• bacterial:
  • erythrasma: brownish patches in intertriginous areas (armpits, under breasts, groin); it is due to Wood's-light fluorescing (because the bacteria contain porphyrins) Corynebacterium minutissimum (Wood's exam can be neg if patient bathed vigorously prior to office exam...therefore, could histology be false neg in same situation?. [clinical photo]¹ Histology is of Gram negative rods & filamentous shapes in the cornified layer [S07-7665]. Treat with topical gel of 2% erythromycin.
• mycobacterial:
  • histologically obvious granulomatous reaction: infectious vs. sarcoid (naked...no surrounding lymphocytes...& non caseous granulomata which tend to be separate & negative for polys, eos, or plasma cells).
  • non-granulomatous reaction:
    • when severely immunocompromised patient (AIDS, etc.).
    • some of the atypical AFBs [S-02-6281].
• viral: HSV vesicles (fever blister); dermatomal hypesthesia with & without vesicles...VZV; small pox (variola virus).
• parasitic:
  • Demodex: a mite living in hair follicles...may cause demodecosis of eyelid margins and nose [HERE]...folliculitis & erythema.
  • babesia: a group of tick-borne protozoan parasites possibly visible in a blood smear within RBCs & causing hemolysis.
• fungal: follicles & hairs [S-04-4151], skin, nails:
• Malassezia yeast/fungal ("tinea versicolor"...caused by the various M. & Pityrosporum species): pustular follicultitis, skin-multi-discolored & faintly scaling dermatosis, Gougerot-Carteau disease (confluent and reticulated papillomatosis), a pigmented eruption occurring mainly on the chest, back and neck of adolescent girls, & seborrheic dermatitis...excellent patient photos, etc., HERE. The fungal stains may show the yeasts in all cases (very scant in seb. derm.) and a mix of yeasts & hyphae in tinea versicolor (in a "spaghetti and meatballs" pattern...the skin lesions colored under Wood's light fluorescent light exam). Treatment
• dermatophytic: dermatophytosis (skin infected); onychomycosis (nails infected).
• spirochetal:
• large pox: the syphilis spirochete (Treponema pallidum).
• erythema chronicum migrans (erythema migrans): of Lyme disease & caused by tick-borne Borrelia burgdorferi (any single tick-borne illness has a reasonable likelihood that that tick or another tick from that tick infested environment might have also passed on an additional agent such as babesia). Other Borrelia cause this lesion (at least one other in S. C. in 2010) and respond to the same treatment; but our Lyme serology test is burdorferi specific.
• leptospirosis: skin jaundice when severe hepatitis.

References:

1. Miller SD, NEJMed 351(16):1666, 14 October 2004.

(posted 2 February 2004; latest addition 28 September 2010)