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Leishmania Species (Leishmaniasis)

Description of the Pathogen The genus Leishmania comprises vector-borne protozoan parasites that cycle asexually between the digestive tract of the sand fly as a motile flagellated promastigote and, in a wide range of vertebrate hosts (e.g., rodents, canids, hyrax, humans), as an immotile intracellular amastigote ( Fig. 267.1 ). Infection can be either zoonotic or anthroponotic (when humans serve as reservoir) and occurs when a female…

Cystoisospora (Isospora) and Cyclospora Species

Cystoisospora belli (formerly Isospora belli ) has been renamed and included in the Cystoisospora genus. Both C. belli and Cyclospora spp. infect the small intestine and have been implicated in diarrheal disease. C. belli, which first was linked with disease in 1915, is believed to infect only humans. Prior reports of diarrhea associated with “Cyanobacteria-like” or “coccidian-like” bodies in stools are now thought to have been…

Giardia intestinalis (Giardiasis)

Giardia intestinalis is a flagellated protozoan that infects the duodenum and upper small intestine. Giardia is the most commonly identified enteric parasite in the US and Canada. Infection can be asymptomatic or associated with a variety of intestinal manifestations. Factors contributing to this variation include genotype of the Giardia strain, the number of cysts ingested, the age of the host, and the status of the host’s…

Other Entamoeba , Amebae, and Intestinal Flagellates

In addition to Entamoeba histolytica ( Chapter 263 ), amebae that can be found in the human intestinal tract include other species of Entamoeba ( E. dispar, E. moshkovskii , E. bangladeshi, E. coli, E. hartmannii, E. polecki), Endolimax nana ( Chapter 262 ), and Iodamoeba bütschlii. Entamoeba gingivalis is found in the human mouth. These are generally nonpathogenic or of unclear pathogenicity and must be…

Entamoeba histolytica (Amebiasis)

Entamoeba histolytica is a “tissue-lysing” ( histo = tissue, lytic = lysing) ameba that can cause disease ranging from asymptomatic colonization to diarrhea, colitis, and liver abscess. It was first discovered in 1875 by Fedor Aleksandrovich Lösch, who described amebic trophozoites in the stool and colonic ulcerations in a farmer with a fatal case of dysentery. In 1993 what was once thought to be a fairly…

Endolimax nana

Endolimax nana is a single-celled parasite of the colon. Although considered nonpathogenic, infection with E. nana is an indicator of exposure to fecal contamination and often is seen in fecal samples positive for other common parasitic intestinal protists. Description of Endolimax Nana E. nana is an intestinal parasitic ameba, which appears to be most closely related to Iodamoeba. , E. nana has a monoxenous life cycle…

Cryptosporidium Species

Human disease caused by Cryptosporidium was first described in 1976 and became recognized increasingly during the HIV epidemic. Improved detection of oocysts in feces has shown Cryptosporidium to be a common cause of diarrhea in both immunocompetent and immunocompromised hosts. , In resource-poor countries, Cryptosporidium is among the four most common causes of moderate to severe diarrhea and diarrhea-associated deaths in young children. The Pathogen Organisms…

Blastocystis Species

Blastocystis is a protist that parasitizes the lower intestine of humans and a vast range of animals. Although Blastocystis has been implicated as a cause of various gastrointestinal tract symptoms, its role as a pathogen, mechanism of transmission, and life cycle have been subject to much debate. A clinical syndrome of self-limited intestinal symptoms has been described in several series of predominantly adult patients, with infection…

Balantioides coli (Formerly Balantidium coli )

Balantioides coli originally was called Balantidium coli (synonym Neobalantidium coli ), but it has been renamed based on new phylogenetic observations. , This ciliate inhabits the intestine of several mammalian species, and it is the only ciliate known to infect humans. Balantiosis is considered a neglected zoonotic disease, and human clinical cases are typically associated with close contact with domestic pigs. Infected people can be symptom…

Babesia Species (Babesiosis)

Acknowledgment The author and editors remember the substantial contributions of Robert W. Tolan to pediatric infectious diseases and to this chapter in previous editions. Colleagues continue to mourn his loss. Babesiosis is an emerging tickborne zoonotic disease, clinically similar to malaria, that is caused by intraerythrocytic protozoa in the genus Babesia . Symptomatic infection is most frequently diagnosed and reported in asplenic adults, those >50 years,…

Ectoparasites (Lice and Scabies)

