Many clinical conditions are sometimes attributed to spiderbites but the most common are ulcers, necrotic lesions or other skin conditions. This gives rise to the term necrotic arachnidism which occurs in many parts of the world with different spiders being blamed for these lesions. In some parts of the world, mainly South America, Loxoscelism occurs and is true necrotic arachnidism because bites from Loxosceles spiders can result in cutaneous lesions. Other than this there is no evidence for other spiders causing necrotic lesions and alternate diagnoses must be looked for.

An approach to the investigation and diagnosis of necrotic skin ulcers, presenting as suspected spider bites (From Isbister and Whyte, 2004).1-3


1. Establish whether there is a history of a definite spider bite:
  • IF a clear history of a spider bite (best if spider is caught) --> Treat as a definite spider bite
  • IF there is NO history of a spider bite --> Investigation and treatment should focus on the ulcer/skin lesion and the diagnosis of a suspected spider bite is not appropriate.
2. Clinical history and examination:
  • Consider features suggestive of infection, malignant disease or vasculitis.
  • Consider underlying disease processes: diabetes, vascular disease
  • Environmental exposure: chemical, infective, soil
  • Prescription medications
  • History of minor trauma
  • Specific historical information about the ulcer may assist in differentiating some conditions:
Painful of painless
Onset and timing of progression
Preceding lesion
3. Investigation:
a. Skin biopsy:
  • Microbiology with appropriate transport media: contact the microbiology laboratory before collecting specimens so that appropriate material and transport conditions are used for organisms such as Mycobacterium spp., fungi and unusual bacterial.
  • Histopathology: discuss with pathologist.

b. Laboratory investigations: may be important for underlying conditions (autoimmune conditions, vasculitis and pyoderma gangrenosum). These could include, but should not be limited to:
  • full blood count, coagulation studies
  • biochemistry (including liver and renal function tests)
  • autoimmune screening tests, cryoglobulins
  • chest radiography
  • colonoscopy
  • vascular function studies of lower limbs
4. Treatment:
  • Local wound management: essential and may require daily dressings.
  • Appropriate treatment based on established pathology.
  • Investigation and treatment of underlying conditions may be important such as diabetes or malignant disease.
5. Follow up and monitoring
  • Diagnosis: may take weeks or months to become clear.
  • Essential that these patients are followed up and reviewed.
  • Continuing management: coordinated with multiple specialities involved

1. Isbister GK, Whyte IM. Suspected white-tail spider bite and necrotic ulcers. InternMedJ. 2004; 34(1-2): 38-44.
2. Isbister GK, Fan HW. Spider bite. Lancet. 2011; 378(9808): 2039-47.
3. Isbister GK. Necrotic arachnidism: the mythology of a modern plague. Lancet. 2004; 364(9433): 549-53.












Table 1: An approach to the investigation and diagnosis of necrotic skin ulcers, presenting as suspected spider bites (From Isbister and Whyte, 2004).1-3

  1. Establish whether there is a history of a definite spider bite:
· IF a clear history of a spider bite (best if spider is caught)
Ø Treat as a definite spider bite
· IF there is NO history of a spider bite
Ø Investigation and treatment should focus on the ulcer/skin lesion and the diagnosis of a suspected spider bite is not appropriate.
  1. Clinical history and examination:
· Consider features suggestive of infection, malignant disease or vasculitis.
· Consider underlying disease processes: diabetes, vascular disease
· Environmental exposure: chemical, infective, soil
· Prescription medications
· History of minor trauma
· Specific historical information about the ulcer may assist in differentiating some conditions:
Ø Painful of painless
Ø Onset and timing of progression
Ø Preceding lesion
  1. Investigation:
· Skin biopsy:
Ø Microbiology with appropriate transport media: contact the microbiology laboratory before collecting specimens so that appropriate material and transport conditions are used for organisms such as Mycobacterium spp., fungi and unusual bacterial.
Ø Histopathology: discuss with pathologist.
· Laboratory investigations: may be important for underlying conditions (autoimmune conditions, vasculitis and pyoderma gangrenosum). These could include, but should not be limited to:
Ø full blood count, coagulation studies
Ø biochemistry (including liver and renal function tests)
Ø autoimmune screening tests, cryoglobulins
Ø chest radiography
Ø colonoscopy
Ø vascular function studies of lower limbs
  1. Treatment:
· Local wound management: essential and may require daily dressings.
· Appropriate treatment based on established pathology.
· Investigation and treatment of underlying conditions may be important such as diabetes or malignant disease.
  1. Follow up and monitoring
· Diagnosis: may take weeks or months to become clear.
· Essential that these patients are followed up and reviewed.
· Continuing management: coordinated with multiple specialities involved