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Trypanosoma Species (American Trypanosomiasis, Chagas Disease): Biology of Trypanosomes

The protozoan genus Trypanosoma consists of several dozen species. Two of the three species that infect humans are pathogenic, and several other species cause severe and economically important diseases in domestic mammals. Broadly defined, the organisms belonging to this genus are protozoan flagellates of the family Trypanosomatidae, order Kinetoplastida, that pass through different morphologic stages (epimastigote, amastigote, and trypomastigote) in their vertebrate and invertebrate hosts. The…

Leishmania Species : Visceral (Kala-Azar), Cutaneous, and Mucosal Leishmaniasis

Overview and General Principles Leishmaniasis refers to a diverse spectrum of clinical syndromes caused by infection with protozoan parasites of the genus Leishmania transmitted by the bite of a phlebotomine sand fly. Leishmaniasis can be separated geographically into Old World and New World disease, referring to the Eastern and Western Hemispheres, respectively. The clinical manifestations of leishmaniasis vary widely but are often divided into three clinically…

Malaria ( Plasmodium Species)

Revised June 24, 2020 The Malaria Problem Malaria remains an overwhelming problem in tropical developing countries, accounting for an estimated 216 million cases and 445,000 deaths in 2016. Nearly 40% of the world's population is at risk for acquiring malaria. In sub-Saharan Africa, most severe cases and deaths occur in children younger than 5 years and in pregnant women. The introduction of chloroquine and dichlorodiphenyltrichloroethane (DDT)…

Free-Living Amebae

Free-living amebae (FLAs) are aerobic, eukaryotic protists that comprise several genera. Clinically apparent infection of humans with FLAs is an infrequent but often fatal occurrence in both normal and immunocompromised individuals. Central nervous system (CNS) invasion by Naegleria fowleri, Acanthamoeba spp., and Balamuthia mandrillaris has been reported in hundreds of patients worldwide, with thousands of Acanthamoeba keratitis cases described. Other FLA species have each been reported…

Entamoeba Species, Including Amebic Colitis and Liver Abscess

Entamoeba histolytica is an invasive enteric protozoan parasite that is the cause of amebiasis. Entamoeba histolytica is morphologically indistinguishable from three other species of human intestinal amebae, Entamoeba dispar, Entamoeba moshkovskii, and the more recently described Entamoeba bangladeshi. E. dispar and E. bangladeshi are nonpathogenic, while E. moshkovskii causes noninvasive diarrhea. Because E. histolytica, E. dispar, E. moshkovskii, and E. bangladeshi cannot be distinguished by a…

Pneumocystis Species

Pneumocystis was discovered in 1909 in Brazil by Carlos Chagas, who mistakenly interpreted the organism as a trypanosome. In 1912 Pierre and Eugénie Delanöe, in Paris, identified Pneumocystis as a separate genus and species and named the organism in honor of Antonio Carini, another researcher. Pneumocystis first came to medical attention when in the 1940s–50s it was identified as the cause of interstitial plasma cell pneumonia…

Uncommon Fungi and Related Species

Scedosporium Apiospermum ( Pseudallescheria Boydii ) Species Complex In humans, infection with Scedosporium apiospermum, Scedosporium boydii (formerly Pseudallescheria boydii ) and Scedosporium aurantiacum (collectively, the S. apiospermum species complex) can produce two distinct diseases: mycetoma and scedosporiosis (pseudallescheriasis). Mycetoma is a chronic subcutaneous infection characterized by the production of grains (see Chapter 261 ), whereas scedosporiosis includes all other infections caused by Scedosporium spp. The most…

Paracoccidioidomycosis

Description of the Pathogen Until 2006 the genus Paracoccidioides was thought to consist of a single species, P. brasiliensis, considered the sole etiologic agent of paracoccidioidomycosis. At present however, molecular and genetic studies have revealed that this genus includes a complex of five distinct phylogenetic species (S1a, S1b, PS2, PS3, PS4) that appear to be confined to distinct regions within the endemic zones. Recently, the P.…

