INTRODUCTION — The spiders of medical importance, an overview of the clinical manifestations of their bites, and diagnosis and differential diagnosis of spider bites will be reviewed here. The assessment and management of spider bites caused by recluse and widow spiders are discussed separately. (See "Bites of recluse spiders" and "Widow spider bites: Clinical manifestations and diagnosis", section on 'Clinical manifestations' and "Widow spider bites: Management".)
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 (mouth appendages) to bite prey (typically insects, other arthropods, or small vertebrates) and inject venom. The arachnology of recluse and widow spiders is discussed in detail separately. (See "Bites of recluse spiders", section on 'Entomology of recluse spiders' and "Widow spider bites: Clinical manifestations and diagnosis", section on 'Arachnology'.)
Spider bites are rare medical events. Of the thousands of spider species that exist around the world, only a handful cause problems in humans . The venom of most spiders has little or no effect on mammalian tissues [2,3]. 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). Thus, most spiders pose no threat to humans.
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 is 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 nonnative spider quickly spreading throughout the southern United States. Widow bites cause unremarkable local lesions that are sometimes accompanied by a characteristic systemic reaction with prominent, proximally-spreading severe pain and localized diaphoresis surrounding the site of the bite. Rigid abdominal musculature may also occur and be mistaken for an acute surgical abdomen. Sialorrhea, diffuse erythema, muscle tremors or weakness, and neurologic manifestations including tetany and seizures may occur in infants and preschool children. (See "Widow spider bites: Clinical manifestations and diagnosis", section on 'Black widow spiders'.)
False black widow spiders — Steatoda or false black widow spiders (found worldwide) are less often implicated in human bites, and typically cause less severe symptoms that those of widows. Significant local reactions accompanied by pain, pruritus, and secondary infection have occurred (picture 3). Steatoda bites are usually painful and may be accompanied by nausea, headache, lethargy, and malaise. Diaphoresis may occur but is less common than in patients with Latrodectus bites. (See "Widow spider bites: Clinical manifestations and diagnosis", section on 'False black widow spiders'.)
Recluse spiders — Loxosceles or recluse spiders are found predominantly in North and South America (picture 4). Their initial fang penetration is typically painless, but development of pain within the next few days can be significant. Their bites may appear as red plaques or papules that develop central pallor (picture 5). Skin examination features are provided in the table (table 2). Although recluse bites are notorious for becoming necrotic (picture 6 and picture 7), this progression happens in a minority of cases.
Systemic reactions to bites are usually mild and consist of nonspecific systemic signs and symptoms. Rarely, patients may develop systemic manifestations including constitutional symptoms and hematologic abnormalities. Systemic response is very rare but can be critical in children who develop hemoglobinuria, hemoglobinemia, and thrombocytopenia. Systemic manifestation is of concern because it can be fatal in 12 to 30 hours, prior to lesion manifestation, confounding accurate diagnosis. (See "Bites of recluse spiders".)
Australian funnel web spiders — Australian funnel web spiders are found in limited areas of eastern coastal Australia [4,5]. Their bites can cause dramatic systemic reactions that mimic organophosphate poisoning and include salivation, diaphoresis, muscle spasms, tachycardia, hypertension, muscle fasciculations (tongue fasciculations are typical), paresthesias, catecholamine-induced myocarditis, and pulmonary edema .
First aid for a suspected funnel web spider envenomation consists of pressure immobilization applied as for elapid snake bites without local tissue necrosis and rapid transport to definitive care . (See "Snakebites worldwide: Management", section on 'Pressure immobilization'.)
Antivenom, derived from rabbit IgG, is typically given to patients with severe signs of envenomation . Dosing consists of two to four vials for severe envenomation and two vials for signs of systemic envenomation . In one case series of eight patients, two vials of antivenom were sufficient to bind all free venom in serum samples from patients with moderate to severe envenomation, although cardiac toxicity in severe cases was not reversed . Allergic reactions are uncommon, occurring in approximately two percent of patients . Nevertheless, antivenom administration should only occur in facilities equipped to rapidly manage anaphylaxis and when epinephrine, antihistamine, and corticosteroid medications are immediately available and ready to administer (table 3).
