Bites of recluse spiders

INTRODUCTION — The entomology of recluse spiders (genus Loxosceles) and the clinical manifestations, diagnosis, differential diagnosis, and management of their bites will be reviewed here.

An overview of spider bites and the management of bites of other spiders are discussed separately. (See "Diagnostic approach to the patient with a suspected spider bite: An overview".)

ENTOMOLOGY OF RECLUSE SPIDERS — Spiders of the genus Loxosceles are known colloquially as recluse spiders, violin spiders, fiddleback spiders, and in South America, by the nonspecific name "brown spiders." These terms are used when describing multiple Loxosceles species as a group. When the common name "brown recluse spider" is used here, it refers only to the one species, L. reclusa, which is widespread and commonly found in homes in the indigenous Central Midwestern United States.

Loxosceles spiders have gained notoriety in the medical literature and lay press because their bites sometimes become necrotic [1,2]. However, this is a relatively uncommon sequela, and is largely limited to areas of the United States where these spiders are endemic (figure 1). Outside of these regions, the vast majority of necrotic skin lesions are caused by other disorders [3-6]. (See 'Differential diagnosis' below.)

Appearance and identification — Recluse spiders are rather nondescript brown spiders (picture 1 and picture 2). The most accurate method of identifying a recluse spider involves counting the eyes. Most spiders have eight eyes in two rows of four. In contrast, recluse spiders have six eyes, with a pair in front, a pair on both sides, and a gap between the pairs (picture 3). With the naked eye or low magnification, the eye pairs (dyads) may appear as individual eyespots.

Identifying a recluse spider on the basis of body markings is less reliable. The brown recluse is described as having a violin pattern on its anterior cephalothorax, although this has led to widespread misidentification of common, harmless spiders as brown recluse when the dark markings on spiders' bodies are mistakenly interpreted as violins [7,8]. Additionally, the violin marking is absent in many juvenile and recently-molted brown recluses as well as in some southwestern United States recluse species. Other features of recluse spiders include monochromatic legs, a monochromatic abdomen, and fine hairs (but not conspicuous spines) on the legs (picture 2).

The most common spider mistaken for Loxosceles in the United States is Kukulcania (picture 4), which has a darkened pattern on the cephalothorax near the eyes of the tan males that can be mistaken for a violin pattern [9]. Female Kukulcania are black or dark brown, velvety in texture, and resemble small tarantulas, although people still mistake them for brown recluse spiders. Kukulcania spiders are found in the southern third of the United States from the San Francisco Bay through southern California, east through Texas to Florida and north to North Carolina and Virginia. They are frequently found in homes, although verified bites from these spiders are virtually unknown.

Geographic location — There are over 140 Loxosceles species in the world, although only a few have extensive distributions and also exist where humans live. Most species are found in North and South America (table 1) [9]. Recluse bites are rare elsewhere, although they have been reported in South Africa and Australia.

●In the United States, recluse spiders are found in limited areas of the South, West, and Midwest (figure 1) and rarely outside these endemic areas [10]. The brown recluse, L. reclusa, is the most widespread and the best known of North American recluse spiders. It is a synanthropic spider (ie, its population numbers increase in association with humans) and these spiders are commonly encountered within homes in endemic areas [11,12].

●In South America, Loxosceles spiders of medical importance are found in Brazil and Chile. The most common species involved in envenomations are L. laeta, L. intermedia, and L. gaucho. Loxosceles laeta is often considered the most dangerous of the recluse spiders, in part because it is the species that attains the largest body size.

●The Mediterranean recluse, L. rufescens, has been transported around the world and continues to establish isolated populations inside buildings on many continents. It has been found in many American cities, where these spiders tend to develop dense populations within isolated buildings. However, despite these infestations, verified bites from Mediterranean recluse are exceedingly rare.

Habitat — Recluses are found mostly inside homes (eg, basements, attics, behind bookshelves and dressers, and in cupboards). As their name implies, these spiders prefer dark, quiet areas that are rarely disturbed (table 1). Out of doors, they are found under objects, such as rocks or the bark of dead trees.

