The epidermis at the periphery of the chancre shows marked acanthosis, but at the center it becomes thin and is eventually lost. The base of the ulcer is infiltrated with lymphocytes and plasma cells, particularly adjacent to the blood vessels, in which there is prominent endothelial swelling. The treponemes can usually be demonstrated by appropriate silver impregnation techniques, such as the Levaditi or Warthin–Starry stains. The May–Grünwald–Giemsa technique can also be used to demonstrate organisms. ³⁸ PCR-based methods may be used on paraffin-embedded skin biopsy specimens to confirm the presence of the organism. ³⁹ On dark-field examination of a smear from a chancre the Treponema pallidum can be seen as a thin, delicate spiral organism 4–15 µm in length and 0.25 µm in diameter. Electron microscopy has shown T. pallidum to be principally in the intercellular spaces in the vicinity of small blood vessels, as well as in macrophages, endothelial cells, and even plasma cells. ⁴⁰ Collagen fibers appear to be damaged by the treponeme. ⁴¹

There is considerable variation in the histological pattern.^{43.80.81. and 82.} Plasma cells may be absent or sparse in up to one-third of all biopsies, and the vascular changes may not be prominent. The infiltrate usually involves both the superficial and the deep dermis, except in the macular lesions, where it is more superficial (Fig. 24.1). Perineural plasma cells are relatively common. ⁸³ Extension of the inflammatory infiltrate into the subcutis is uncommon. ⁴³ The infiltrate is predominantly lymphocytic, with some histiocytes and variable numbers of plasma cells (Fig. 24.2). The histiocytic cells express both CD4 and CD14, while the majority of lymphocytes are CD8⁺. ⁸⁴ Plasma cells are less numerous in macular lesions. ⁸⁵ Angiogenesis is common. ⁸⁶ Early lesions often show a neutrophilic vascular reaction, which is presumed to be related to immune complex deposition. ⁸⁷ A heavy neutrophil infiltrate resembling that in Sweet’s syndrome has been reported. ⁸⁸ Follicles and sweat glands may be sleeved by inflammatory cells. ⁸⁰ In alopecia syphilitica, a lymphoid infiltrate around hair follicles and follicular keratotic plugging are almost invariable.^{75. and 76.} Peribulbar lymphoid aggregates may be present. ⁷⁵ Immunohistochemical studies have shown the presence of T. pallidum limited to the peribulbar region and penetrating into the follicle matrix. ⁸⁹ Sometimes the dermal infiltrate in secondary syphilis is dense and diffuse, and it has been likened to cutaneous lymphoma,^{90. and 91.} although its heterogeneous nature is against lymphoma. Approximately 5–10% of the cells may be CD30⁺. ⁹² A granulomatous pattern may also be seen in secondary syphilis, ⁹³ particularly after about 16 weeks of the disease. ⁸⁰ Epithelioid granulomas may be found in late secondary syphilis (Fig. 24.3), and there is a report of a palisading granuloma similar to that seen in granuloma annulare. ⁹⁴ Perineural granulomas simulating leprosy may be present. ⁹⁵ Sarcoidal granulomas are rare. ⁴⁹ The epidermis is frequently involved. It may show acanthosis with spongiosis, psoriasiform hyperplasia, and spongiform pustulation, with considerable exocytosis of neutrophils (Fig. 24.4). ⁸¹ A lichenoid tissue reaction may be present, particularly in late lesions (Fig. 24.5). ⁹⁶ In the rare ulcerative form there is necrosis of the upper dermis, ⁵⁹ whereas in the follicular type there is microabscess formation in the outer root sheath of the hair follicle, or a follicular pustule (Fig. 24.6). There may be perifollicular granulomas. ⁵⁶ Condylomata lata have marked epidermal hyperplasia and a dermal infiltrate which is similar to that seen in other lesions of secondary syphilis. T. pallidum may be identified in tissue sections using silver stains such as the Warthin–Starry stain, or by immunoperoxidase techniques. ⁹⁷ The latter are more sensitive than silver stains, ⁹⁸ with a sensitivity of 71% in one study compared to 41% for the silver stain. ⁸³ Organisms are found in the dermis more often than the epidermis using immunohistochemistry. ⁸³ The molecular detection of Treponema pallidum, using PCR-based techniques, is likely to become the ‘gold standard’.^{39. and 99.}

Subepidermal vesicles have been reported, uncommonly, as a manifestation of the Jarisch–Herxheimer reaction. ⁴⁴ The bullae reported in a case of bullous secondary syphilis were also subepidermal. ⁵⁵

