Spider Bites

Spider bites are a rare cause of morbidity and mortality. Almost all spiders are venomous, but the majority do not have the ability to penetrate skin for envenomation. Brown recluse and black widow spiders are the most common causes of serious systemic reactions in the United States. The brown recluse spider contains a necrotizing venom that can lead to a painful, blistering, necrotic wound; fevers; myalgias; hemolysis; seizures; and renal failure. Black widow spiders have a neurotoxic venom that can cause muscle cramping and rigidity (often manifesting as severe abdominal pain), vital sign instability, lacrimation and salivation, ptosis, and respiratory distress. The diagnosis is clinical. Management includes wound care, pain management, supportive care for systemic symptoms, antivenom for black widow bites, and delayed debridement of necrotic tissue in brown recluse bites.

Last update:

Table of Contents

Share this concept:

Share on facebook
Share on twitter
Share on linkedin
Share on reddit
Share on email
Share on whatsapp

Epidemiology and Species


  • Spider bites are rarely a cause of serious morbidity or mortality.
  • < 3 deaths per year in the United States
  • Severe symptoms and death are more likely to occur in children.

Clinically relevant species

In the United States, the following spider bites can result in serious systemic symptoms: 

Brown recluse spider:

  • Found in central and southern United States
  • Occupy dry, secluded areas including: 
    • Basements
    • Attics
    • Wood piles
Brown recluse spider

The highly venomous brown recluse spider Loxosceles reclusa:
Note the characteristic violin-shaped marking visible on its dorsal cephalothorax.

Image: “1125” by Margaret Parsons. License: Public Domain

Black widow spider:

  • Found worldwide, including the southern and western United States
  • Prefers outdoor habitats:
    • Firewood piles
    • Garages
    • Gardening equipment
    • Trash
    • Outdoor furniture
Female black widow spider

A female black widow spider, Latrodectus mactans, in the process of spinning her web upon a tree branch:
Note the characteristic red hourglass located on her inferior abdominal surface, which can vary in coloration from yellowish to shades of orange and red. The female’s body is a shiny, jet-black color.

Image: “20260” by James Gathany. License: Public Domain


Brown recluse spider

  • Necrotizing venom
  • Cytotoxic and hemolytic
  • Components:
    • Sphingomyelinase D: 
      • Complement-mediated erythrocyte destruction
      • Tissue destruction
    • Proteases → degradation of:
      • Collagen
      • Fibronectin
      • Fibrinogen
      • Elastin

Black widow spider

  • Neurotoxic venom
  • Affects neuromuscular transmission and autonomic function
  • Alpha-latrotoxin is the primary toxin:
    • Binds irreversibly to receptors on presynaptic neurons
    • Causes an influx of calcium ions
    • Results in the release of:
      • Acetylcholine
      • Dopamine
      • Norepinephrine
      • Epinephrine
      • Glutamate

Clinical Presentation

Brown recluse spider

  • Bite site:
    • Erythema
    • 2 small puncture wounds may be noted.
    • Center of the bite will become paler.
    • Central blister with surrounding ecchymosis
    • Blister fills with blood and ruptures.
    • Ulceration and eschar formation
  • Localized symptoms:
    • May be painless initially
    • Severe pain of the entire extremity develops 30‒60 minutes later.
    • Pruritis
  • Systemic symptoms:
    • Fever and chills
    • Nausea and vomiting
    • Arthralgias and myalgias
    • Generalized, morbilliform rash
  • Complications:
    • Seizure
    • Hypotension
    • Disseminated intravascular coagulation
    • Thrombocytopenia
    • Hemolysis
    • Renal failure

Black widow spider

  • Bite site:
    • 2 small puncture wounds may be seen.
    • Erythema and edema around the bite
    • Central, pale clearing develops (target appearance).
  • Local symptoms:
    • Immediate, sharp, stinging pain
    • Piloerection
    • Isolated diaphoresis
  • Systemic symptoms:
    • Vital signs:
      • Hypertension
      • Tachycardia
      • Tachypnea
      • Hyperthermia
    • Severe cramping pain and rigidity of large muscle groups:
      • Abdomen
      • Back
      • Chest
      • Thighs
    • Headache
    • Nausea and vomiting
    • Salivation and lacrimation
    • Anxiety
    • Ptosis
    • Facial and extremity swelling
  • Complications:
    • Respiratory distress
    • Hypertensive emergency
    • Rhabdomyolysis
    • Myocarditis
    • Ileus



Spider bites are a clinical diagnosis.

  • Spider bites are often falsely suspected by patients, especially in nonendemic regions.
  • Witnessing the bite or identification of the spider is required for confirmation.
  • MRSA skin infections should be excluded.


NOT RECLUSE” can help exclude the diagnosis of a brown recluse spider bite:

  • N: Numerous lesions are not typical of a spider bite.
  • O: occurrence (bites tend to occur when disturbing a spider)
  • T: timing (most occur between April and October)
  • R: red center (recluse bites have a pale center)
  • E: Elevated lesions are not typical.
  • C: Chronic lesions are rare.
  • L: Large lesions are rare.
  • U: ulcerates too early
  • S: Swollen lesions usually occur only on the feet or face.
  • E: Exudative lesions do not occur.