CLINICAL MANIFESTATIONS OF BITES — Loxoscelism is the term for the medical manifestations of bites by recluse spiders. In this review, the term is used to refer to both local and systemic symptoms resulting from bites, although toxicologists sometimes use the term to refer exclusively to the systemic symptoms.

The literature pertaining to loxoscelism is in general inadequate because of the lack of documentation of a proven spider bite, which is the proven presence of the culprit spider in the vicinity [13]. Reports of larger series of patients carrying the diagnosis of loxoscelism only have proven bites in 7 to 14 percent of cases [14,15].

Venom properties — Loxosceles venom contains a large number of enzymes and biologically active substances that can be divided into two major groups: insecticidal toxins (inhibitor cystine knot peptides), metalloproteases (astacins), and phospholipases D; and the toxins expressed in lower quantities such as hyaluronidases, serine proteases, serine protease inhibitors (serpins), allergenic factors, and a translationally controlled tumor protein family of toxins [16]. Of these, the phospholipases D (formerly designated as sphingomyelinases) cause the majority of the clinical entity loxoscelism, including necrotic loxoscelism, platelet disorders, hemolysis, and acute renal failure. The phospholipases D are unique in nature to Loxosceles and its sister genus, Sicarius, but are absent in all other spiders, including other closely related haplogyne spiders [17].

Clinical history — Recluse spiders (like most spiders) typically bite humans only as a desperate last line of defense as they are being crushed between flesh and some object. This happens most frequently indoors, as a result of rolling over on the spider in bed or putting on clothing or footwear that has been left in closets or on the floor, in which the spider has sought refuge.

Recluse spider bites typically occur on the upper arm, thorax, or inner thigh. Bites on the hands or face (ie, uncovered areas) are rare.

Recluse bites can be sustained out of doors, although these spiders are not known to be found in living vegetation. Patients reporting a painful bite while reaching into living foliage are more likely to have sustained an insect sting or puncture by thorns or other sharp plant matter.

Findings following bites — Symptoms may be divided into local signs at the bite site, skin necrosis, and, less commonly, systemic effects.

Local effects — The initial bite of a Loxosceles spider is usually painless, although they can occasionally be painful or cause a burning sensation [18].

The site can sometimes be identified by two small cutaneous puncture marks with surrounding erythema. The bite is usually a red plaque (picture 5) or papule, which often develops central pallor. Occasionally, vesiculation around the site may occur (picture 6). The pain typically increases over the next two to eight hours, and may become severe. It may develop a livedoid, vasculitic appearance. In most cases, this lesion is self-limited and resolves without further complications in approximately one week [19]. However, in some patients, the lesion will develop a dark, depressed center over the ensuing 24 to 48 hours, culminating in a dry eschar that subsequently ulcerates (picture 7). (See 'Necrosis' below.)

Some patients develop urticaria or a morbilliform rash in the hours after the bite; this has been suggested by some as evidence of a prior bite, although the phenomenon has not been studied formally.

Necrosis — Experts in the past have estimated that approximately 10 to 20 percent of recluse spider bites become necrotic [13,20,21]. However, many necrotic skin lesions are erroneously attributed to spider bites. Thus, the true frequency of skin necrosis after a bite is unclear [22].

The progression of necrosis from a recluse spider bite typically occurs over several days. The original papule or plaque develops a dusky red or blue color in the center of the lesion, and a dry, depressed center may herald necrosis. There may be anesthesia in the center. An eschar forms and subsequently breaks down to form an ulcer. The lesion may enlarge in a gravitational manner (picture 8) [23].

A fully developed necrotic lesion is usually 1 to 2 cm in diameter, although skin loss can be more extensive and ulcers as large as 40 cm or more have rarely been described. These most typically occur over fatty tissue on the buttocks and thighs.

Lesions usually stop extending within 10 days of the bite, and most lesions heal by secondary intent over several weeks, without scarring (picture 9) [23]. Some necrotic lesions take months to heal fully. Permanent scarring or requirement for surgical repair is uncommon [24]. (See 'Dermal necrosis' below.)

