I’m carrying so much pork, I’m beginning to get trichinosis.
—Phil Gramm, American Politician
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.
Four of them—Toxocara canis, Baylisascaris procyonis, Trichinella spiralis, and Ancylostoma braziliense—are zoonotic, meaning natural parasites of domestic and wild animals. 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 are anthroponotic, meaning they use humans as their 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.
Table 55–1 summarizes these nematodes, diseases they cause, their definitive host, and usual routes of human infection.
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 |
|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 |
|Dracunculus mediensis (guinea worm) ||Dracunculiasis ||Human ||Drinking water contaminated by cyclops |
TOXOCARA CANIS: PARASITOLOGY AND LIFE CYCLE
Toxocara canis is a large, intestinal ascarid of canines, including dogs, foxes, and wolves (Figure 55–1). Occasionally, a related organism found in cats (T cati) can behave in a similar fashion. Each female worm discharges approximately 200 000 thick-shelled eggs daily into the fecal stream. After reaching the soil, these eggs embryonate for a minimum of 2 to 3 weeks. Thereafter, the eggs are infectious to canines, humans, and other mammals. The eggs may remain infectious in the soil for months ...