General measures

  • Wound cleansing
  • Elevation of the extremity
  • Ensure tetanus immunization is up to date.
  • Hospitalization and management for systemic manifestations

Additional measures for brown recluse spider bites

  • Cold compresses:
    • Sphingomyelinase D is temperature dependent.
    • Slows the necrosis process
  • Pain management:
    • NSAIDs
    • Opioids
  • Surgical excision of necrotic regions should be delayed until fully demarcated.

Additional measures for black widow spider bites

  • Pain management:
    • Opioids
    • Benzodiazepines for muscle relaxation
  • Black widow spider antivenom

Differential Diagnosis

  • Insect sting: bee, wasp, and ant stings can cause envenomation with localized swelling. Some patients will develop a severe allergic reaction, including anaphylaxis. Neurologic manifestations and tissue necrosis are not usually present. The diagnosis is clinical. Management includes removing the stinger (if present), antihistamines, pain control, and emergency care for anaphylaxis.
  • Scorpion sting: most scorpion stings are harmless; however, the bark scorpion is venomous. Patients will have pain and swelling at the site of the sting. Systemic manifestations can include muscle spasms, diaphoresis, abnormal neck and head movements, tachycardia, hypertension, and respiratory distress. The diagnosis is clinical. Management includes supportive care, pain control, benzodiazepines for muscle spasms, and antivenom.
  • Cellulitis: a common bacterial skin infection that affects the deeper layers of the dermis and subcutaneous tissue. This condition is most commonly caused by Staphylococcus aureus and Streptococcus pyogenes. Cellulitis presents as an erythematous, edematous area that is warm and tender to the touch. Bullae, necrosis, and systemic toxicity are not usually present. The diagnosis is clinical, and management involves antibiotics tailored to the suspected organism.
  • Necrotizing fasciitis: a life-threatening infection that causes rapid destruction and necrosis of the fascia and subcutaneous tissues. Patients may have significant pain and rapidly progressive erythema of the affected area. Fever, hypotension, altered mental status, and multisystem organ failure may also be present. The diagnosis is primarily clinical. This type of infection is a surgical emergency and requires emergent surgical debridement, parenteral antibiotics, and close hemodynamic monitoring.
  • Anthrax: an infection caused by Bacillus anthracis. Cutaneous infection presents with a painless, pruritic papule that enlarges to form an ulcer with a black eschar. Systemic symptoms such as fever, lymphadenopathy, myalgia, and nausea are common. Hematologic, neurologic, and renal complications are not seen. The diagnosis requires identification of the organism on culture, direct fluorescent antibody testing, or PCR. Antibiotics are used for treatment.
  • Mesenteric ischemia: interruption in blood flow through the mesenteric arteries resulting in ischemia of the bowel. Patients present with abdominal pain out of proportion to the clinical exam. Peritonitis, hematochezia, and shock occur with bowel infarction. No skin lesion will be seen. Computed tomography with angiography is the diagnostic modality of choice. Management is often surgical and focuses on re-establishing blood flow to the intestines.
  • Hypocalcemia: a low plasma calcium concentration that may be due to hypoparathyroidism, vitamin D deficiency, or renal disease. Patients with severe hypocalcemia may present with paresthesias, muscle cramping, tetany, hyperreflexia, and seizures. The skin lesion and other systemic symptoms of a black widow spider bite will not be seen. Low serum calcium levels provide the diagnosis, and management requires calcium supplementation and correction of the underlying etiology.


  1. Centers for Disease Control and Prevention. Venomous spiders. National Institute for Occupational Safety and Health (NIOSH). Reviewed May 31, 2018. https://www.cdc.gov/niosh/topics/spiders/
  2. Bush, S.P., Cohen, J.P., Green, S. (2020). Widow spider envenomation. In Alcock, J. (Ed.). Medscape. Retrieved March 24, 2021, from https://emedicine.medscape.com/article/772196-overview
  3. Arnold, T.C. (2018). Brown recluse spider envenomation. In Alcock, J. (Ed.). Medscape. Retrieved March 24, 2021, from https://emedicine.medscape.com/article/772295-overview
  4. Anoka, I.A., Robb, E.L., Baker, M.B. (2020). Brown recluse spider toxicity. StatPearls. Retrieved March 24, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK537045/
  5. Williams, M., Anderson, J., Nappe, T. (2020). Black widow spider toxicity. StatPearls. Retrieved March 24, 2021, from https://www.statpearls.com/ArticleLibrary/viewarticle/29298
  6. Barish, R.A., Arnold, T. (2020). Spider bites. MSD Manual Professional Version. Retrieved March 24, 2021, from https://www.msdmanuals.com/professional/injuries-poisoning/bites-and-stings/spider-bites

Study on the Go

Lecturio Medical complements your studies with evidence-based learning strategies, video lectures, quiz questions, and more – all combined in one easy-to-use resource.

Learn even more with Lecturio:

Complement your med school studies with Lecturio’s all-in-one study companion, delivered with evidence-based learning strategies.

🍪 Lecturio is using cookies to improve your user experience. By continuing use of our service you agree upon our Data Privacy Statement.