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# Spider bites (loxoscelism and latrodectism)

Poisoning by the venom of recluse (corner) spiders, which kill [skin](https://durgical.com/en/anatomy/skin/) and can destroy red blood cells, or widow spiders, which cause severe muscle cramps.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Skin](https://durgical.com/en/anatomy/skin/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Rectus abdominis](https://durgical.com/en/anatomy/rectus-abdominis-muscle/)
- [Gastrocnemius (inner head)](https://durgical.com/en/anatomy/medial-head-of-gastrocnemius/)
- [Gastrocnemius (outer head)](https://durgical.com/en/anatomy/lateral-head-of-gastrocnemius/)
- [Spleen](https://durgical.com/en/anatomy/spleen/)
- [Liver](https://durgical.com/en/anatomy/liver/)

## Why it happens

- [Patterns of necrosis: coagulative, liquefactive, caseous, fat, fibrinoid, gangrenous](https://durgical.com/en/general-pathology/patterns-of-necrosis-coagulative-liquefactive-caseous-fat-fibrinoid-gangrenous/)
- [Synaptic transmission](https://durgical.com/en/physiology/synaptic-transmission/)
- [Sphingomyelin](https://durgical.com/en/biochemistry/molecules/sphingomyelin/)

## What it is

Envenomation by medically important spiders. Recluse or corner spiders (Loxosceles) cause loxoscelism: a painful bite that can turn into a spreading area of dead skin and, less often, a systemic form with destruction of red blood cells and [kidney](https://durgical.com/en/anatomy/kidney/) injury. Widow spiders (Latrodectus) cause latrodectism: a neurotoxic syndrome with intense pain, muscle cramps, abdominal rigidity, sweating and high blood pressure.

## Who gets it

Loxosceles species live inside homes across Latin America, hidden behind furniture, pictures and clothing, and Chile, Peru, Brazil and Argentina report the most cases; in Colombia bites are reported but are less frequent. Widow spiders live in fields, crops, woodpiles and outdoor latrines, so farm workers are most often bitten. Many skin lesions blamed on spiders are actually bacterial infections, especially community MRSA.

## Causes

- Loxosceles bite: venom rich in sphingomyelinase D, which damages cell membranes and activates complement
- Latrodectus bite: alpha-latrotoxin, which forces massive release of neurotransmitters at nerve endings
- Bites usually happen when the spider is pressed against the skin while dressing, in bed or handling stored items

## Risk factors

- Clothes, shoes or bedding left unchecked in houses with corners, cracks and clutter
- Rural and agricultural work, wood stacking and outdoor latrines (widow spiders)
- Young children and older adults (more severe systemic disease)
- Delay in seeking care

## Pathophysiology

Recluse venom sphingomyelinase D damages endothelium and skin cells and triggers complement, neutrophil influx and thrombosis of small vessels, producing ischaemic necrosis that spreads by gravity. In the systemic form it attacks red cell membranes, causing intravascular haemolysis, haemoglobinuria and acute kidney injury. Widow alpha-latrotoxin opens presynaptic channels and empties vesicles of acetylcholine and noradrenaline, causing painful muscle spasm and an adrenergic storm.

## Classification

- Cutaneous loxoscelism: local lesion only (most cases)
- Cutaneous-visceral (systemic) loxoscelism: haemolysis, jaundice, kidney injury, sometimes coagulopathy
- Latrodectism: local pain only, or systemic neuromuscular and autonomic syndrome

## Symptoms and signs

- Loxosceles: bite often painless at first, then burning pain over hours
- Loxosceles: livedoid plaque with a pale centre, red ring and purple areas ('red, white and blue' sign), later a dark eschar and ulcer
- Systemic loxoscelism: fever, malaise, dark red or brown urine and jaundice in the first days
- Latrodectus: sharp pain at the bite, spreading cramps of the limbs, back, chest and abdomen
- Latrodectus: board-like abdominal rigidity, sweating (sometimes only around the bite), hypertension, tachycardia, restlessness and facial grimacing

## Red flags

- Dark urine, jaundice or falling haemoglobin (haemolysis)
- Reduced urine output or rising creatinine
- Lesion on the face or a rapidly spreading lesion
- Severe hypertension, chest pain or breathing difficulty with latrodectism
- Children, pregnant women and older adults with systemic signs

## Diagnosis

**Clinical**
Mostly clinical: history of a spider seen or captured, place and time of the bite, and the typical lesion or neuromuscular picture. A captured spider helps identify it. Many 'spider bites' without a seen spider are infections.

