Spider Bite (Brown Recluse)

Brown Recluse Spider Bite (Loxoscelism)

Brown recluse spider (Loxosceles reclusa) bites are most often self-limited and self-healing, but approximately 10% become necrotic after 24–48 hours, and up to 10% of cases develop systemic loxoscelism with potentially life-threatening hemolysis.[1-3] Management is primarily conservative with local wound care and tetanus prophylaxis; no specific therapy has proven efficacy in controlled trials.[1-2]

1. History

• Was the spider seen or captured? (Definitive diagnosis requires spider identification by an arachnologist)[2]
• Timing of bite — often painless initially; patients may be unaware of the bite event[3]
• Circumstances: rolling onto spider in bed, putting on stored clothing, reaching into dark undisturbed areas[1]
• Progression of the wound: initial erythema → central pallor → vesicle/blister → eschar over days[3-4]
• Pain character: initially painless or mild burning, progressing to significant pain over hours[5]
• Constitutional symptoms: fever, chills, malaise, myalgias, arthralgias, nausea, headache, rash (present in up to 50% within 24–48 hours)[3]
• Dark or red urine (suggests hemolysis/hemoglobinuria)[3]
• Geographic location — endemic to southern Midwest and Southwestern United States[1]

2. Alarm Features

• Systemic loxoscelism (up to 10% of cases): fever, jaundice, dark urine, hemolytic anemia, rhabdomyolysis, acute renal failure[3][6]
• Myalgia and malaise are independently associated with development of hemolysis (aOR 7.1 and 12.76, respectively)[7]
• Rapidly expanding necrosis or hemorrhagic blisters[3]
• Signs of DIC: petechiae, mucosal bleeding, prolonged clotting times — odds of death are significantly higher if DIC develops (OR 82.9)[8]
• Hemoglobin <4 g/dL associated with complications including hyperkalemia, metabolic acidosis, hypotension requiring vasopressors, and hypoxia requiring intubation[7]
• Children and younger adults are at higher risk for systemic loxoscelism (median age 14 years in one large cohort)[9]

3. Medications

• First-line: OTC analgesics (acetaminophen, NSAIDs) for pain[10-11]
• Tetanus prophylaxis if not up to date[1]
• Dapsone: Historically used but no prospective human study supports efficacy; causes dose-dependent hemolysis in all patients and severe hemolysis in G6PD-deficient patients — use is controversial and should be prescribed judiciously if at all[2]
• Not recommended / no proven benefit: Corticosteroids, antihistamines, hyperbaric oxygen, antibiotics (unless secondary infection), early surgical excision[1-2]
• Antivenom: Not available in the United States[1][3]
• Avoid: Suction devices, tourniquets[11]

4. Diet

• No specific dietary triggers or restrictions
• Maintain adequate hydration, especially if hemolysis is present, to support renal perfusion and prevent acute kidney injury
• In hospitalized patients with systemic loxoscelism, aggressive IV hydration is appropriate

5. Review of Systems

• Constitutional: Fever, chills, malaise, fatigue
• Skin: Rash (generalized pruritus, morbilliform eruption), wound progression[3][12]
• GU: Dark or red urine (hemoglobinuria), decreased urine output
• MSK: Myalgias, arthralgias[7][12]
• GI: Nausea, vomiting
• Neuro: Headache
• Heme: Easy bruising, petechiae (suggests coagulopathy)

6. Collateral History and Family History

• Confirm geographic plausibility — brown recluse is endemic to south-central US; bites outside this range are extremely unlikely[2]
• Home environment: old homes, storage areas, woodpiles, undisturbed spaces
• Was the spider captured? Identification by an arachnologist is the only way to confirm diagnosis[2]
• G6PD deficiency history (relevant if dapsone is considered)[2]
• No hereditary predisposition to loxoscelism

7. Risk Factors

• Living in or traveling to endemic areas (south-central US: Missouri, Kansas, Oklahoma, Arkansas, Tennessee, and surrounding states)[1-2]
• Indoor exposure in dark, quiet areas (closets, attics, basements, stored clothing, bedsheets)[1]
• Warm seasons — bites more frequent in warmer months[3]
• Slightly more women than men are bitten (domestic environment exposure)[3]
• Children and young adults at higher risk for systemic loxoscelism[7][9]

8. Differential Diagnosis

• This is a critical section — loxoscelism is vastly overdiagnosed, and many conditions mimic necrotic spider bites:[2]
• MRSA/Staphylococcal abscess — most common mimic; look for purulence, fluctuance, surrounding cellulitis
• Streptococcal skin infection / necrotizing fasciitis — rapidly progressive, systemic toxicity, pain out of proportion
• Herpes simplex / herpes zoster — grouped vesicles on erythematous base, dermatomal distribution
• Pyoderma gangrenosum — undermined violaceous borders, associated with IBD/autoimmune disease
• Diabetic ulcer — chronic, neuropathic distribution
• Cutaneous anthrax — painless black eschar with surrounding edema
• Erythema migrans (Lyme disease) — expanding erythematous patch with central clearing
• Sporotrichosis — nodular lymphangitis, gardening exposure
• Squamous cell carcinoma — chronic non-healing ulcer
• Factitious injury — atypical distribution, inconsistent history[2]