South American Phoneutria — Phoneutria or armed spiders are large spiders found predominantly in South America, especially urban areas of Brazil. The bites of these spiders typically cause mild envenomation consisting of pain at the site, redness, piloerection, and primary (localized) diaphoresis [4,8]. However, systemic reactions, consisting of nausea, vomiting, dizziness, visual disturbances and autonomic effects (ie, tachycardia, hypertension, generalized diaphoresis, salivation, and, especially in young boys, priapism) may occur in up to 10 percent of bites with a higher frequency of systemic findings in young children. Fatalities from pulmonary edema and shock can occur but are rare .
Pain control is the primary treatment for mild envenomation . Polyvalent Fab fragment antivenom is available and indicated for patients with systemic envenomation.
Yellow sac spiders — Yellow sac spiders (Cheiracanthium spp.) (picture 8) are common indoor resident spiders in North America, but can occupy indoor and outdoor niches and are distributed worldwide. In human environments, they can be found in garden sheds, garages, house foundations, window sills, between folds of curtains, and baseboards . While reputed in prior literature to be a cause of skin necrosis, more recent case series have not identified skin necrosis in association with documented yellow sac spider bites. They can, however, cause painful bites similar to bee stings (unlike the painless bites of Loxosceles and Latrodectus) . This species is very aggressive and is known to bite suddenly while crawling over its victims. Because the bite is painful, it is often rapidly recognized with recovery of the spider.
A case series of 20 documented Cheiracanthium spider bites in the United States and Australia reported pain or discomfort in all cases . Redness at the bite site was also common. Other local effects included swelling and pruritus. The median duration of pain was approximately two hours but pain lasted 2 days in one patient. Systemic effects (eg, nausea, vomiting, or headache) occurred in three patients.
Treatment consists of washing of the bite site, pain control with oral analgesics (eg, ibuprofen), and, when present, symptomatic treatment of pruritus with an antihistamine.
DIAGNOSIS — There are no commercially available laboratory tests for identifying the presence of spider venom. Thus, the diagnosis is made clinically. 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 "Widow spider bites: Clinical manifestations and diagnosis", section on 'Geographic distribution' and "Widow spider bites: Clinical manifestations and diagnosis", section on 'Geographic distribution'.)
●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 predominantly limited to South America with one minimally toxic species extending north to Costa Rica.
●Australian funnel web spiders are limited to southeastern and coastal Australia.
A presumptive diagnosis of a spider bite is most often based upon the history and clinical presentation. However, the diagnosis of a spider bite can be considered definitive only if the criteria below are fulfilled :
●A spider was observed inflicting the bite.
●The spider was recovered, collected, and properly identified.
●A skin lesion and/or systemic finding typically associated with the type of spider bite are present.
The setting in which the patient sustained the alleged bite should also be carefully reviewed to see if it is consistent with the known habitat and behavior of the toxic spiders that live in the area. (See "Widow spider bites: Clinical manifestations and diagnosis", section on 'Epidemiology' and "Bites of recluse spiders", section on 'Habitat' and "Bites of recluse spiders", section on 'Clinical history'.)
If the above 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). 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.
Unfortunately, the diagnostic criteria for spider bites 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 . 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 . 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. Similarly, in an observational study of 182 patients who came to an emergency department with a chief complaint of a spider bite, 84 percent were diagnosed with a skin or soft tissue infection; only seven patients were confirmed by a clinician to have a spider bite, and of these, only three had the spider identified, none by an arachnologist . (See 'Differential diagnosis' below.)
