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Ticks, Including Tick Paralysis

Ticks are the most competent and versatile of all arthropod vectors of zoonotic infectious diseases for several reasons. First, ticks are not afflicted by most of the microorganisms that they may transmit or the paralytic salivary toxins that they may transfer during blood-feeding. Second, and unlike mosquitoes, ticks can transmit the broadest range of infectious microbes among all arthropods, including bacteria, viruses, and parasites. In addition,…

Mites, Including Chiggers

Mites, including chigger and scabies mites, are among the smallest arthropods, with most barely visible without magnification. Only about 20 species of the more than 3000 species of chigger, animal, plant, and scabies mites are of any medical importance, and most of these are simply biting nuisances and do not transmit infectious diseases. Mites are closely related to ticks but not as prodigious at blood-feeding. They…

Myiasis and Tungiasis

Flies and fleas are mostly bothersome biting nuisances of humans and animals that can also transmit infectious diseases and deeply invade living tissues, causing amputation, disfigurement, and, rarely, death. Flies can serve as mechanical vectors of shigellosis, and rat fleas can transmit bubonic plague and murine typhus. Flies may lay their eggs on human flesh, and their developing larvae, or maggots, can invade subcutaneous tissues and…

Scabies

Scabies, an infection by the itch or scabies mite, Sarcoptes scabiei var. hominis, remains a major public health problem throughout the developing world ( Fig. 293.1 ). Scabies in its most severe form, crusted or Norwegian scabies ( Fig. 293.2 ), has now become a significant reemerging ectoparasitosis in the developed world, especially among homeless people, institutionalized older adults, individuals with intellectual disability, and immunocompromised individuals.…

Lice (Pediculosis)

Pediculosis is a complex of three different human infestations with two species of blood-sucking lice of the insect order Phthiraptera, suborder Anoplura: Pediculus humanus and Phthirus pubis. Sometime after early humans began to wear clothes, P. humanus evolved into two clinically distinct ectoparasitic variants, P. humanus var. corporis, the body louse ( Fig. 292.1 ), and P. humanus var. capitis, the head louse ( Fig. 292.2…

Introduction to Ectoparasitic Diseases

Ectoparasites infest the skin and its appendages, such as the hair and sebaceous glands, and most external orifices, especially the ears, nares, and orbits. Like endoparasites, ectoparasites may be obligatory parasites, programmed to feed on human hosts to complete their life cycles, or facultative parasites, preferring to feed on nonhuman hosts, infesting humans only as accidental or dead-end hosts. Over the past 2 decades, there have…

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.…