Pediculosis The Parasites, Pathogenesis, and Epidemiology Pediculosis is an infestation by one of several species of sucking lice of the phylum Arthropoda, class Insecta, order Phthiraptera, suborder Anoplura, and family Pediculidae or family Pthiridae. The three types of lice that infest humans are Pediculus humanus capitis (the head louse), Pediculus humanus humanus (the body louse), and Phthirus pubis (the crab louse). Louse infestation spreads readily, and…

Classification of Parasites

Although all infectious agents of humans are parasites, by convention, parasitic diseases are often defined as diseases caused by ectoparasites, parasites that live on the outside or within the skin of the host, and endoparasites, parasites that live on the inside of the host. Parasites occupy a diverse array of eukaryotic organisms that span many phyla ( Box 256.1 ). Together, ectoparasites and endoparasites exert an…

Agents of Eumycotic Mycetoma: Pseudallescheria boydii and Scedosporium apiospermum

Mycetoma (maduromycosis, Madura foot) is a neglected tropical disease that has its greatest impact on severely impoverished, remote rural communities. It is a noncontagious, localized chronic debilitating infection that affects skin, subcutaneous tissue, fascia, muscle, and bone. The foot is most commonly involved, but any site can be affected. Characteristic clinical features of mycetoma include localized swelling, fistulous tracts, and suppurative drainage that contains dense colonies…

Superficial Fungal Infections

Superficial fungal infections of the skin, nails, and mucosa are mainly caused by three groups of fungi—the dermatophytes, Candida spp., and Pityrosporum spp. Non-dermatophytic molds form a fourth group that contributes a minority of such infections. Superficial fungal infections are mostly amenable to antifungal treatment, but under certain circumstances, chronic and recalcitrant disease may develop. Further, they contribute significantly to the economic burden on healthcare. Dermatophytes…

Coccidioides immitis and Coccidioides posadasii (Coccidioidomycosis)

Coccidioidomycosis is an infection caused by the dimorphic fungi Coccidioides immitis and Coccidioides posadasii . These fungi are endemic to the southwestern US, Mexico, Guatemala, and sporadic regions within Central and South America. Infection is caused by inhalation of highly infectious and environmentally resistant spores following soil disruption and aerosolization. Primary infection is often manifest as an acute respiratory illness although the spectrum of disease ranges…

Blastomyces Species (Blastomycosis)

Blastomyces , the etiologic agent of blastomycosis, can infect both people with healthy immune systems and those with impaired immune defenses. The Blastomyces genus consists of 7 different species, of which B. dermatitidis and B. gilchristii cause the majority of infections in North America. Most blastomycosis infections are sporadic; however, at least 13 outbreaks have involved pediatric patients. An estimated 2%–13% cases of blastomycosis occur in…

Pneumocystis jirovecii

Description of Pathogen Pneumocystis jirovecii (formerly called P. carinii ) is an opportunistic human respiratory pathogen. Although Pneumocystis organisms share some of the common features of protozoa, molecular genetic comparisons have definitively placed Pneumocystis among the ascomycetous fungi. , Pneumocystis organisms display strict host-species specificity, and the name P. jirovecii designates a genetically and phenotypically distinct form capable of infecting humans but not other mammals. In…

Histoplasma capsulatum (Histoplasmosis)

Histoplasmosis is the most common primary systemic mycosis in the Americas, affecting hundreds of thousands of people living in urban and rural areas of the Ohio and Mississippi river valleys, some mideastern states, and parts of Central and South America. , The fungus is also endemic in other continents such as Australia, Africa, and Asia. The Pathogen Histoplasmosis is caused by the dimorphic fungus Histoplasma capsulatum…

Cryptococcus Species

The majority of human cryptococcal infections are caused by one of two species: Cryptococcus neoformans and Cryptococcus gattii. These species were previously separated into 3 varieties: C. neoformans var. neoformans, C. neoformans var. grubii, and C. neoformans var. gattii, which were classified into 8 molecular genotypes and 5 serotypes based on capsular polysaccharide antigens. Serotypes A and D and the hybrid diploid AD strains belong to…

Sporothrix schenckii Complex (Sporotrichosis)

Sporotrichosis is a chronic subcutaneous fungal infection caused by thermally dimorphic fungal species in the Sporothrix schenckii complex . This species complex is found worldwide in soil, on decomposing vegetation, and on plant material, such as sphagnum moss, hay bales, rosebushes, and wood. In nature, S. schenckii grows as a filamentous mold, but in tissue it forms small budding yeast cells. Sporotrichosis has a worldwide distribution…