There is the elephant disease which is generated beside the streams of the Nile in the midst of Egypt and nowhere else. In Attica the feet are attacked and the eyes in Achean lands. And so different places are hurtful to different parts and members. —Lucretius (99-55 bc)
The nematodes discussed in this chapter cause disease through their presence in the tissues and lymphohematogenous system of the human body. Some migrate through the human gastrointestinal tract on their way there, but because this is a temporary part of their life cycle, they are not considered to be “intestinal” nematodes.
They are a heterogeneous group. Four of them—Toxocara canis, Baylisascaris procyonis, Trichinella spiralis, and Ancylostoma braziliense—are natural parasites of domestic and wild carnivores. Although they are capable of infecting humans, they cannot complete their life cycle in the human host. Humans therefore serve only as “accidental hosts,” injured bystanders rather than major participants in the life cycle of these parasites.
The remaining four major tissue nematodes—Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, and Loa loa—are members of a single superfamily (Filarioidea). All use humans as their natural definitive host. The thin, thread-like adults live for years in the subcutaneous tissues and lymphatic vessels, where they discharge their live-born offspring called “microfilariae.” These progeny circulate in the blood or migrate in the subcutaneous tissues until they are ingested by a specific bloodsucking insect. Within this insect, they transform into filariform larvae capable of infecting another human when the vector again takes a blood meal.
The nematodes considered, diseases caused, natural definitive host, and usual routes of infection in humans are listed in Table 55–1.
TABLE 55–1General Characteristics of Tissue Nematodes ||Download (.pdf) TABLE 55–1 General Characteristics of Tissue Nematodes
|PARASITE ||DISEASE ||NATURAL DEFINITIVE HOST ||USUAL SOURCE OF HUMAN INFECTION |
|Toxocara canis ||Toxocariasis (visceral or ocular larva migrans) ||Dog ||Ingestion of ova from canine stools |
|Baylisascaris procyonis ||Eosinophilic CNS or ocular disease ||Raccoon ||Ingestion of ova from raccoon stools |
|Trichinella spiralis ||Trichinosis ||Pig ||Ingestion of improperly cooked pork |
|Ancylostoma braziliense ||Cutaneous larva migrans ||Cat ||Soil contaminated with dog or cat feces |
|Major Filarial Worms || || || |
|Wuchereria bancrofti, Brugia malayi ||Lymphatic filariasis (elephantiasis) ||Human ||Mosquito |
|Onchocerca volvulus ||Onchocerciasis (river blindness, dermatitis) ||Human ||Simulium fly |
|Loa loa (eye worm) ||Loiasis (Calabar swellings) ||Human ||Chrysops fly |
TOXOCARA CANIS: PARASITOLOGY AND LIFE CYCLE
Life cycle of Toxocara canis. (Reproduced with permission from Roberts RL, Janovy J, Nadler S: Foundations of Parasitology, 9th edition. McGraw-Hill, 2013.)
Toxocara canis is a large, intestinal ascarid of canines, including dogs, foxes, and wolves. ...