe symptoms that those of widows (picture 3).

Recluse spiders — Loxosceles, or recluse spiders are found predominantly in North and South America (picture 4). Their bites are notorious for becoming necrotic, although this happens in a minority of cases. Systemic reactions to bites are usually mild, and consist of non-specific systemic signs and symptoms. (See "Bites of recluse spiders".)

Australian funnel web spiders — Australian funnel web spiders are found in limited areas of eastern coastal Australia. Their bites can cause dramatic systemic reactions that mimic organophosphate poisoning and include salivation, diaphoresis, muscle spasms, tachycardia, hypertension, and pulmonary edema. An antivenom is available.

South American Phoneutria — Phoneutria or armed spiders are large spiders found in South America, especially urban areas of Brazil. The bites of these spiders can lead to severe systemic reactions, with occasional fatalities in children. An antivenom is available.

TYPES OF REACTIONS — A spider bite usually presents acutely as a solitary papule, pustule, or wheal. Systemic symptoms can accompany some envenomations, particularly those of widow spiders, funnel web spiders, and less often, recluse spiders. Allergic reactions typically result from contact with spiders (rather than bites).

Local reactions — Photographs of verified spider bites are rare in the medical literature, although dramatic images of necrotic lesions attributed to spider bites are commonplace in both medical journals and on the Internet. In reality, the majority of spider bites result in unremarkable wheals, papules, or pustules (picture 5). Local redness with a tender nodule at the site of the bite appears within minutes. The lesions are similar to those induced by a bee sting. In some cases, the markings of the fangs (one or two small puncture marks) are visible. Some bites also itch or burn.

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Spider bites may or may not be painful, and some go unnoticed. Pain can develop gradually over the ensuing hours after a bite, and can range from a slight prickly sensation to severe pain. The variability among bites and patients limits the clinical utility of this information in implicating a specific type of spider.

Most local reactions to spider bites resolve spontaneously in approximately 7 to 10 days. They occasionally become secondarily infected with skin-derived bacteria.

Necrotizing local reactions — Recluse (Loxosceles) spiders inflict bites that may become necrotic, although this is an uncommon complication. Other types of spiders have been implicated in causing necrotic bites, but this is based largely upon circumstantial evidence. The management of necrotic recluse spider bites is discussed separately. (See "Bites of recluse spiders".)

Systemic reactions — Systemic symptoms are reported in a minority of patients, and occur when venom enters the circulation in sufficient amounts.

The bites of certain spiders are known for distinct and potentially severe systemic reactions, including bites of the widow, Australian funnel web, and Phoneutria spiders.

Allergic reactions — Allergic reactions to spiders are rare and have been reported mostly in response to contact with spiders [3-6] . In the United States, tarantulas are increasingly popular pets (picture 6). These nonaggressive spiders rarely bite. When threatened, they dislodge small (about 1 mm long) barbed hairs at the posterior of their abdomens and launch them at their attacker. These hairs, as well as airborne material from crushed tarantulas, may cause irritation or urticaria if they come in contact with skin, eyes, or mucous membranes [1,6] . In addition, airborne material from tarantulas can cause foreign body reactions in the eye [7] .

Contact with tarantulas has also induced rare anaphylactic reactions in sensitized individuals [8] . The acute management of anaphylaxis (from any cause) is reviewed separately. (See "Anaphylaxis: Rapid recognition and treatment".)

DIAGNOSIS — A presumptive diagnosis of a spider bite is most often based on the history and clinical presentation. However, the diagnosis of a spider bite can be considered definitive ONLY if both of the criteria below are fulfilled:

  • A spider was observed inflicting the bite.
  • The spider was recovered, collected, and properly identified by an expert entomologist.

If these criteria are not met, then other conditions such as vasculitis, infection, vascular problems, or other relevant disorders must be ruled out. (See 'Differential diagnosis' below.)

Unfortunately, the criteria above are rarely met, even in published medical reports. This has resulted in a body of literature and considerable media attention falsely attributing various lesions and symptoms to spider bites [1] . The extent of this problem was illustrated in a review of 600 cases of suspected spider bites, which found that 80 percent of presumed bites could be more reasonably attributed to other causes [9] . These other causes included bites of different arthropods such as ants, fleas, bedbugs, ticks, mites, mosquitoes, and biting flies, as well as erysipelas, cellulitis, ecthyma, vasculitis, pyoderma, ophthalmic zoster, urticaria, angioedema, and burns. (See 'Differential diagnosis' below.)