9. Past Medical History

• G6PD deficiency — critical to identify before any consideration of dapsone[2]
• Immunocompromised states — may affect wound healing
• Diabetes — impaired wound healing, higher infection risk
• Bleeding disorders or anticoagulant use — relevant if coagulopathy develops
• Prior spider bite history
• Tetanus immunization status

10. Physical Exam

• Vital signs: Fever, tachycardia, hypotension (suggest systemic loxoscelism)

Local wound

• Early: single, flat, erythematous lesion with central pallor ("red, white, and blue" sign — erythema, ischemia, ecchymosis)[1][3]
• Distinguished from other bites by single, flat lesion without significant surrounding swelling[1]
• 24–72 hours: painful edema with induration, irregular ecchymosis, hemorrhagic blisters[3]
• 72 hours: necrotic eschar with well-defined borders[3]
• Common locations: upper arm, thorax, inner thigh, trunk[1][3][5]
• Oedematous variant: Facial bites may show extensive edema/erythema with little necrosis[3]
• Systemic exam: Jaundice, generalized rash, petechiae, hepatosplenomegaly

11. Lab Studies

• For uncomplicated cutaneous bites: Labs generally not necessary[1]

If systemic loxoscelism suspected

• CBC with differential (hemoglobin drop averages 3.1 g/dL over ~2 days)[9]
• Total bilirubin + LDH — most sensitive and specific combination for detecting preclinical hemolysis (sensitivity 94%, specificity 91%)[13]
• Reticulocyte count, haptoglobin, peripheral smear
• Direct antiglobulin test (DAT/Coombs) — positive in ~56% of hemolysis cases (positive to C3 and IgG)[3][9]
• Urinalysis — hemoglobinuria (positive for blood without microscopic RBCs); hematuria in 32% of hemolytic cases[7]
• BMP (creatinine, potassium — renal function monitoring)
• CK (rhabdomyolysis occurred in 61% of hemolytic cases)[7]
• PT/PTT, fibrinogen, D-dimer if DIC suspected[8]
• LFTs (elevated transaminases in ~29% of hemolytic cases, but with normal synthetic function)[7]
• If dapsone considered: G6PD level, CBC, LFTs at baseline and weekly during therapy[2]

12. Imaging

• Imaging is generally not indicated for brown recluse bites
• Consider imaging only if concerned about deep tissue infection, abscess, or necrotizing fasciitis as alternative diagnoses (CT or MRI of affected area)
• Ultrasound may help differentiate abscess from cellulitis if MRSA is in the differential

13. Special Tests

• No commercially available venom assay exists for humans; ELISA has been tested only in animal models[2]
• Spider identification by an arachnologist is the gold standard for confirming the diagnosis[2]
• Wound culture if secondary infection suspected (secondary infection is uncommon even with extensive necrosis)[3]

14. ECG

• Not routinely indicated for uncomplicated bites

Obtain ECG if

• Severe hemolysis with hyperkalemia (peaked T waves, widened QRS)
• Hemodynamic instability
• Significant metabolic acidosis

15. Assessment

Brown recluse bites present on a spectrum

• Mild (majority): Self-limited erythema and mild pain, resolves within 1 week[1]
• Moderate (cutaneous loxoscelism with necrosis, ~10%): Necrotic ulcer developing over 24–72 hours; healing takes weeks to months depending on severity (mean 5.6 weeks; severe lesions average 74 days)[5]
• Severe (systemic/viscerocutaneous loxoscelism, up to 10%): Intravascular hemolysis, rhabdomyolysis, acute renal failure, DIC; mortality 3.5% in one large cohort[9]
• Key clinical pearl: The diagnosis is vastly overdiagnosed — maintain a broad differential for any necrotic skin lesion, especially outside endemic areas.[2]

16. Treatment Plan

Initial stabilization (all patients)

• Elevation and immobilization of affected limb[2]
• Application of ice to bite site[2]
• Local wound care with soap and water[1]
• Tetanus prophylaxis if indicated[1]
• OTC analgesics (acetaminophen, NSAIDs)[10-11]

Cutaneous loxoscelism

• Conservative wound care is the mainstay — most lesions heal without intervention[2]
• Antibiotics only if secondary bacterial infection develops[1]
• Avoid early surgical excision — associated with delayed wound healing and objectionable scarring[2][14]
• Delayed surgical revision of scar may be considered after necrosis has fully demarcated (typically weeks later)[1]
• Dapsone remains controversial; if used, dose is typically 50–100 mg/day with G6PD screening and weekly CBC/LFTs[2]