The gummas of tertiary syphilis have large areas of gummatous necrosis with a peripheral inflammatory cell infiltrate which includes lymphocytes, macrophages, giant cells, fibroblasts, and plasma cells. The giant cells are often of Langhans type. There is usually prominent endothelial swelling and sometimes proliferation involving small vessels. Attempts to demonstrate T. pallidum by silver staining techniques are usually unrewarding, although indirect immunofluorescence and PCR-based assays have been used with success.^{99. and 109.} The nodular or tuberculoid lesions show hyperkeratosis, often overlying an atrophic epidermis. There is a superficial and deep mixed inflammatory cell infiltrate, which usually includes plasma cells and tuberculoid granulomas. ¹¹⁰ Plasma cells may be sparse in an occasional case. ¹¹¹

The appearances of the primary and secondary lesions are similar. There is usually prominent epidermal hyperplasia, which is usually of pseudoepitheliomatous rather than psoriasiform type, with overlying scale crust and superficial epidermal edema (Fig. 24.7). There are intraepidermal abscesses and a heavy superficial and mid-dermal infiltrate of plasma cells, lymphocytes, macrophages, neutrophils, and often a few eosinophils. The neutrophils are more prominent superficially. Blood vessels show minimal endothelial swelling. The ulcerative lesions resemble those of syphilis.

The spirochete of yaws, like that of pinta, is most often demonstrated by silver techniques in the epidermis, in contrast to that of syphilis, which is found in the upper dermis (Fig. 24.8).

Primary and secondary lesions are identical and show hyperkeratosis, parakeratosis, and acanthosis. There is exocytosis of inflammatory cells, sometimes with intraepidermal abscesses. Hypochromic areas show loss of basal pigmentation with numerous melanophages in the upper dermis. The dermal infiltrate, like the other changes, is heavier in established than in early lesions, and includes lymphocytes, plasma cells, and sometimes neutrophils. The infiltrate is predominantly superficial and perivascular. The treponemes can be demonstrated by silver methods: they are present mainly in the upper epidermis and are seldom, if ever, found in the dermis.

There is a superficial and deep perivascular and interstitial infiltrate of lymphocytes, sometimes with abundant plasma cells and eosinophils. ¹⁸² Plasma cells are an inconsistent finding. ¹⁷⁰ Eosinophils are only prominent adjacent to the site of the initial tick bite. ¹²⁹ Rarely there are scattered neutrophils. In more than half of the cases the infiltrate extends into the subcutis. The dermal infiltrate can be equally sparse or moderately dense to dense. ¹⁷⁰ When heavy, the infiltrate is more likely to involve the subcutis. Pseudolymphomatous features with germinal centers are uncommon. Interstitial histiocytes (CD68⁺) are common and perivascular dendritic cells much less frequent. ¹⁷⁰ Epidermal changes include spongiosis, vacuolar change, apoptotic keratinocytes, and lymphocyte exocytosis. ¹⁷⁰ With the Warthin–Starry silver stain a spirochete can be found in nearly half of the specimens in the papillary dermis near the dermoepidermal junction.^{183.184. and 185.} The diagnosis requires an index of suspicion based on the clinical features. The small size of the organism places some limits on its identification using conventional microscopic techniques. ¹⁷⁸ The diagnosis has also been confirmed using an indirect immunofluorescence technique with a monoclonal antibody to the axial filaments of several Borrelia species. ¹⁸⁶ Monoclonal antibodies can also be used in immunoperoxidase techniques. A technique that uses PCR allows the molecular detection of B. burgdorferi in formalin-fixed paraffin-embedded lesions of this condition.^{170.187. and 188.}

Focus floating microscopy (FFM) has been described recently as the new ‘gold standard’ for the diagnosis of cutaneous borreliosis. ¹⁷¹ It detected organisms in 30 of 32 cases of erythema migrans. This immunohistochemical method involves staining sections with a B. burgdorferi antibody and then simultaneously scanning sections through two planes, using standard histological equipment. ¹⁷¹ FFM ‘scans through the sections in 2 planes: horizontally in serpentines as in routine cytology and, simultaneously, vertically by focusing through the thickness of the cut (usually 3–4 µm)’. ¹⁷¹

The early stages of the disease show a superficial and deep chronic inflammatory cell infiltrate in the dermis which is moderately heavy and composed predominantly of lymphocytes with some histiocytes and plasma cells. There is often accentuation around blood vessels, which may show telangiectasia, and also around adnexae. Sometimes there is a superficial band-like infiltrate of inflammatory cells with a thin zone of collagen separating the inflammatory cells from the basal layer. Scattered vacuoles or groups of vacuoles that morphologically resemble fat cells (but which do not appear to stain for fat – pseudolipomatosis cutis) have been reported in the upper dermis in some cases (see p. 848). ¹⁸⁹ As the lesions progress there will be atrophy of the dermis to about half its normal thickness or less (Fig. 24.9). This is usually accompanied by loss of elastic fibers and pilosebaceous follicles, atrophy of the subcutis, and variable epidermal atrophy with loss of the rete pegs. ¹⁹⁰