**Tests**
In loxoscelism: complete blood count, haemolysis markers (bilirubin, LDH, haptoglobin, reticulocytes), urinalysis for haemoglobin, creatinine and electrolytes, coagulation tests and creatine kinase, repeated in the first days. In latrodectism: ECG, creatine kinase, and tests to rule out other causes of acute [abdomen](https://durgical.com/en/anatomy/region/abdomen/) or [chest](https://durgical.com/en/anatomy/region/chest/) pain.

**Imaging**
Not needed to confirm the diagnosis; used to exclude surgical abdomen or other causes when latrodectism mimics them.

## Differential diagnosis

- Skin and soft tissue infection or abscess, including MRSA
- Necrotizing fasciitis
- Pyoderma or vasculitic ulcer
- Acute abdomen (appendicitis, pancreatitis, perforation) for latrodectism
- Acute coronary syndrome, tetanus or scorpion sting for latrodectism

## Goal of treatment

Control pain, limit the skin lesion, detect haemolysis and kidney injury early, and give specific antivenom when indicated.

## Treatment

**Acute**
Clean the wound, cold compresses, elevate and rest the limb, tetanus prophylaxis, and observation for systemic signs.

**Medical**
Analgesia; in latrodectism, opioids and benzodiazepines for spasm. Specific antivenom (anti-Loxosceles or anti-Latrodectus) where available and indicated, most useful early. Corticosteroids are used in some Latin American protocols for systemic loxoscelism. Antibiotics only for secondary infection.

**Follow-up**
Repeated blood count, urinalysis and creatinine in suspected loxoscelism; blood pressure, cardiac monitoring and fluid balance in systemic latrodectism; nursing watch of urine colour and output.

**Surgical**
Early excision of a recluse lesion is avoided; once the necrosis is delimited, debridement and grafting may be needed.

## Complications

- Skin ulcer, scarring and secondary infection
- Intravascular haemolysis with acute kidney injury
- Disseminated intravascular coagulation (rare)
- Hypertensive crisis, arrhythmias or myocarditis in latrodectism
- Rhabdomyolysis

## Prognosis

Most bites cause local disease that heals in weeks, sometimes with a scar. Systemic loxoscelism carries a real risk of death, mainly in children, from haemolysis and kidney failure. Latrodectism is rarely fatal, but pain can last days.

## Prevention

- Shake out clothes, shoes and bedding before use and move beds away from walls
- Clean corners and remove clutter; wear gloves when handling wood or stored items
- Seal cracks and use screens

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme in recluse venom causes skin necrosis and haemolysis?

Sphingomyelinase D, which damages membranes and activates complement, leading to small-vessel thrombosis, necrosis and, in the systemic form, intravascular haemolysis.

## What laboratory follow-up does suspected loxoscelism need in the first days?

Repeated blood count, haemolysis markers, urinalysis for haemoglobin and creatinine, because systemic haemolysis and kidney injury appear within 24 to 72 hours.

## Which spider bite can mimic an acute abdomen with board-like rigidity?

The widow spider (Latrodectus): its alpha-latrotoxin causes massive neurotransmitter release, painful spasm, abdominal rigidity, sweating and hypertension.

## What is the most common true cause of a necrotic skin lesion blamed on a spider bite that nobody saw?

Bacterial skin infection, often community MRSA; spider bite should not be diagnosed without a compatible history.

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## Related conditions

- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Rhabdomyolysis](https://durgical.com/en/conditions/rhabdomyolysis/)
- [Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene)](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/)
- [Cellulitis of the leg](https://durgical.com/en/conditions/cellulitis-of-the-leg/)
- [Autoimmune haemolytic anaemia](https://durgical.com/en/conditions/autoimmune-haemolytic-anaemia/)
- [Disseminated intravascular coagulation](https://durgical.com/en/conditions/disseminated-intravascular-coagulation/)
- [Scorpion envenomation (Tityus)](https://durgical.com/en/conditions/scorpion-envenomation/)
- [Snakebite envenomation (Bothrops, Lachesis, Crotalus)](https://durgical.com/en/conditions/snakebite-envenomation/)
- [Acute appendicitis](https://durgical.com/en/conditions/acute-appendicitis/)
- [Acute pancreatitis](https://durgical.com/en/conditions/acute-pancreatitis/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Brown recluse spider](https://medlineplus.gov/ency/article/002859.htm) (MedlinePlus, US National Library of Medicine, 2026); [Black widow spider](https://medlineplus.gov/ency/article/002858.htm) (MedlinePlus, US National Library of Medicine, 2026).

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Source: https://durgical.com/en/conditions/spider-bites/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/spider-bites/