Systemic loxoscelism

• Hospital admission with close monitoring[3]
• Aggressive IV hydration to maintain renal perfusion
• Serial CBC, LDH, total bilirubin, renal function monitoring[13]
• RBC transfusion as needed (76.9% of hemolytic patients required transfusion)[7]
• Monitor for and treat hyperkalemia, metabolic acidosis
• Systemic corticosteroids (prednisone 40–80 mg/day for 5 days) are used in some protocols, though evidence is limited[3]
• Renal replacement therapy if acute renal failure develops

17. Disposition

• Discharge: Uncomplicated cutaneous bites with no systemic symptoms, normal vital signs, and no laboratory evidence of hemolysis

Observation/admission criteria

• Constitutional symptoms (fever, myalgia, malaise) — especially in children[7]
• Any laboratory evidence of hemolysis (elevated LDH, bilirubin, hemoglobinuria)
• Rapidly expanding necrosis or hemorrhagic blisters
• Hemodynamic instability
• ICU admission: Severe hemolytic anemia (Hgb <4 g/dL), DIC, acute renal failure, hemodynamic instability[7-8]
• Specialist consultation: Toxicology (for management guidance), dermatology or plastic surgery (for significant necrotic wounds), hematology (for severe hemolysis), nephrology (for renal failure)

18. Follow Up / Return Precautions

• Follow-up: Wound recheck in 48–72 hours to assess for necrosis progression, then weekly until healing[5]

Return immediately for

• Dark or red urine
• Fever, chills, worsening malaise or myalgias
• Rapidly expanding wound, increasing pain
• Jaundice
• Decreased urine output
• Signs of wound infection (increasing warmth, purulent drainage, expanding erythema)
• Expected course: Mild bites resolve in ~1 week; necrotic wounds take 5–17+ weeks to heal (mean 5.6 weeks)[5]
• Patient counseling: Most bites heal without long-term consequences; scarring may occur with necrotic lesions; surgical scar revision can be considered after full healing[1-2]
• Prevention: Shake out clothing and shoes stored in dark areas; use gloves when moving stored items; seal cracks in home; use DEET-based repellents[1]

References

1. Arthropod Bites and Stings. — Herness J, Snyder MJ, Newman RS. American Family Physician. 2022.

2. Bites of Brown Recluse Spiders and Suspected Necrotic Arachnidism. — Swanson DL, Vetter RS. The New England Journal of Medicine. 2005.

3. Spider Bite. — Isbister GK, Fan HW. Lancet. 2011.

4. Necrotic Ulcers Caused by Loxosceles Rufescens Bites: A Report of Seven Patients and Scanning Electron Microscopy of the Spider. — Veraldi S, Schianchi R, Nazzaro G. European Journal of Dermatology : EJD. 2024.

5. Nineteen Documented Cases of Loxosceles Reclusa Envenomation. — Sams HH, Hearth SB, Long LL, et al. Journal of the American Academy of Dermatology. 2001.

6. Systemic Loxoscelism, Less Frequent but More Deadly: The Involvement of Phospholipases D in the Pathophysiology of Envenomation. — Gremski LH, da Justa HC, Polli NLC, et al. Toxins. 2022.

7. Cutaneous-Hemolytic Loxoscelism Following Brown Recluse Spider Envenomation: New Understandings. — Loden JK, Seger DL, Spiller HA, Wang L, Byrne DW. Clinical Toxicology. 2020.

8. Coagulation Abnormalities Following Brown Recluse Spider (Loxosceles Reclusa) Envenomation: A Description of 2 Cases and Review of the Literature. — Hart SA, Gailani D, Bibb LA, et al. American Journal of Clinical Pathology. 2025.

9. Defining the Complex Phenotype of Severe Systemic Loxoscelism Using a Large Electronic Health Record Cohort. — Robinson JR, Kennedy VE, Doss Y, et al. PloS One. 2017.

10. 2024 American Heart Association and American Red Cross Guidelines for First Aid. — Hewett Brumberg EK, Douma MJ, Alibertis K, et al. Circulation. 2024.

11. First Aid: Guidelines From the American Heart Association and American Red Cross. — Nelson M. American Family Physician. 2026.

12. Atypical Systemic and Dermatologic Loxoscelism in a Non-Endemic Region of the USA. — Downs JW, Gould KT, Mclaughlin RC, Cumpston KL, Rose SR. Clinical Toxicology. 2021.

13. Laboratory Predictors of Hemolytic Anemia in Patients With Systemic Loxoscelism. — Jacobs JW, Bastarache L, Thompson MA. American Journal of Clinical Pathology. 2022.

14. Brown Recluse Spider Bites. A Comparison of Early Surgical Excision Versus Dapsone and Delayed Surgical Excision. — Rees RS, Altenbern DP, Lynch JB, King LE. Annals of Surgery. 1985.