Systemic findings — Systemic symptoms are an infrequent complication of recluse bites, and do not correlate with local findings. The following nonspecific signs and symptoms may appear over several days following a recluse bite [18]:

●Malaise

●Nausea and vomiting

●Fever

●Myalgias with dark urine (rhabdomyolysis)

●Pallor, jaundice, icterus, and painless dark urine (acute hemolytic anemia)

Life-threatening effects — Rare complications following a recluse spider bite include angioedema, acute hemolytic anemia, disseminated intravascular coagulopathy, rhabdomyolysis, myonecrosis, renal failure, coma, and death [25-28]. The risk of these complications from Loxosceles bites is low in the United States (approximately 1 percent of confirmed bites) [13].

By contrast, these complications are more common with loxoscelism from the South American species L. laeta [23]. (See 'South American recluse spider bites' below.)

Evidence is limited regarding outcomes of Loxosceles bites in pregnant patients and children:

●Pregnant patients – In a case series of five pregnant patients, toxicity was confined to transient toxic erythema of the skin without evidence of fetal toxicity [19].

●Children – Based upon small case series, Loxosceles bites can be very severe in children [23]; however, the literature is not clear on the frequency of complications because of reporting ambiguities and lack of documentation of the actual culprit spider.

One retrospective case series from the southern United States reported 26 children with the diagnosis of recluse spider bite at discharge [29]. All had an admission diagnosis of cellulitis and documentation was not clear on how the discharge diagnosis was confirmed. In three cases, a "spider" was seen. Clinical findings included 22 patients with skin necrosis with three patients developing total desquamation several days after the bite, 13 who developed hemolytic anemia, seven with rhabdomyolysis, and three with progression to acute renal failure. There was a bimodal peak for hemolysis at days 2 through 3 and days 4 through 9. Five patients required management in an intensive care unit. Therapy was supportive, and there were no deaths.

In a case report of a suspected recluse spider bite in a six-year-old child, profound hemolysis (hemoglobin <2.0) and vascular collapse developed but responded to nine single-volume plasma exchanges, aggressive transfusions, inotropes, and mechanical ventilation [27]. Another case report of a suspected bite in a 17-year-old woman with persistent hemolysis responded to a single exchange [30].

Unexplained severe hemolysis in an indigenous area (figure 1 and table 1) may suggest a surreptitious Loxosceles envenomation [31]. There are rare reports of death in small children occurring within 24 hours of a presumed recluse spider bite [26,32].

DIAGNOSIS — A presumptive diagnosis of a spider bite is most often based on the history of feeling a bite and clinical presentation of a wound that may be necrotic. An assay for Loxosceles venom has been developed but is not commercially available [33]. Of note, the diagnosis of a spider bite can be considered definitive only if the patient has a consistent skin lesion (picture 5 and picture 6 and picture 8) and both of the following criteria are fulfilled:

●A spider was observed inflicting the bite.

●The spider was recovered, collected, and properly identified by an expert entomologist or an accurate identification of the culprit spider is made by the medical provider.

If both of the above conditions are not met, then other conditions such as vasculitis, infection, vascular problems, or other relevant disorders must be excluded. (See 'Differential diagnosis' below.)

Because systemic effects may precede skin findings, a working diagnosis of recluse spider bite may be made for children with acute hemolysis of unknown etiology in regions where the recluse spider is found. However, other etiologies for acute, severe hemolytic anemia must also be excluded. (See 'Pediatric considerations' below.)

The general approach to a patient suspected of having a spider bite, as well as the differential diagnosis of an uncomplicated (lacking signs of necrosis) spider bite, are reviewed separately. (See "Diagnostic approach to the patient with a suspected spider bite: An overview".)

Further evaluation — Patients with clinical manifestations limited to local effects do not require laboratory evaluation.

