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Skin infections may be with bacteria, fungi or viruses. Infestations with parasites are also included in this section. In some of these disorders, histology is a vital part of routine diagnosis particularly when organisms are difficult to identify or culture.
The pathology of the following skin diseases will be outlined:
Descriptions refer to routine sections stained with haematoxylin and eosin (H&E), unless otherwise specified.
|Subcorneal pustule||Neutrophils clustered just below the stratum corneum|
|Subcorneal blister||Fluid filled cavity just below the stratum corneum|
|Ulceration||Complete epidermal loss|
|Gram-positive||Tissue gram stain reveals organisms e.g. staphylococci, streptococci|
|Gram-negative||Tissue gram stain does not reveal organisms e.g. pseudomonas|
|Acid-fast||Organisms revealed by Ziehl-Neilson, auramine or Fite stain|
|Tuberculoid granuloma||Well-demarcated granuloma surrounded by a lymphocytic infiltrate.|
|Caseation||Cheese-like necrosis of collagen|
|Ballooning degeneration||Enlarged and pale keratinocytes, sometimes with pink intranuclear inclusions, acantholysis or multinucleated keratinocyte formation|
|Molluscum bodies||Eosinophilic to basophilic intracytoplasmic inclusions that push the nucleus and keratohyaline granules aside|
The most common bacterial infections of the skin are those caused by streptococci and staphylococci. Tuberculosis is described because it has characteristic granulomatous pathology.
Impetigo is generally recognised by its clinical features and confirmed by culture of gram-positive organisms found on microscopy of a skin swab. However, the histological features of impetigo should be familiar as this infection can be confused with several other skin conditions. Some inflammatory dermatoses and tumours may be secondarily infected with the same organisms as cause impetigo. Link to a clinical description of impetigo.
The histological features of impetigo are:
Cutaneous tuberculosis is uncommon in New Zealand. It has a variety of clinical presentations so the histology is often valuable in diagnosis. Link to a clinical description of cutaneous tuberculosis.
The histological features of tuberculosis are:
Fungal infections are very common and frequently misdiagnosed as inflammatory skin diseases particularly eczema.
Tinea refers to infection with dermatophyte organisms (Microsporum, Trichophyton or Epidermophyton). Link to a clinical description of tinea.
The histological features of tinea are:
Pityriasis versicolor is an infection with Malassezia yeasts. Link to a clinical description of pityriasis versicolor.
The histological features of pityriasis versicolor are:
Candidiasis is due to infection with Candida yeasts. It may have a variety of clinical presentations. Link to clinical descriptions of candida infections.
The histological features of candida infections are:
The most common viral infections are papilloma and herpes viruses, and molluscum contagiosum.
Warts are due to human papillomavirus infection. Link to a clinical description of warts.
The histological features of wart infections are:
The histological features of herpes infections are:
Molluscum shows unique histological features and is very easy for a pathologist to diagnose. Link to a clinical description of molluscum contagiosum.
The histological features of molluscum contagiosum are:
In New Zealand, scabies remains a common cause of severe itching. Link to clinical description of scabies. Skin biopsy is often unhelpful in diagnosis, as on average a patient only has 10 mites and many of the skin lesions are as nonspecific histologically as they are clinically.
The histological features of scabies are:
Histopathology is rarely employed to diagnose skin infections. Describe when it may be useful.
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