Most patient 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 . Even when a bite is witnessed by the patient, the "spider" is commonly found to be some other arthropod . Spider bites capture the imagination. Reports exist of patients both feigning spider bites as part of drug seeking behavior  and attempting suicide with genuine spider bites .
Patients may also 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 a person working in food transport and handling, but it is minimal in the general community. The most common event occurring after the discovery of a large spider in transported produce is misidentification of a harmless tropic spider as that of a potentially dangerous Phoneutria spider .
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 'Management overview' below). 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 recluse spiders" and "Widow spider bites: Clinical manifestations and diagnosis", section on 'Differential diagnosis'.)
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 'Diagnosis' 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 . 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 "Methicillin-resistant Staphylococcus aureus (MRSA) in adults: Epidemiology" and "Methicillin-resistant Staphylococcus aureus infections in children: Epidemiology and clinical spectrum", section on 'Epidemiology and risk factors'.)
●Erythema migrans, the target-like skin lesion of early Lyme disease, may be mistaken for a spider bite (picture 9). 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 10). Acute onset is associated with vesicles, vesicopustules, severe edema, erythema, or pain. Tzanck staining for multinucleated giant cells and viral culture are useful to confirm the diagnosis. (See "Epidemiology, clinical manifestations, and diagnosis of herpes zoster" and "Paronychia".)
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 4) . 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.
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. Stings are instantaneously painful, and patients usually capture or at least clearly witness the offender [19,20]. Local pain is the most common presenting symptom. Systemic symptoms include hypertension, tachycardia, diaphoresis, and salivation . (See "Scorpion envenomation causing neuromuscular toxicity (United States, Mexico, Central America, and Southern Africa)".)
Dermatoses — Common dermatoses mistaken for spider bites include 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 11). These lesions tend to be pruritic, rather than painful. (See "Poison ivy (Toxicodendron) dermatitis".)
Rarer dermatoses confused for spider bites include pyoderma gangrenosum and neutrophilic dermatoses:
●Pyoderma gangrenosum (picture 12 and picture 13) causes a painful, rapidly enlarging ulcer that patients not infrequently attribute to a spider bite. It has associations with several conditions such as inflammatory bowel disease, pyogenic arthritis pyoderma gangrenosum, and acne (PAPA) syndrome, rheumatoid arthritis, myeloma, leukemia, lymphoma, and myelodysplasia, but it also occurs as an isolated phenomenon in about half of cases. The diagnosis is often difficult and is initially misdiagnosed in up to 40 percent of patients . Skin biopsy findings are nonspecific but helpful in excluding other diseases such as neoplasms and infection. Patients with pyoderma gangrenosum should have a full vasculitis and coagulation workup, including methylhydrofolate reductase levels. A diagnostic algorithm exists to help rule in the diagnosis . (See "Pyoderma gangrenosum: Pathogenesis, clinical features, and diagnosis", section on 'Diagnosis'.)
●Neutrophilic dermatosis of the dorsal hands is considered by some authors to be a variant of Sweet syndrome, but the presence of leukocytoclastic vasculitis in some otherwise similar cases has led others to classify it as a "pustular vasculitis". The clinical hallmarks are painful nodules and plaques located on the dorsal aspect of the hands; these may proceed to form pustules and/or ulcerate (picture 14). The diagnosis is established by skin biopsy which typically shows dermal infiltration by neutrophils. Some of the stock images of Loxosceles bite reactions posted on the internet may actually be misidentified pictures of neutrophilic dermatosis of the hands. (See "Neutrophilic dermatoses", section on 'Neutrophilic dermatosis of the dorsal hands'.)
MANAGEMENT OVERVIEW — 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. 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 (picture 15) although this is not always a diagnostic feature. Some bites also itch or burn.
Spider bites may or may not be painful, and some go unnoticed. Discomfort 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 pain 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 (picture 5). 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, Loxosceles and Phoneutria spiders. (See 'Medically important spiders' above.)