Dermatophytosis (Ringworm) and Other Superficial Mycoses

The superficial fungal infections include some of the most common infectious conditions, such as ringworm, tinea corporis, and pityriasis versicolor, and rare disorders such as tinea nigra. Their prevalence varies in different parts of the world, but in many tropical countries they are the most common causes of skin disease. Dermatophyte infections and other superficial mycoses are described in this chapter. Superficial candidiasis is discussed in…

Coccidioidomycosis ( Coccidioides Species)

Although the systemic fungal infection now known as coccidioidomycosis has been recognized for more than a century, its endemic domains continue to be expanded. A medical intern is credited with first identifying in 1892 a patient who had widespread disease. Organisms seen microscopically were mistakenly thought to be parasites, and only several years later was the true mycotic etiology determined and the agent given the name…

Blastomycosis

Revised October 30, 2020 The etiologic agents of blastomycosis belong to a group of fungi that are characterized by thermal dimorphism. Advances in genomics and phylogenetics have expanded the number of Blastomyces species to include B. dermatitidis, B. gilchristii, B. percursus , B. helicus, B. parvus, and B. silverae . Infection is primarily acquired through the inhalation of infectious conidia and hyphal fragments following the disruption…

Histoplasma capsulatum (Histoplasmosis)

Histoplasma capsulatum is one of the more common causes of infection in the US Midwest and Southeast. Histoplasmosis, acquired through inhalation of mycelial fragments and microconidia, is most often self-limiting but can cause potentially lethal infection in patients with preexisting conditions. It remains a frequent cause of opportunistic infection in patients whose immune system is impaired by pharmaceutic agents or by the human immunodeficiency virus (HIV).…

Cryptococcosis ( Cryptococcus neoformans and Cryptococcus gattii )

Cryptococcus neoformans and Cryptococcus gattii are encapsulated, heterobasidiomycetous fungi that have progressed from being rare human pathogens, with just over 300 cases of cryptococcosis reported in the literature before 1955, to becoming a common worldwide opportunistic pathogen as immunocompromised human populations have dramatically increased over the past 2 decades. Cryptococcosis crosses the entire spectrum of patient populations, from the apparently immunocompetent host without an underlying disease…

Agents of Mycetoma

Mycetoma is a chronic progressive granulomatous infection of the skin and subcutaneous tissue most often affecting the lower extremities, typically a single foot. The disease is unique from other cutaneous or subcutaneous diseases in its triad of localized swelling, underlying sinus tracts, and production of grains or granules (composed of aggregations of the causative organism) within the sinus tracts. These infections may be caused by fungi…

Agents of Chromoblastomycosis

Chromoblastomycosis (chromomycosis) is a chronic, localized fungal infection of the skin and subcutaneous tissue that produces raised scaly lesions, usually of the lower extremities. The lesions of chromoblastomycosis are frequently warty or cauliflower-like in appearance, with pathognomonic muriform cells (also called “copper penny” or sclerotic bodies) found at histologic examination. This disease of tropical and subtropical distribution is produced by inoculation of the infecting fungi in…

Sporothrix schenckii

Mycology Sporothrix schenckii sensu lato (meaning in the broadest sense) is a dimorphic fungus that exists in a hyphal form in vitro at temperatures less than 37°C. Colonies are initially white but gradually become brown to black due to the production of pigmented conidia. In vivo or at 37°C on rich media such as brain heart infusion agar, the organism converts to an oval- or cigar-shaped…

Agents of Mucormycosis and Entomophthoramycosis

The disappearance of the class Zygomycetes from current taxonomy has made the term zygomycosis obsolete. The term zygomycosis encompassed both mucormycosis and entomophthoramycosis. These two infections are so different that no new name has been proposed to include both infections. Most of this chapter discusses mucormycosis, a group of filamentous fungi in the subphylum Mucoromycotina that belong to the order Mucorales. Mucorales can cause life-threatening infections…

Aspergillus Species

Revised August 7, 2021 Invasive aspergillosis (IA) remains a major cause of morbidity and mortality in the immunosuppressed population. This infection is caused by Aspergillus , a hyaline mold, and is responsible for a variety of noninvasive or semiinvasive conditions. These syndromes range from colonization, such as a fungus ball due to aspergillosis (also known as aspergilloma); allergic responses to Aspergillus , including allergic bronchopulmonary aspergillosis…