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Protozoa are unicellular parasites that infect millions of people each year. Infections are often latent but may progress to active disease when the host-parasite balance shifts in favor of the parasite. Parasite burden (at initial infection or later), host immunity, and other factors influence this balance. International travel, extension of vector habitats, and increased numbers of immunocompromised hosts (patients undergoing chemotherapy, organ transplant recipients, HIV-infected individuals) have increased the pool of susceptible hosts.

Protozoa may cause cutaneous lesions at a primary, secondary, or incidental site. Leishmaniasis, the most common form of protozoal dermatitis, affects millions of people in the tropical and subtropical belts of the world. Up to 1 billion people harbor Entamoeba histolytica, but cutaneous lesions are rare and occur as secondary complications of chronic diarrhea or fistulas. Lesions may develop at the inoculation site of visceral infections (kala-azar or Chagas disease). Iatrogenic and HIV-associated immune suppression have resulted in an increase in toxoplasmosis and acanthamebiasis. Cutaneous lesions in these patients often represent a manifestation of disseminated disease. Many protozoal diseases cause cutaneous immune reactions in the absence of cutaneous organisms.

Algae (both chlorophyllic and achlorophyllic) cause infections in wild and domestic animals but rarely cause disease in humans.

Classically, protozoal infections of the skin are diagnosed by clinical correlation with skin smears; routine histology; and in some cases, microbiologic culture. Molecular assays increase the specificity and in some instances may increase the sensitivity of histopathologic tests.1 In this regard, molecular methods of detection may be particularly useful when protozoa are undetectable by histochemical methods, are present in low numbers, stain poorly, cannot be cultured, or exhibit an atypical morphology. Free trophozoites and protozoa present in large areas of necrosis may be impossible to distinguish from apoptotic nuclei. In some cases, molecular methods are important for the rapid, specific, and more rarely, quantitative detection of protozoa.1,2 Molecular tests and other special techniques are typically employed following histopathologic examination when clarification of nonspecific findings is needed. These special techniques include immunohistochemistry, in situ hybridization (ISH), and nucleic acid amplification technologies, mainly polymerase chain reaction (PCR).1-6 After histochemical staining, immunohistochemistry is the most commonly used ancillary diagnostic technique for the diagnosis of protozoa in histologic sections.7-12 This technique uses monoclonal or polyclonal antibodies directed against specific protozoa antigens. The specificity of this method is dependent on the specificity of the antigen binding (Fab) portion of the immunoglobulin molecule used.12 Whereas immunofluorescent immunohistochemistry is usually performed for detection of protozoa on fresh, frozen tissue, immunoperoxidase methods are usually performed to detect protozoa on formalin-fixed, paraffin-embedded tissue. ISH has many of the same advantages as immunohistochemistry. However, this method uses the complementary nature of protozoa nucleic acids, rather than an antibody raised against a protozoal antigen, to impart specificity. The nucleic acid probe, which may be labeled by a variety of methods, anneals to a specific target sequence ...

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