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Visceral Larva Migrans and Other Uncommon Helminth Infections

Most helminths that infect humans are relatively host specific to humans, undergo characteristic migration and development, and are found in typical anatomic locations. However, these helminths sometimes undergo atypical or aborted migrations and cause symptoms or signs because of their unusual or ectopic location. A good example of this is the deposition of schistosomal ova and the subsequent granulomatous inflammatory lesions in the spinal cord or…

Tapeworms (Cestodes)

In humans, parasitic cestode infections occur in either of two forms: as mature tapeworms residing in the gastrointestinal tract or as one or more larval cysts (variously called hydatidosis, cysticercosis, coenurosis, or sparganosis) embedded in liver, lung, muscle, brain, eye, or other tissues. The form taken by the infecting parasite depends on which cestode species causes the infection and, to a lesser extent, on the route…

Trematodes (Schistosomes and Liver, Intestinal, and Lung Flukes)

The trematode flatworms that infect humans include the schistosomes, which live in venules of the gastrointestinal (GI) or genitourinary tract, and other flukes, which inhabit the bile ducts, intestines, or lungs. The geographic distribution of each species of trematode parallels the distribution of the specific freshwater snail that serves as its intermediate host ( Table 288.1 ). Six species of schistosomes infect more than 230 million…

Tissue Nematodes, Including Trichinellosis, Dracunculiasis, Filariasis, Loiasis, and Onchocerciasis

Tissue-dwelling nematode (roundworm) infections are widely distributed throughout the world. The health and socioeconomic impacts of these infections are greatest in resource-poor settings in the tropics and subtropics, although populations in temperate and industrialized regions of the world continue to be at risk for infection and morbidity. Like all parasitic nematodes, the life cycle of these multicellular organisms includes five distinct stages: adult male or female…

Intestinal Nematodes (Roundworms)

Intestinal Nematodes More than 1 billion people are infected with intestinal nematodes worldwide. Also referred to as soil-transmitted helminths (STHs), intestinal nematodes are complex, nonsegmented, multicellular worms. Each organism is surrounded by a species-specific acellular cuticle composed of structural proteins, enzymes, and lipids that permit tissue migration through the host and evasion of the host immune response. Under the cuticle lies a muscular ring of tissue…

Introduction to Helminth Infections

The helminthiases are among the most prevalent infections in the world and a leading cause of morbidity, particularly in low-income and resource-constrained regions. An estimated 1.5 billion persons harbor at least one species of parasitic worm. The helminths that parasitize humans include the nematodes (roundworms) and platyhelminths (flatworms); the latter group consists of cestodes (tapeworms) and trematodes (schistosomes and other flukes). Leeches, ectoparasites belonging to the…

Human Illness Associated With Harmful Algal Blooms

During the past several decades, the association of human health and environmental problems with harmful and toxic algae has been increasingly recognized, as has awareness of the complex range of natural toxins (and toxin congeners) that can be produced by these microorganisms. Toxic species constitute a small percentage of the thousands of species of microscopic algae at the base of the marine food chain. However, when…

Cyclospora cayetanensis, Cystoisospora belli, Sarcocystis Species, Balantidium coli, and Blastocystis Species

Coccidia Other Than Cryptosporidia Cyclospora, Cystoisospora, and Sarcocystis are coccidian parasites belonging to the phylum Apicomplexa, family Eimeriidae. Coccidian protozoan infections are well recognized but still relatively uncommon causes of diarrheal disease. Cryptosporidium parvum and Cyclospora cayetanensis, two of the more commonly identified coccidian pathogens in diarrheal illness, account for a small but increasing proportion of laboratory-confirmed diarrheal disease reported to the Foodborne Diseases Active Surveillance…

Cryptosporidiosis ( Cryptosporidium Species)

Protozoan parasites of the genus Cryptosporidium were first identified in the stomach of mice in 1907. The species name Cryptosporidium parvum was proposed in 1912 to describe parasites identified in murine intestines. Although Cryptosporidium was linked to gastrointestinal disease in turkeys in 1955 and to bovine diarrhea in 1971, the first human cases were described only in 1976. Only a handful of cases had been reported…

Babesia Species

The first recorded mention of babesiosis is believed to date to biblical times. In the Book of Exodus, the fifth plague is described as “very grievous murrain” that fell “upon thy cattle which is in the field, upon the horses, upon the asses, upon the camels, upon the oxen and upon the sheep.” Bovine babesiosis is still referred to as murrain in the Irish countryside. In…

Trichomonas vaginalis

Trichomonas vaginalis is the causative agent of trichomoniasis, a common cause of vaginitis. In men, it may cause urethritis but is more commonly asymptomatic. Despite being a readily diagnosed and treatable sexually transmitted disease (STD), trichomoniasis is not a reportable infection and its control has received relatively little emphasis from public health STD control programs. More recently, however, recognition of the high rates of disease and…

Giardia lamblia

Description of the Pathogen Giardia was probably described in the late 1600s, when van Leeuwenhoek likely discovered it in his own stool. It was in the early 1900s that the parasite received the genus name Giardia. The designated species name for the human parasite has been lamblia ; intestinalis and duodenalis are also used. The genus Giardia is in the category of intestinal flagellates in the…

Toxoplasma gondii

Although Toxoplasma gondii infects a large proportion of the world's human population, it is an uncommon cause of disease. Certain individuals, however, are at high risk for severe or life-threatening disease because of this parasite. These include congenitally infected fetuses and newborns and immunologically impaired individuals. Congenital toxoplasmosis is the result of maternal infection acquired during gestation, an infection that is most often clinically inapparent. In…

Agents of African Trypanosomiasis (Sleeping Sickness)

Parasites and Their Transmission The agents of human African trypanosomiasis (HAT, sleeping sickness) are flagellated protozoan parasites that belong to the genus Trypanosoma, subgenus Trypanozoon . A general description of the members of this genus and specific characteristics of the subgenus are presented in the introduction to Chapter 276 . Three trypanosome subspecies, Trypanosoma brucei brucei, T. brucei rhodesiense, and T. brucei gambiense, are considered here.…

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…