History — Most patients' reports of spider bites are unreliable. The bite history is often speculative and retrospective and a spider was never visualized, either inflicting the bite or even present [10] . Even when a bite is witnessed by the patient, the "spider" is commonly found to be some other arthropod [9] .

The diagnosis of a spider bite is thus highly suspect unless the patient actually observed a spider inflicting the bite and can retrieve it for identification. In the absence of this history and supporting evidence, another explanation should be sought.

The setting in which the patient sustained the alleged bite should be carefully reviewed to see if it is consistent with the known habitat and behavior of the toxic spiders that live in the area. People may worry about the possibility that a toxic spider was transported into a non-indigenous area on fruit or other produce. However, it is rare for spiders to survive intact through the many steps involved in produce transportation, and then end up in a situation in which they would bite. The risk may be more significant for people working in food transport and handling, but it is minimal in the general community. (See "Bites of recluse spiders" and "Bites of widow spiders".)

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Clinical clues that essentially EXCLUDE the diagnosis of spider bite include the following:

  • Multiple lesions or more than one lesion on widely-separated parts of the body suggest another etiology. Spider bites are typically single lesions.
  • Bites are generally not simultaneously sustained by multiple residents of the same household.

Spider bites capture the imagination. Reports exist of patients both feigning spider bites as part of drug seeking behavior [11]  and attempting suicide with genuine spider bites [12] .

Influence of geographic location — Each of the toxic spiders lives in specific parts of the world (table 1). Clinicians should know which spiders are indigenous to their area.

  • Widows and false black widows are found worldwide. (See "Bites of widow spiders".)
  • Recluse spiders are found predominantly in North and South America. Within the United States, they are limited to the mid-western and southern portions of the country (figure 2). (See "Bites of recluse spiders".)
  • Phoneutria spiders are limited to South America.
  • Australian funnel web spiders are limited to southeastern and coastal Australia.

Laboratory data — There are no commercially available laboratory tests for identifying the presence of spider venom. Thus, the diagnosis is made clinically.

DIFFERENTIAL DIAGNOSIS — A spider bite usually presents as a local lesion (possibly with necrosis in the case of recluse bites) with or without systemic symptoms. (See 'Types of reactions' above.) The differential diagnoses for local and systemic symptoms are reviewed in this section. Disorders that can mimic the bites of specific spiders are discussed in the appropriate topic reviews. (See "Bites of widow spiders" and "Bites of recluse spiders".)

As discussed previously, many other conditions are more common than spider bites and pose a more immediate threat to the patient's health if not accurately diagnosed. A patient who did not clearly witness a spider inflicting the bite should be presumed to have some other disorder, and the presence of multiple lesions essentially excludes the diagnosis of spider bite. (See 'History' above.)

The clinician can usually determine whether a spider bite is possible based upon a careful history of the patient's recent activities, details of the onset and evolution of the lesion, and knowledge of biting spiders found in the area. Despite this, it is not uncommon for patients to present with nondescript lesions, suggest that it might be a spider bite because of some circumstantial detail, and have that history accepted without further questioning.

A spider bite may present as a papule, pustule, wheal, plaque (possibly ecchymotic), or ulcer. The most common disorders that are mistaken for local reactions to spider bites include infections and the bites of other insects.

Infections — Papules and pustules should be carefully unroofed and cultured to identify infectious causes. Common infections that could be mistaken for spider bites include staphylococcus and streptococcal infections, the skin lesion of early Lyme disease, and atypical presentations of herpes zoster or herpes simplex.

  • Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) skin infections can begin with singular papules or pustules that may evolve to necrotic lesions [13] . CA-MRSA is far more prevalent than spider bites. Infections occur both sporadically and as institutional epidemics in nursing homes, prisons, military barracks, and athletic facilities. Risk factors and epidemiology of CA-MRSA are discussed separately. (See "Epidemiology of methicillin-resistant Staphylococcus aureus infection in adults" and "Epidemiology and clinical spectrum of methicillin-resistant Staphylococcus aureus infections in children".)
  • Erythema migrans, the target-like skin lesion of early Lyme disease, may be mistaken for a spider bite (picture 7). Southern tick-associated rash illness (STARI) is a similar infection with similar skin findings, which occurs in the southern United States (below Maryland). (See "Diagnosis of Lyme disease" and "Southern tick-associated rash illness (STARI)".)
  • Herpes zoster and herpes simplex infections (especially herpetic whitlow) may occasionally present with singular lesions (picture 8). Acute onset is associated with vesicles, vesicopustules, severe edema, erythema, or pain. Tzanck staining of vesicles will demonstrate multinucleated giant cells and viral culture will grow HSV. (See "Epidemiology and pathogenesis of varicella-zoster virus infection: Herpes zoster" and "Paronychia and ingrown toenails".)

