Chapter 13

Most are free-living, with well-developed systems. These are absent or reduced in parasitic forms (except reproductive system). Life cycle can be complex, with definitive and intermediate hosts. Sexual reproduction only occurs in the definitive host asexual reproduction may occur in the intermediate host. May be dioecious (separate sexes) or hermaphroditic (reproductive organs of both sexes in one body). Most parasitic forms are transmitted by ingestion of eggs (gastrointestinal route).

flatworms; incomplete digestive system. 2 medically important classes.

Leaf-shaped bodies. Oral and ventral suckers for attachment and obtaining food.

Asian liver fluke

lung fluke. Man- definitive host. Snail and crayfish- intermediate hosts

blood flukes. Intermediate forms penetrate skin

Tapeworms. Intestinal parasites.
Obtain nutrients by absorption.

head with suckers or hooks

Body segments. Gravid (mature) are most distal.

beef tapeworm. Human is definitive host. Eggs shed in feces are ingested by cattle. Larvae hatch from eggs, migrate to muscle and encyst. Human eats infected meat and cannot digest scolex. Contracted by ingestion of encysted larvae in raw or poorly cooked beef.

pork tapeworm. Human-pig life cycle similar to T. saginata, but rare in U.S

Human to human transmission- Eggs hatch and larvae encyst in the brain, causing

Dogs and coyotes are definitive hosts.
Humans become infected by the oral route.

Eggs hatch, migrate to liver or lungs and develop to

X-rays detect cysts, but diagnosis usually at

Unsegmented roundworms.
Complete digestive system. Most are dioecious. Entire life cycle of some can occur in a single host.
Clinically relevant forms are classified on the basis of the infective form: eggs or larvae.

Eggs are infective form: pinworms. Ingested eggs hatch, worms mature in the colon, mate, and eggs are deposited near the anus or passed in feces.
Diagnosis by "tape" or "flashlight" methods.

Eggs are in infective form: Eggs excreted in feces survive in soil. Ingested eggs hatch in intestines, mature in lungs, and migrate to intestine.
Diagnosis by finding eggs (in stool sample) or appearance worms.

Larvae are infective form: Hookworm. Adult worms inhabit the intestine.
Eggs are excreted in feces; larvae hatch and develop in soil.
Larvae penetrate the skin, and travel through the blood or lymphatics to the lungs.
From there they are coughed up and swallowed. Eventually they attach to the lining of the intestine.
Diagnosis is by finding eggs by microscopic examination of feces.

Larvae are infective form: Causative agent of trichinellosis (trichinosis). Transmission is usually by ingestion of encysted larvae in poorly cooked pork or bear meat. Larvae mature to adults in intestine and reproduce. Second generation larvae hatch, migrate throughout the body, and become encysted in muscle tissue. Diagnosis is by microscopic examination of encysted larvae in muscle tissue.