ECVD HistoRound

Mucocutaneous pyoderma

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  • 定義不清；黏膜皮膚交界的紅斑、糜爛、潰瘍、痂、色素變化，抗菌治療會消退、且超出表皮（Fig 7b）。
  • 病原不明，可能是皮膚常在菌＋Pseudomonas（潮濕處）。
  • 口周／陰門周時難與間擦傷 Intertrigo 區分；德國牧羊犬好發；須與黏膜皮膚紅斑性狼瘡（MCLE）鑑別。olivryMucocutaneousLupusErythematosus2015
  • 本指引在「病灶類似間擦傷但更嚴重（偶潰瘍代表更深）」時，將其歸為表淺膿皮症。

Pathogenesis

Wiemelt et al. 2004 — The exact pathomechanism of MCP is not fully elucidated. Key hypotheses proposed to explain overlapping histopathology with DLE/LLDN:

  • Plasma cells resulting from secondary infection may complicate identification of lymphocytes, shifting a lymphocyte-rich (LLDN) pattern toward a plasmacytic (MCP) pattern
  • Mucocutaneous tissue responds to any form of chronic injury (rubbing, licking, scratching) with a plasmacellular subepidermal infiltrate — meaning any trauma to the nasal planum of a dog with LLDN may mimic MCP histologically
  • The conditions may occur along a histopathological continuum rather than representing discrete entities
  • There may be more than one histopathological pattern consistent with autoimmune nasal disease, further blurring the line

Fleischman et al. 2023 — MCP was found as a concurrent comorbidity in 5/14 (35%) GSDs with nasal alar arteriopathy. Bacteria were identified cytologically in 5/14 dogs, but always at geographically distinct foci (mucocutaneous junction crusting), not within the arteriopathy fissures themselves — suggesting MCP and the vasculopathy are separate, co-occurring processes in GSDs.

Critical finding: Pattern M (classically “MCP”) was seen in both antibiotic-responsive AND immunomodulatory-responsive cases — histopathology could not predict treatment response or distinguish infection from autoimmunity in most cases (P values all non-significant for individual features).

Duration effect: A predominantly plasmacytic infiltrate was significantly associated with longer disease duration (mean 28.5 months vs. 13.5 months; P = 0.026). This suggests that even LLDN/DLE cases may shift toward a plasmacytic pattern over time, mimicking MCP.

Fleischman et al. 2023 used the following histopathological definition of MCP:

  • Lichenoid lymphoplasmacytic dermatitis WITHOUT basal cell damage (to distinguish from facial DLE, which requires robust lymphoplasmacytic lichenoid interface dermatitis with basal cell apoptosis and epidermal thinning)
  • Cases with only subtle or focal interface change were classified as overlapping MCP/DLE
  • MCP co-existed with nasal alar arteriopathy in 5/14 GSDs; bacteria (cocci) were found in crusts or follicular infundibula in 5/14, always at mucocutaneous sites rather than the arteriopathic fissures

  • Treatment Comparison (Wiemelt 2004)

Antibiotic-Responsive (AbR) — n = 4:

  • Complete/near-complete resolution on oral antibiotics ± topical antibiotics (mupirocin in 1 case)
  • One dog improved only mildly on oral steroids, then resolved completely on oral antibiotics
  • Duration of antibiotic treatment: 4 to >10 weeks
  • Note: recurrence after stopping topical antibiotic in 1 case

Immunomodulatory-Responsive (ImR) — n = 11:

  • Required immunomodulatory/immunosuppressive drugs for resolution or maintenance
  • Regimens included: oral prednisone, tetracycline + niacinamide ± vitamin E/fatty acids, topical tacrolimus 0.1%, topical betamethasone, azathioprine
  • 4/11 received adequate antibiotics first → only mild-moderate improvement → full resolution after adding immunomodulatory therapy
  • All 11 required ongoing immunomodulatory maintenance (disease controllable, not curable)

Key clinical conclusion (Wiemelt 2004): Histopathology alone cannot predict treatment response. A stepwise approach — antibiotics first, re-evaluate, then add immunomodulatory treatment if response is incomplete — is the practical strategy.