Patients with systemic findings (eg, malaise, nausea and vomiting, fever, and myalgias), especially children, warrant evaluation for acute hemolysis, rhabdomyolysis, and acute kidney injury as follows (see "Rhabdomyolysis: Clinical manifestations and diagnosis", section on 'Evaluation and diagnosis' and "Diagnosis of hemolytic anemia in adults"):

●Complete blood count with peripheral smear

●Reticulocyte count

●Type and screen with Coombs testing if signs of hemolytic anemia

●Total and direct serum bilirubin

●Aspartate aminotransferase and alanine aminotransferase

●Serum lactate dehydrogenase

●Serum haptoglobin if signs of hemolytic anemia

●Serum electrolytes

●Serum calcium and phosphate

●Serum uric acid if signs of rhabdomyolysis

●Blood urea nitrogen and creatinine

●Creatine kinase

●Rapid urine dipstick for blood and for urobilinogen with reflex to urinalysis if positive

●Prothrombin time (PT) with international normalized ratio (INR)

●Activated partial thromboplastin time (aPTT)

●Fibrinogen and D-dimer if INR or aPTT is prolonged

●Electrocardiogram if signs of rhabdomyolysis and electrolyte abnormalities

DIFFERENTIAL DIAGNOSIS — Numerous conditions have been mistaken for a necrotic recluse spider bite (table 2). The most common disorders in the differential diagnosis are presented in this section (table 3).

The following mnemonic (NOT RECLUSE) may assist in differentiating brown recluse spider bites from other skin lesions [34]:

●N – Numerous (recluse bites are typically a single focal lesion)

●O – Occurrence (recluse bites typically occur in secluded locations in the home such as attic space, garage, or closet rather than outside)

●T – Timing (lesions appearing from November to March are much less likely to be caused by recluse spider bites)

●R – Red center (recluse bites typically have a pale center)

●E – Elevated (recluse bites are flat or sunken)

●C – Chronic (lesions presenting longer than several weeks are unlikely to be recluse spider bites)

●L – Large (lesions >10 cm are uncommon after a recluse spider bite)

●U – Ulcerates too early (<7 days) suggests infection or pyoderma gangrenosum rather than a recluse spider bite

●S – Swollen (except for bites to the face or feet, significant swelling is not typical for recluse spider bites)

●E – Exudative (other than bites on eyelids or toes, recluse spider bites are not moist or exudative; frank pus suggests infection)

Solitary ulcerated lesion — Conditions that can cause single ulcerated lesions include infections, trauma, vascular diseases, pyoderma gangrenosum, and vasculitides.

Infections — Common infections that can become necrotic include staphylococcus and streptococcal infections, deep fungal infections, and atypical mycobacterial infections (table 2). However, in most instances of cutaneous infection, there is initial swelling with elevation of the central region above the level of the surrounding skin and ulceration is rare [34]. By contrast, necrotic recluse bite wounds tend to have limited swelling above the skin surface with the exception of vesicles and ulcerate early on in the process (picture 8).

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) skin infections can begin with singular papules or pustules that may evolve to necrotic lesions [35,36]. CA-MRSA is far more prevalent than spider bites. CA-MRSA strains both in the United States and in Europe have an enhanced virulence that has resulted in the more striking clinical manifestations, compared with non-MRSA. 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'.)

A patient who had recently traveled to the tropics and presented with a skin ulcer would require evaluation for the various infections that can cause ulcerating lesions (table 4). (See "Skin lesions in the returning traveler".)

Vascular disease — Foot and ankle ulcers can be seen in patients with chronic venous insufficiency, arterial insufficiency, or diabetes mellitus, and a history of the underlying disease should alert clinicians to this possibility. These ulcers are distinguishable by various characteristics (table 5). (See "Diagnostic evaluation of lower extremity chronic venous disease".)

Pyoderma gangrenosum — Pyoderma gangrenosum is an idiopathic disorder in which dark, blue-red papules progress to necrotic ulcerating lesions. Patients may report a history of antecedent trauma at the site or have signs of systemic illness. Borders are typically irregular with undermined edges, and the lesion may have a purulent base (picture 10 and picture 11). Lesions may be bullous. (See "Pyoderma gangrenosum: Pathogenesis, clinical features, and diagnosis".)

