The term congenital melanocytic nevus (CMN) refers to a melanocytic nevus that either was present at birth or became apparent by the end of the first year of life. The term tardive CMN is used for a melanocytic nevus not visible at birth that becomes clinically apparent in early childhood. Congenital melanocytic nevi can have a variety of presentations and may not involve only the face,…
Acquired melanocytic nevi are thought to result from activation of oncogenes either through an activating mutation or translocation, allowing for clonal proliferation of the affected melanocytes. In the majority of common acquired, including the so-called dysplastic, nevi, the initiating oncogenic event is either an activating mutation in BRAF (80%) or NRAS (5%). Mutations in GNAQ or GNA11 are associated with blue nevi, whereas most Spitz nevi…
The term melanotic macule is used to refer to benign flat pigmented lesions, which are histopathologically characterized by melanin pigment deposited in basilar keratinocytes without or at times with a slight increase in the density of solitary units of junctional melanocytes. Cutaneous and mucosal melanotic macules are often labeled lentigo . The conjunctival equivalent has historically been termed acquired melanosis . In contrast to melanocytic nevi,…
Introduction A variety of common and uncommon structures may be identified in histologic and cytologic preparations that bear a striking resemblance to parasites, fungi, bacteria, or viral cytopathic changes. These infectious mimics may confuse the examiner and thus lead to erroneous diagnoses and potentially unnecessary or harmful treatments. Misinterpretation of special stains may also contribute to diagnostic errors. It is therefore essential for the pathologist to…
Introduction The protozoa that infect humans encompass a vast spectrum of organisms from several different phyla. Although all are eukaryotic, single-celled, microscopic, and nonphotosynthetic, they exhibit diverse life cycles, means of infectivity, types of motility, pathogenicity, and location within the human host. This chapter focuses on the most important human pathogens within this complex group of organisms. Malaria and Babesiosis Malaria and babesiosis are two different…
Introduction Helminthic infections comprise a great variety of worms that parasitize virtually every organ in the human body. Although helminthic infections occur throughout the world, they are more common in the subtropics and tropics. This is usually related to climatic factors, human customs, and occurrence in countries with less well developed systems of sanitation and education. Human infections are usually acquired when someone ingests the parasite,…
Introduction Yeasts and yeastlike fungi are commonly encountered in human tissue specimens from patients with fungal infections. Alterations in normal anatomy, due to the presence of indwelling catheters, and alterations in normal microbiota, which occur with antibiotic use, provide opportunities for fungi to cause disease. It is not surprising then that yeasts from our own microbiota, such as Candida albicans, have become one of the most…
Introduction The term dematiaceous is used to describe fungi that are dark in coloration, typically olive colored, dark brown, or black, due to the presence of melanin or melanin-like pigment. The dematiaceous fungi are a large and diverse group and are usually saprotrophs or plant pathogens. They most often cause opportunistic infections in the immunocompromised host with a notable exception of central nervous system infections, which…
Introduction A reclassification of the phylum Zygomycota into the phylum Glomeromycota has been proposed based on molecular phylogenetic studies. Acceptance of this reclassification would change the class name, Zygomycetes, to the new class of Glomerulomycetes. Instead of zygomycetes causing zygomycosis, under the proposed classification, Glomerulomycetes would cause glomerulomycosis. In that this new classification has just been proposed and is not completely accepted, this text is using…
Introduction Hyalohyphomycosis is fungal disease caused by a variety of molds that produce hyaline, septate hyphae with narrow hyphae that range from 2 to 6 μm in diameter. These include Aspergillus species, Fusarium species, Scedosporium species, and others. Zygomycete hyphae are also hyaline but are much broader with sparse septations, and these are discussed in a separate chapter. Hyalohyphomycetes are the counterpart to the phaeohyphomycetes, which…
Introduction Mycobacterium ulcerans infection, commonly called Buruli ulcer (BU), is an indolent, necrotizing disease largely limited to skin and subcutaneous tissue, but it may disseminate to lymph nodes and bone. Molecular studies suggest that M. ulcerans diverged from Mycobacterium marinum in geologic antiquity. Cook wrote the first description of BU, in Uganda in 1897. MacCallum and coworkers (1948) were first to isolate M. ulcerans from patients,…
Introduction Clinical Features Prevalence Globally, approximately 200,000 new cases of leprosy are diagnosed annually, according to figures available from the World Health Organization, based on passive case reporting. Active case-finding surveys, however, indicate that the actual number is likely to be much higher. In the United States, approximately 250 new cases are diagnosed annually. The majority of patients in the United States have a history of…
Tuberculosis Clinical Features Prevalence Tuberculosis remains one of the most important diseases in the world. Although the DNA of the microorganism responsible for tuberculosis has been recovered from ancient Egyptian mummies, the profound increase in the incidence of the disease occurred in conjunction with the Industrial Revolution. The overcrowding, poor living conditions, and lack of effective therapy created perfect conditions for the effective transmission of the…
Introduction Rickettsial infections are caused by related obligate intracellular bacteria in the families Rickettsiaceae (vasculotropic rickettsioses) and Anaplasmataceae (ehrlichiosis and anaplasmosis). Clinical similarities among these infections result from alterations or injury to vascular endothelial cells, either by direct infection and inflammation or by induction of vascular leakiness mediated by proinflammatory cytokine release. Natural infections by Rickettsia prowazekii (epidemic typhus), Rickettsia typhi (murine typhus), Rickettsia rickettsii (Rocky…
Introduction Several bacterial species included in the genus Bartonella, and the pathogens Coxiella burnetii and Tropheryma whipplei, cause a remarkably wide and diverse spectrum of infectious disease syndromes that pose particularly difficult diagnostic challenges for anatomic pathologists. These agents stain poorly or not at all with conventional tissue Gram stains, have fastidious culture requirements, and are uncommonly considered in most differential diagnoses. In addition, each of…
Introduction Anaerobic bacteria are normal flora of human skin and mucous membranes of the gastrointestinal tract including the mouth. Anaerobic bacteria can be classified according to their oxygen requirement in culture. By definition, anaerobic bacteria do not grow on solid media in room air (10% carbon dioxide and 18% oxygen), whereas facultative anaerobic bacteria can grow in the presence or absence of room air. Anaerobic bacteria…
Enteric pathogens are the leading cause of childhood death in the world and the second leading cause of death for people of all ages (second only to cardiovascular disease). In many areas of the world, problems with water quality, sanitation, and food hygiene practice contribute to the transmission of gastrointestinal infectious diseases. Many other aspects of modern living also contribute to the transmission of enteric pathogens,…
Introduction Botryomycosis, from the Greek botryo, meaning “grapes,” is a rare, chronic infection of the skin and viscera. This ancient name for the disease with the use of “mycosis” is actually inappropriate and misleading, because the infection is caused not by fungi but by bacteria. It involves the formation of granules and grains in tissues due to the presence of a large spectrum of nonfilamentous bacteria.…
Introduction Clinical Features Mycetoma is a much neglected tropical disease affecting the poorest people of countries in which it is endemic. Mycetoma may be defined as “a pathological process induced by exogenous fungi and actinomycetes producing grains” (Barquisimeto, 1978). If we also consider clinical features and topographic patterns, a broader definition can be proposed: a chronic, commonly painless, progressive, and insidious infection of the skin and…
Introduction The microorganisms covered in this chapter all have the common characteristic of being partially acid fast but unable to retain carbol fuchsin in the presence of a strong acid decolorizer (i.e., they are not fully acid fast). This chapter, therefore, should serve as a reference for those attempting to limit the differential diagnosis of organisms with such characteristics. Many of the other aerobic actinomycetes, such…