Fleischman et al. 2023 (NAA with concurrent MCP — GSD cohort):

  • No dogs received antimicrobials alone
  • Initial treatments: tacrolimus, prednisone, ciclosporin-modified, pentoxifylline, doxycycline/niacinamide, cefpodoxime, surgical excision
  • Of 7 dogs with long-term follow-up: 5/7 (71%) complete response, 2/7 (29%) partial response
  • 6/7 (86%) required ongoing immunomodulatory treatment to maintain remission
  • Surgical excision (6–8 mm punch biopsy with closure) was used in 5/10 dogs and contributed to resolution in the NAA component

  • Histopathological Features of MCP — Full Picture

Epidermal/Epithelial Changes

  • Hyperplasia of epidermis or mucosal epithelium
  • Lymphocytic exocytosis with variable spongiosis — extending to superficial hair follicles
  • Scattered eosinophilic shrunken keratinocytes (apoptosis), typically above the basal cell layer
  • Neutrophilic serocellular crusting in lesions with self-trauma/erosion/ulceration
  • Neutrophils may form superficial exudative crusts ± parakeratosis cap

Dermal Changes

  • Perivascular to lichenoid infiltrate
    • Perivascular pattern in milder/peripheral lesions
    • May still be perivascular even in severely ulcerated lesions
  • Infiltrate composition: predominantly plasma cells + lymphocytes, neutrophils, macrophages ± eosinophils
  • Infiltrate closely approximates overlying epidermis, but dermal-epidermal interface is NOT obscured ← key distinguishing feature from DLE
  • Dilated blood vessels
  • Pigmentary incontinence (mild to severe)
  • In haired skin: inflammation surrounds adnexal appendages; may form a continuous deep lichenoid band to the level of deep adnexal dermis

Integration with Wiemelt 2004 & Fleischman 2023

FeatureTextbook (Gross et al.)Wiemelt 2004Fleischman 2023
Infiltrate typePredominantly plasmacytic, perivascular to lichenoidPattern M: band-like plasmacytic; Pattern M/L: lymphoplasmacyticLichenoid lymphoplasmacytic
DEJ obscured?No — infiltrate approximates but does not obscurePattern M: not obscured; Pattern L (DLE): obscuredNot obscured in MCP; obscured = DLE
Basal cell damageApoptosis above basal layer (not in basal layer)Pattern M: minimal/absent; Pattern L: vacuolar degeneration + Civatte bodiesMCP = no basal cell damage; DLE = present
ExocytosisLymphocytic, with spongiosisLymphocytic exocytosis scored (not statistically discriminatory)Not specifically reported
Adnexal involvementCommon — may form deep confluent lichenoid bandNot specifically quantifiedNot specifically reported

Key Diagnostic Points

What distinguishes MCP from DLE histologically:

The single most reliable differentiator is basal cell integrity:

  • MCP → apoptosis occurs above the basal layer; DEJ is close but not obscured
  • DLE → vacuolar degeneration + Civatte bodies in the basal layer; DEJ obscured by lymphocytic interface infiltrate

Why this still fails clinically (Wiemelt 2004):
Chronic MCP or secondarily infected DLE can both produce a plasmacytic band-like infiltrate. When disease is longstanding, the infiltrate becomes predominantly plasmacytic in both entities (P = 0.026 for duration correlation), erasing the lymphocytic interface pattern of DLE — making histopathologic separation unreliable.

What the textbook adds beyond the papers:

  • Confirms that lip-fold and vulvar-fold intertrigo at mucocutaneous junctions is histopathologically identical to MCP — only clinical context separates them
  • Other chronic mucocutaneous diseases (mucous membrane pemphigoid, facial pemphigus foliaceus, DLE) can all share the same pattern; primary lesion features (subepidermal clefting, acantholytic pustules, basal cell degeneration) must be actively sought but may be subtle or absent in chronic cases
  • This stereotypic plasmacellular response represents a non-specific mucocutaneous reaction pattern to any persistent antigenic stimulus — including chronic bacterial infection

Bottom line: MCP’s histopathology is a diagnosis of exclusion with clinical correlation. Confirm infection cytologically, treat, and confirm resolution — consistent with the ISCAID 2025 stepwise approach.

 fleischmanClinicalHistopathologicalFeatures2023a wiemeltRetrospectiveStudyComparing2004a [[]]

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