Vasculitis — Cutaneous vasculitis results from inflammation of the small or medium-sized blood vessels in the skin. Small blood vessels are capillaries, postcapillary venules, and nonmuscular arterioles in the superficial and mid-dermis (<50 micrometers). Medium-sized vessels consist of 50 to 150 micrometer vessels with muscular walls in the deep dermis and subcutis. Cutaneous vasculitis occurs in a wide variety of clinical settings (table 6). Ulceration and tissue necrosis occur when vasculitis results in reduced vascular perfusion in the skin (picture 12A-B). Superficial ulcers can occur in patients with small vessel vasculitis; deep ulcers are usually the result of medium vessel disease. Less commonly, vasculitis and necrotizing vasculitis may present with singular lesions that show eschar or ulceration (picture 13) that may resemble pyoderma gangrenosum. (See "Evaluation of adults with cutaneous lesions of vasculitis", section on 'When to suspect cutaneous vasculitis'.)

Patients with vasculitis may have the presence of systemic findings consistent with connective tissue disease, recent symptoms of infection, or introduction of a medication within the past 7 to 10 days, which helps to differentiate them from victims of recluse spider bites. However, skin biopsy is most definitive to establish the diagnosis of vasculitis. (See "Evaluation of adults with cutaneous lesions of vasculitis", section on 'Patient assessment' and "Evaluation of adults with cutaneous lesions of vasculitis".)

Neutrophilic dermatosis of the dorsal hand — Neutrophilic dermatosis ("pustular vasculitis") of the dorsal hand is an ulcerating condition that is characterized by one or more ulcers on the hand and is felt by some to be a variant of Sweet disease. The lesions may be bullous or bullous hemorrhagic initially (picture 14). Skin biopsy shows neutrophilic infiltration. (See "Neutrophilic dermatoses", section on 'Neutrophilic dermatosis of the dorsal hands'.)

Exaggerated arthropod bite — This lesion can result from the assault by a number of common bloodsucking insects. It may occur without an underlying explanation, but may also occur in the context of an underlying hematologic malignancy [37].

Systemic reactions — The systemic symptoms of recluse spider envenomation (eg, malaise, nausea and vomiting, fever, and myalgias) are sufficiently nonspecific that an accompanying lesion with an identifiable biting spider is essential for making the diagnosis of a spider bite. (See 'Diagnosis' above.)

TREATMENT — For patients with ulceration or systemic complaints, the evidence supporting the use of Loxoscelism specific treatments (eg, dapsone, antivenom) is lacking [38]. Care providers should weigh the relative risks versus potential benefits with the understanding of the controversies surrounding effectiveness.

Patients with local effects — The treatment of acute local findings following a recluse spider bite involves local wound care, pain management, and, if indicated, tetanus prophylaxis.

Wound care and general measures — Initial treatment measures following any spider bite include:

●Clean the bite with mild soap and water.

●Apply cold packs, taking care not to freeze the tissue.

●Maintain the affected body part in an elevated or neutral position (if possible).

●Administer pain medication as needed. Some patients will respond to nonsteroidal antiinflammatory medications, while others may require opioids.

●Administer tetanus prophylaxis if indicated (table 7).

Most bites can be managed with minimal intervention and heal without scarring. Resolving bites should be monitored for the development of secondary bacterial infection.

Antibiotics are prescribed only if there are signs of infection such as increased erythema, fluctuation, and suppuration. If infection is suspected, it should be treated with antibiotics as recommended for uncomplicated cellulitis. (See "Acute cellulitis and erysipelas in adults: Treatment", section on 'Acute cellulitis' and "Skin and soft tissue infections in children >28 days: Evaluation and management", section on 'Cellulitis'.)

Dermal necrosis — For patients with recluse spider bites that have a dusky center or other signs of developing necrosis, no proven therapy, other than antivenom administration, exists. Antivenom is suggested for patients with moderate to severe dermal necrosis who present for care within 48 hours after recluse spider bite occurring in South America. (See 'South American recluse spider bites' below.)

In the absence of antivenom (not available in the United States), we suggest that patients with dermal necrosis receive only symptomatic and supportive wound care; the use of dapsone should be avoided. Dapsone has been advocated by some, but there is no clear benefit from existing evidence and substantial risk of adverse effects including aplastic anemia, methemoglobinemia, and dapsone hypersensitivity [39,40]. In addition, adverse side effects may confound the monitoring of patients with possible systemic loxoscelism.