Other bites and stings — A wide variety of insects sting or bite humans, including triatomid bugs, ants, fleas, bedbugs, blister beetles, ticks, mites, mosquitoes, and biting flies (table 2) [9] . Spiders are less likely to do so than many others. With the exception of tick-borne illnesses and allergic reactions, the exact insect inflicting the bite is of little clinical importance and local care suffices.

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Scorpion stings are more common than spider envenomations worldwide, and most stings have been reported in Africa, the Middle East, southern Asia, and Central and South America. In the United States, scorpion stings are most common in Arizona and nearby areas of the Southwest. (See "Scorpion stings in the United States and Mexico".) Stings are instantaneously painful, and so patients usually capture or at least clearly witness the scorpion inflicting the sting [14,15] . Local pain is the most common presenting symptom. Systemic symptoms include hypertension, tachycardia, diaphoresis, and salivation [14] .

Other common dermatoses — Poison ivy, poison oak, and other plants in the Anacardiaceae family may occasionally cause dermatitis that presents as a single lesion, although linear lesions are more typical (picture 9). These lesions tend to be pruritic, rather than painful. (See "Poison ivy (Toxicodendron) dermatitis".)

SUMMARY AND RECOMMENDATIONS — Spider bites are uncommon medical events, since there are limited number of spiders worldwide with fangs strong enough to pierce human skin, and most spiders bite humans only as a final defense when being crushed between skin and another object. Thus, most lesions attributed to spider bites are caused by some other etiology. (See 'Introduction' above.)

  • The spiders that can cause medically significant bites include widow and false widow spiders (worldwide), recluse spiders (mostly North and South America), Australian funnel web spiders (eastern coastal Australia), and Phoneutria spiders (Brazil) (table 1). (See 'Medically important spiders' above.)
  • Acute spider bites most commonly result in a solitary papule, pustule, or wheal (picture 5). Systemic symptoms can accompany envenomations of widow, funnel web, and Phoneutria spiders, and less often, those of recluse spiders. The bites of recluse spiders can become necrotic, although most bites do not necrose. Allergic reactions to contact with spiders (rather than bites) occur most often in response to tarantulas. (See 'Types of reactions' above.)
  • Clinicians should know which of the biting spiders (if any) are found in the areas in which they practice, and have a basic understanding of the entomology of those species (table 1). (See 'Influence of geographic location' above.)
  • The working diagnosis of a spider bite is based upon suggestive history and clinical presentation. However, definitive identification of a spider bite requires all of the following: a spider was observed inflicting the bite, the spider was recovered, collected, and properly identified by an expert entomologist, and other disorders have been ruled out. (See 'Diagnosis' above.)
  • In the majority of cases, another etiology is responsible for the lesion, other than a spider bite. The differential diagnosis includes infections, bites and stings of other arthropods, and several other more common dermatoses. A culture should be performed in most cases. (See 'Differential diagnosis' above.)


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  7. Hered RW, Spaulding AG, Sanitato JJ, Wander AH. Ophthalmia nodosa caused by tarantula hairs. Ophthalmology 1988; 95:166.
  8. Wong RC, Hughes SE, Voorhees JJ. Spider bites. Arch Dermatol 1987; 123:98.
  9. Russell FE, Gertsch WJ. For those who treat spider or suspected spider bites. Toxicon 1983; 21:337.
  10. Vetter RS, Bush SP. Additional considerations in presumptive brown recluse spider bites and dapsone therapy. Am J Emerg Med 2004; 22:494.
  11. Spiller HA, Schultz OE. Envenomations as a novel drug-seeking method. Vet Hum Toxicol 2002; 44:297.
  12. Fisher DP. Letter: Attempted suicide by black widow spider bite. JAMA 1976; 235:2718.
  13. Dominguez TJ. It's not a spider bite, it's community-acquired methicillin-resistant Staphylococcus aureus. J Am Board Fam Pract 2004; 17:220.
  14. Al-Asmari AK, Al-Saif AA. Scorpion sting syndrome in a general hospital in Saudi Arabia. Saudi Med J 2004; 25:64.
  15. Isbister GK, Volschenk ES, Seymour JE. Scorpion stings in Australia: five definite stings and a review. Intern Med J 2004; 34:427.

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