Overview of Parasitology

Reading assignment: Please look up all of these in your textbook!

The Helminths - The Worms

These are parasitic animals that spend at least part of their life inside a human host. They are multicellular and they possess digestive, circulatory, nervous and reproductive systems.

The medically important worms are either ROUNDWORMS (Nematodes) or FLATWORMS. The Flatworms are divided into two groups, the Cestodes (TAPEWORMS) and the Trematodes (the FLUKES).

THE CESTODES - THE TAPEWORMS

Adult tapeworms inhabit the intestines of vertebrates and the larva inhabit the tissues. The head of the adult (the SCOLEX) is really an attachment device with hooks and suckers. The long body is really a long sequence of repeating segments (PROGLOTTIDES) each with its own set of male and female reproductive organs. New proglottids are constantly being formed by the scolex.

There are a number of tapeworms that infect humans. Taenia saginata can be contracted by eating rare beef. Taenia solium can be caught from undercooked pork.

THE TREMATODES - THE FLUKES

Adult flukes are usually long, flat leaf-shaped creatures ranging in size from 1 mm to several centimeters. Many species are hermaphroditic although there are a few that have individual male and female organisms.

Humans can be infected by a variety of fluke species and these are catagorized by the infection site. Therefore there are liver flukes, pulmonary flukes, intestinal flukes and blood flukes.

SCHISTOSOMIASIS is caused by a number of Schistosoma species blood flukes. The adult worms live in the blood vessels surrounding the large intestine and the urinary bladder. Mating occurs at these sites and the female lays thousands of large (1 mm) spiked eggs onto the vascular endothelium. These eggs eventually work their way through the walls of blood vessels as well as the walls of the intestines and bladder whereupon they are defecated or urinated out and into some fresh water. The eggs hatch and if the water contains the appropriate species of snail the schistosome can complete its life cycle.

Most of the pathology in schistosomiasis is due the inflammation caused by the thousands of eggs moving through the victim's tissues.

THE NEMATODES - THE ROUNDWORMS

PINWORMS - Enterobius vermicularis

Just about anyone with school age children has some familiarity with this organism. Adult worms live in the large intestine and the female makes nightly visits to the outside anal region of the human host to lay her eggs. Little human fingers scratching itchy butts easily allow these eggs to be spread (oral-fecal transmission) to others' mouths where they are swallowed and a new cycle of transmission begins.

ANISAKIS

The larvae of these worms live in the intestines of marine fish. After the death of the fish the larvae may migrate out into the tissues and humans become infected upon eating such fish raw. Primarily fish from colder waters seem to carry these worms although this may not be totally exclusive. Humans have been infected from eating raw herring and sashimi. The infection in humans is incomplete and the worms eventually die, however while they are around they can cause severe gastric pain, nausea and vomiting.