As discussed further below, early surgical excision and/or curettage of a necrotic lesion as the lesion is still evolving should be avoided. However, once the lesion is demarcated and clinically stable, debridement and wound care may permit better healing. In one series of eight patients, vacuum-assisted wound closure was used to promote healing of necrotic lesions [41] and, in one animal trial, this method was associated with more rapid wound healing [42]. A small minority of necrotic lesions later require surgical revision of scars, including skin grafting.

Several other therapies have been proposed or performed but are not recommended:

●Tetracycline – Application of topical tetracycline has been shown to reduce the progression of dermonecrotic lesions in rabbits exposed to Loxosceles intermedia venom but awaits further study in humans [43].

●Insufficiently studied treatments – Therapies that have not been adequately studied in humans include antihistamines, glucocorticoids, empiric administration of topical or systemic antibiotics, vasodilators, heparin, nitroglycerin, hyperbaric oxygen, dextran, and local electric shock [40,43-45].

●Early surgical interventions – Early surgical excision and/or curettage of a necrotic lesion is potentially harmful [44,46-48]. One retrospective study of 31 patients with bites affecting the upper extremity or hand concluded that painful and recurrent wound breakdown occurred more often with early surgical excision [47].

Patients with systemic toxicity — Patients with systemic toxicity may demonstrate nausea, fever, malaise, vomiting, myalgias, or pallor within a few days of a spider bite. The severity of systemic toxicity does not correlate with toxicity at the bite site. Patients with systemic findings warrant laboratory studies to assess for hemolytic anemia, rhabdomyolysis, and kidney injury. Hospital admission is indicated for patients with signs of hemolytic anemia, rhabdomyolysis, or disseminated intravascular coagulopathy. (See 'Further evaluation' above.)

Acute hemolytic anemia — Although hemolytic anemia following a recluse spider bite can be severe, it is typically self-limited. The combination of elevated serum total bilirubin and lactate dehydrogenase will detect hemolytic anemia before the fall in hemoglobin, with a sensitivity of 94 percent and a specificity of 91 percent [49]. The primary treatment consists of blood transfusions for patients with a rapidly falling hematocrit or uncompensated anemia. Consultation with a hematologist is encouraged.

Antivenom is suggested for patients with acute hemolytic anemia after South American recluse spider bites. (See 'South American recluse spider bites' below.)

Plasma exchange has been performed in one case of severe hemolytic anemia in a child with profound hemolysis (hemoglobin <2.0) and vascular collapse with ultimate survival [27]. However, the need for this invasive treatment is rare following recluse bites and should only be performed in consultation with a pediatric hematologist.

Rhabdomyolysis — The recommended approach to rhabdomyolysis following spider bites is extrapolated from crush injuries and is discussed in detail separately. Initial therapy consists of rapid infusion of isotonic saline to establish urine output of 200 to 300 mL/hour (4 ml/kg per hour in children) with a goal of preventing renal failure. (See "Prevention and treatment of heme pigment-induced acute kidney injury (including rhabdomyolysis)", section on 'Volume administration'.)

Antivenom is suggested for patients with rhabdomyolysis after a South American recluse spider bite. (See 'South American recluse spider bites' below.)

Disseminated intravascular coagulopathy — Patients with systemic toxicity after a recluse spider bite warrant studies to assess for the presence of disseminated intravascular coagulopathy (DIC). (See 'Further evaluation' above.)

The treatment of DIC is discussed separately. (See "Disseminated intravascular coagulation in infants and children", section on 'Management' and "Evaluation and management of disseminated intravascular coagulation (DIC) in adults", section on 'Treatment'.)

Antivenom is suggested for patients with DIC after a South American recluse spider bite. (See 'South American recluse spider bites' below.)

South American recluse spider bites — The bites of South American Loxosceles species (eg, L. gaucho) are more severe than those of recluse spiders found in the United States and carry a higher risk of dermal necrosis and systemic effects. Consultation with a physician experienced with the management of bites by South American recluse spiders is encouraged prior to the administration of antivenom.

Management is determined by the severity of effects and whether the patient has acute local skin findings alone or also has signs of systemic envenomation. (See 'Patients with local effects' above and 'Acute hemolytic anemia' above and 'Rhabdomyolysis' above.)