The management of systemic reactions (eg, muscle pain/spasm, tachycardia, hypertension, diaphoresis) from widow spider bites is discussed separately. (See "Widow spider bites: Management".)
Urticating hairs and allergic reactions — In the United States, tarantulas are increasingly popular pets (picture 16). 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 (picture 17 and picture 18). These urticating hairs, as well as airborne material from crushed tarantulas, may cause irritation if they come in contact with skin, eyes, or mucous membranes [1,24-26]. In addition, airborne material from tarantulas can cause foreign body reactions in the eye .
Allergic reactions to spiders are rare and have been reported mostly in humans who have frequent contact with urticating hairs from large spiders, including tarantulas [23-26,28]. Anaphylaxis has been described .
The acute management of anaphylaxis (from any cause) is reviewed separately. (See "Anaphylaxis: Emergency treatment".)
Regional poison control centers — Regional poison control centers in the United States are available at all times for consultation on patients with known or suspected poisoning, and who may be critically ill, require admission, or have clinical pictures that are unclear (1-800-222-1222). In addition, some hospitals have medical toxicologists available for bedside consultation. Whenever available, these are invaluable resources to help in the diagnosis and management of ingestions or overdoses. Contact information for poison centers around the world is provided separately. (See "Society guideline links: Regional poison control centers".)
Society guideline links — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Envenomation by snakes, arthropods (spiders and scorpions), and marine animals".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, “The Basics” and “Beyond the Basics.” The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on “patient info” and the keyword(s) of interest.)
●Basics topic (see "Patient education: Spider bites (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Epidemiology – Spider bites are uncommon medical events, as there are limited number of spiders worldwide with fangs strong enough to pierce human skin. 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 'Medically important spiders' above.)
●Entomology – 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.)
●Clinical features – Acute spider bites most commonly result in a solitary papule, pustule, or wheal (picture 5) (see 'Local reactions' above). More serious reactions are rare and occur with specific spiders:
•The bites of recluse spiders can become necrotic. (See "Bites of recluse spiders".)
•Systemic symptoms can accompany envenomations of the following spiders:
-Widow spiders (picture 1 and picture 2) – Prominent, proximally spreading, severe pain and localized diaphoresis surrounding the site of the bite. Rigid abdominal musculature may also occur. (See "Widow spider bites: Clinical manifestations and diagnosis".)
-False widow spiders (picture 3) – Similar to widow spiders but less severe in most cases. (See "Widow spider bites: Clinical manifestations and diagnosis".)
-Funnel web spiders – Findings mimic organophosphate poisoning and include salivation, diaphoresis, muscle spasms, tachycardia, hypertension, muscle fasciculations (tongue fasciculations are typical), paresthesias, catecholamine-induced myocarditis, and pulmonary edema. (See 'Australian funnel web spiders' above.)
-Phoneutria spiders – Bites can cause nausea, vomiting, dizziness, visual disturbance and autonomic effects (ie, tachycardia, hypertension, generalized diaphoresis, salivation, and, especially in young children, priapism). (See 'South American Phoneutria' above.)
-Recluse spiders – Systemic features can occur and include acute hemolytic anemia, disseminated intravascular coagulopathy, rhabdomyolysis, and acute kidney injury, especially in children and after bites of South American species. (See "Bites of recluse spiders".)
•Skin, eye, or mucous membrane irritation can occur after contact with urticating hairs from large spiders such as tarantulas or wolf spiders. (See 'Urticating hairs and allergic reactions' above.)
●Diagnosis – 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). 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 (see 'Diagnosis' above):
•A spider was observed inflicting the bite.
•The spider was recovered, collected, and properly identified by an expert entomologist.
•There is a skin lesion or systemic findings associated with the spider bite, and other medical conditions that may cause similar clinical manifestations have been ruled out.
●Differential diagnosis – 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 skin conditions. (See 'Differential diagnosis' above.)
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