Skin-nontumor
Granulomatous but non-infectious disorders
Arthropod bites

Authors: Ifeoma U. Perkins, M.D. (see Authors page)
Editors: Jerad M. Gardner, M.D., Lauren Stuart, M.D.

Revised: 13 May 2016, last major update May 2016

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Arthropod bite
Definition / General
  • Seen in all age groups in various cutaneous sites
  • Caused by fleas, bed bugs, biting flies, mosquitoes, ants, ticks and spiders
  • Most bites are clinically self limited and not biopsied
  • Bites from brown recluse and black widow spiders have the greatest potential morbidity, causing necrosis requiring operative debridement and occasionally amputation
Essential Features
  • Variable clinical presentation from erythematous papule to nodular lesion with induration, ulceration and vesicles
  • Often self limited; extensive or persistent cutaneous reactions may be treated with intralesional steroids
  • Typically manifests microscopically with a mixed infiltrate of lymphocytes, histiocytes and eosinophils
  • Exuberant cases with dense deep lymphocyte rich infiltrate may mimic a low grade lymphoma
  • Spider bites often show necrosis and vasculitis
Sites
  • Variable, relating to site of exposure, including:
    • Flea bites: ankles
    • Mosquito bites: exposed skin
    • Chigger bites: socks and belt lines
Clinical Features
  • Varied clinical presentation ranging from small clusters of erythematous papules to large deep violaceous nodules with induration, ulceration or vesicles
  • May resemble lymphoma or ulcerated carcinoma
  • A solitary punctum may be found at the site where insect mouth parts contact the skin
  • Spider bites may have necrosis with nonviable tissue present at the ulcer base
Treatment
  • Most bites are self limited
  • Antihistamines may provide symptomatic relief
  • Severe mosquito reactions ("Skeeter syndrome") may require prednisone (Am Fam Physician 2013;88:841)
Clinical Images

Images hosted on other servers:

Various images

Micro Description
  • Typically wedge shaped superficial and deep mixed inflammatory infiltrate composed of lymphocytes, histiocytes, eosinophils and sometimes neutrophils
  • Scattered eosinophils in interstitial areas (away from vessels), especially in deep dermis, are a useful clue
  • May also have a prominent granulomatous component
  • Dermal edema commonly seen; vessels may be prominent
  • Variable epidermal features including spongiosis, acanthosis, parakeratosis; these are most prominent in center of lesion where mouth parts penetrate the epidermis
  • Excoriated lesions may have prominent parakeratosis with scale crust formation, epidermal erosion or overt ulceration
  • May have lymphoid germinal centers resembling lymphoma, with destruction of adnexae, particularly sweat glands (J Cutan Pathol 2009;36:26)
  • Mouth parts may be identified in center of lesion, particularly in tick bites
  • May have secondary vasculitis
  • Spider bites: often show marked necrosis with extensive suppurative neutrophilic dermal inflammation, often extending into underlying subcutaneous tissue; small and large vessel vasculitis is also common
  • Fire ant stings:
    • Acute phase: may be characterized by an urticarial reaction
    • Later stages: mixed inflammatory infiltrate consisting of eosinophils and neutrophilic pustules
Micro Images

Images hosted on other servers:

Wedge shaped and perivascular inflammatory infiltrate of lymphocytes and eosinophils

Differential Diagnosis
  • Allergic contact dermatitis: may occasionally have increased eosinophils but infiltrate is usually mostly in superficial not deep dermis; often has more broad epidermal changes including spongiosis, parakeratosis and Langerhans cell microabscesses; necrosis and vasculitis are uncommon
  • Eosinophilic folliculitis: perifollicular infiltrate with numerous eosinophils; may be associated with HIV and thus diagnosis should be made with caution; clinical correlation very important
  • Dermal hypersensitivity reaction (e.g. drug eruption): can have very similar appearance to arthropod bite reaction; clinical correlation is best way to separate; presence of epidermal changes or wedge shaped infiltrate may favor arthropod bite
  • Lymphomatoid papulosis: scattered or numerous CD30+ atypical lymphocytes
  • Well's syndrome (eosinophilic cellulitis): can have very similar appearance; superficial and deep infiltrate with eosinophils that may be so numerous that they degranulate, and free granules coat degenerated collagen bundles ("flame figures")
    • Well's syndrome is a diagnosis of exclusion - must rule out arthropod bite reaction (may have flame figures) and drug eruption clinically