In addition, antivenoms for the treatment of recluse spider bites are available in Brazil, Chile, and Peru, although not in the United States. We suggest that patients with presumed spider bites by Loxosceles gaucho, L. laeta, and L. intermedia (ie, species indigenous to Brazil, Chile, and Peru) who have systemic findings at any time after a bite or who have moderate to severe dermal necrosis and present for care within 48 hours receive antivenom rather than supportive care alone [50,51]. Mild allergic reactions consisting of nausea or urticaria have been described in 7 to 20 percent of patients who receive the antivenom [50,52]. Given the frequency of reactions and the potential for anaphylaxis, antivenom administration in an acute care setting (eg, emergency department or intensive care unit) is recommended, if at all possible.

Observational studies in humans and animal trials suggest that South American recluse antivenoms may reduce the risk of dermatonecrosis, as well as systemic envenomation and its severe complications (eg, hemolysis, renal failure, and disseminated intravascular coagulation). However, definite benefit in humans is not well established [18,50,52-54]. This antivenom is not US Food and Drug Administration (FDA)-approved for use and is not available in North America.

PEDIATRIC CONSIDERATIONS — Systemic loxoscelism is rare, but may occur more commonly in children. Because systemic effects may precede skin findings, loxoscelism should be considered in the differential diagnosis of acute hemolysis of unknown etiology occurring in the pediatric age group in regions in which recluse spiders are indigenous. (See 'Life-threatening effects' above.)

All children with any systemic signs after a presumed recluse spider bite warrant hospitalization and evaluation for acute hemolysis and rhabdomyolysis. Treatment is supportive. (See "Prevention and treatment of heme pigment-induced acute kidney injury (including rhabdomyolysis)".)

Children who do not have systemic findings may be discharged home after local wound care and general measures. However, because hemolysis has been described up to seven days after a spider bite [29], caretakers must have clear instructions to promptly seek medical care if any systemic findings (eg, vomiting, fever, myalgias, or hematuria) occur. In addition, re-evaluation of the child at three and seven days after the bite, regardless of symptoms, is suggested.

Case reports of Loxosceles bites during pregnancy have documented no adverse effects on fetal outcomes [19].

DISCHARGE INSTRUCTIONS AND AFTER CARE — Patients should be counseled about how to care for the bite site and advised to watch the site for signs of secondary bacterial infection (eg, fever, spreading redness, pus formation or drainage), as well as progressive skin changes that suggest early necrosis (ie, enlargement of the lesion or black/blue color changes).

Parents/caregivers of children with a presumed recluse spider bite should be instructed to seek prompt medical care if systemic findings (eg, fever, vomiting, myalgias, or hematuria) occur.

Patients who are concerned about avoiding future recluse bites should be counseled to shake out clothes, shoes, gloves, and other items that have been unused or lying on the floor before putting them on.

Beds should be modified so that only the legs of the bed touch the floor: they should be moved away from the wall, bedding should be tucked in and ruffles removed, and items should not be stored beneath it.

Insecticides may be effective for controlling recluse populations within the home; however, these must be properly administered by a pest control professional. Placement of sticky traps next to baseboards but out of reach of curious children and pets is another useful method to eliminate brown recluse spiders from a building.

ADDITIONAL RESOURCES

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 5^th to 6^th 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 10^th to 12^th 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

●Entomology – Recluse spiders (genus Loxosceles) are one of a handful of spiders throughout the world that are capable of inflicting medically significant bites in humans. They are nondescript brown spiders (picture 1) that can be more reliably identified by their distinctive eye pattern (three pairs) (picture 3). They are found mostly in certain parts of North and South America. In the United States, recluse spiders are limited to areas of the South, West, and Midwest (figure 1). (See 'Entomology of recluse spiders' above.)

●Clinical history – Recluse spider bites are usually sustained indoors. These spiders are not aggressive and generally bite humans only when being crushed between flesh and some object. (See 'Habitat' above and 'Clinical history' above.)

Patients presenting with possible spider bites should always be questioned carefully regarding the circumstances surrounding the bite. Unless the patient witnessed the spider inflicting the bite and can retrieve the spider for identification by an entomologist, other disorders are responsible for most lesions attributed to spider bites (table 2). (See "Diagnostic approach to the patient with a suspected spider bite: An overview" and 'Diagnosis' above.)

