Impetigo

Impetigo recruits neutrophils to the stratum corneum. Organisms are commonly visible in hematoxylin and eosin sections. Gram stain and culture may be required.

Bullous impetigo

Suppurative folliculitis

The follicle may not be visible in every plane of section. In some sections, only a focus of inflammatory cells may be noted in the dermis. The microscopic differential diagnosis includes bacterial infection (including furunculosis and hot-tub folliculitis), fungal infection, chemical folliculitis, acne, rosacea, and pustular drug eruption.

Botryomycosis

Clinically, botryomycosis may resemble a mycetoma. The grains represent huge staphylococcal colonies.

Pitted keratolysis

Pitted keratolysis is rarely biopsied because it is readily distinguished by its appearance and smell (so-called “toxic sock syndrome”). Both micrococci and diphtheroids are generally present (Kytococcus sedentarius and Corynebacterium).

Erythrasma

Ecthyma gangrenosum

Ecthyma gangrenosum is usually a manifestation of Pseudomonas sepsis.

Rhinoscleroma

Rhinoscleroma is caused by Klebsiella rhinoscleromatis. The inflammatory infiltrate is mixed, and contains many plasma cells. Russell bodies are plasma cells filled with bright pink immunoglobulin (“pregnant plasma cells”). The nucleus may no longer be visible. Mikulicz cells are histiocytes containing large round collections of bacilli.

Chancroid

Granuloma inguinale

The organism may be seen best in very thin, plastic-embedded sections, processed as for electron microscopy.

Leprosy

Indeterminate leprosy may only demonstrate mild inflammation and onion skin fibrosis around nerves. Borderline leprosy shows features intermediate between lepromatous and tuberculoid disease. New classification systems often divide disease into multibacillary (lepromatous end of spectrum) and paucibacillary (tuberculoid end) forms.

Tuberculosis

Syphilitic chancre

Syphilitic chancres and chancroid have a similar histology. Chancroid typically produces a deeper, softer, more painful ulcer, as well as suppurative lymphadenitis. Stains, culture, and serology are typically required.

Secondary syphilis

Secondary syphilis is highly variable in its clinical presentation, and almost as variable in its histologic appearance. Any suspicious feature should prompt an immunostain or silver stain, clinical evaluation, and serologic studies. False-negative prozone reactions are the result of massive antibody excess. Antigen–antibody complexes form most efficiently with mild antigen excess and can be inhibited by massive antibody excess. The serum may need to be diluted many-fold in order to test positive.

A biopsy frequently suggests the diagnosis. While most other forms of vacuolar interface dermatitis are associated with effacement of the rete, secondary syphilis characteristically demonstrates a combination of vacuolar interface dermatitis and acanthosis with long slender rete. This pattern has been referred to as an “icicle” or “icepick” pattern of acanthosis. Some students have found it helpful to remember that syphilis (a sexually transmitted disease) produces long, slender, “sexy” rete ridges with associated vacuolar interface dermatitis. Neutrophils are commonly present in the stratum corneum. Plasma cells are present in about two-thirds of cases. Small dermal blood vessels appear to have no lumen because of endothelial swelling. Another helpful feature is the presence of perivascular lymphocytes and histiocytes with visible cytoplasm. It may be difficult to decide at scanning power whether the infiltrate is granulomatous or lymphoid. A subtle interstitial infiltrate is often present as well.

Tertiary syphilis

Lyme disease

Acrodermatitis chronica atrophicans

In the evolving stage, lesions with a morphea-like appearance may show an interstitial granulomatous infiltrate, resembling granuloma annulare.

Leishmaniasis

The organisms are similar in size and shape to histoplasmosis. A kinetoplast is present, but may be difficult to see. The distribution of the organisms within the histiocyte is helpful. Histoplasma organisms are evenly spaced and surrounded by a pseudocapsule. In contrast, Leishmania organisms lack a pseudocapsule. They may be randomly spaced, or lined up at the periphery of a vacuole, like light bulbs on a make-up mirror or movie marquee. Culture, polymerase chain reaction, and quantitative nucleic acid sequence-based amplification are sensitive techniques used to detect, type, and quantify Leishmania in tissue.

Acanthamoeba

At first glance, amoeba trophozoites may look like large histiocytes with somewhat refractile nuclei. Glance again. They have a characteristic appearance, as noted in Figure 17.23. Vascular invasion and necrosis are typical. Lobular fat necrosis has been reported. Disseminated disease is frequently associated with human immunodeficiency virus (HIV) infection or immunosuppressive drugs. Cultures on an agar plate seeded with a lawn of Escherichia coli will demonstrate characteristic tracks.

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