●Clinical manifestations – Loxoscelism is the term for the medical manifestations of bites by recluse spiders. The initial bite of a Loxosceles spider is typically painless, although some patients describe sharp pain or burning. The bite is usually a red plaque (picture 5), which can sometimes be identified by two small cutaneous puncture marks with surrounding erythema. Occasionally, vesiculation around the site may occur (picture 6). In most cases, this lesion is self-limited and resolves without further complications. Patients with clinical manifestations limited to local effects do not require laboratory evaluation. (See 'Local effects' above and 'Further evaluation' above.)

The progression of necrosis from a recluse spider bite typically occurs over several days (picture 7 and picture 8). These generally heal by secondary intent over several weeks, without scarring (picture 9). (See 'Necrosis' above.)

Systemic toxicity may appear over several days following a bite and include malaise, nausea and vomiting, fever, and myalgias. Patients with systemic findings warrant ancillary studies to identify hemolytic anemia, rhabdomyolysis with acute kidney injury (AKI), and/or disseminated intravascular coagulopathy (DIC). Systemic toxicity is more common in small children than adults. (See 'Systemic findings' above and 'Further evaluation' above and 'Pediatric considerations' above.)

●Diagnosis – A presumptive diagnosis of a spider bite is most often based on the history of feeling a bite and clinical presentation of a wound that may be necrotic. Of note, the diagnosis of a spider bite can be considered definitive only if the patient has a consistent skin lesion (picture 5 and picture 6 and picture 8) and both of the following criteria are fulfilled:

•A spider was observed inflicting the bite.

•The spider was recovered, collected, and properly identified by an expert entomologist or an accurate identification of the culprit spider is made by the medical provider.

If both conditions are not met, then other etiologies such as vasculitis, infection, vascular problems, or other relevant disorders must be excluded. (See 'Differential diagnosis' above.)

A working diagnosis of recluse spider bite may be made for children with acute hemolysis of unknown etiology in regions where the recluse spider is found once other causes for acute, severe hemolytic anemia are excluded. (See 'Pediatric considerations' above.)

●Management – Management is determined by the severity of cutaneous effects and whether systemic toxicity is present:

•Local effects only – For patients with bites by recluse spiders who do not have dermal necrosis or systemic toxicity, management consists of general wound measures and tetanus prophylaxis, as needed (table 7). (See 'Wound care and general measures' above.)

•Dermal necrosis – For patients with dermal necrosis after a bite by a recluse spider other than a South American species (Loxosceles gaucho, L. laeta, and L. intermedia [ie, species indigenous to Brazil, Chile, and Peru]), we suggest symptomatic and supportive wound care; no antivenom exists for these species, and the use of dapsone should be avoided. (Grade 2C). Only patients with signs of cellulitis or abscess (eg, redness, swelling, suppuration) should receive antibiotics. The clinician should avoid early surgical excision and/or curettage of a necrotic lesion as it is still evolving. However, once the lesion is demarcated and no longer progressing, debridement and wound care may permit better healing. (See 'Dermal necrosis' above.)

For patients with presumed spider bites by a South American species and who have systemic findings at any time after a bite or who have significant dermal necrosis and present for care within 48 hours, we suggest antivenom (Grade 2C). Consultation with a physician experienced with the management of bites by South American recluse spiders is encouraged prior to the administration of antivenom. (See 'South American recluse spider bites' above.)

●Systemic toxicity – Hospital admission is indicated for all patients with recluse bites complicated by systemic toxicity. For bites by species other than South American recluse spiders, treatment is condition-specific and includes supportive care of:

•Acute hemolytic anemia (see 'Acute hemolytic anemia' above)

•Rhabdomyolysis and AKI (see "Prevention and treatment of heme pigment-induced acute kidney injury (including rhabdomyolysis)")

•DIC (see "Disseminated intravascular coagulation in infants and children", section on 'Management' and "Evaluation and management of disseminated intravascular coagulation (DIC) in adults", section on 'Treatment')