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Lecture 4

Medical Parasitology Course (MLAB 362)

Dr. Mohamed A. El-Sakhawy

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Introduction to Parasitic Helminths

Parasitic Helminths

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Eukaryotic, multicellular animals that usually have

digestive, circulatory, nervous, excretory, and reproductive

systems.

Worms with bilateral symmetry, head and tail, and tissue

differentiation (endoderm, mesoderm, and ectoderm).

Type of parasites

Protozoa

Nematodes

Parasites Helminths Trematodes Endo-

Cestodes

Arthropods ---------------- Ecto-

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Taxonomic classification of helminths

Sub

kingdom

Phylum Class Genus – examples

Metazoa Nematodes

Round worms; appear round in

cross section, they have body

cavities, a straight alimentary canal

and an anus

Ascaris (roundworm)

Trichuris (whipworm)

Ancylostoma (hookworm)

Necator (hookworm)

Enterobius (pinworm or

threadworm)

Strongyloides

Platyhelminthes

Flat worms; dorsoventrally

flattened, no body cavity and, if

present, the alimentary canal is

blind ending

Cestodes Adult tapeworms are found in the

intestine of their host

They have a head (scolex) with

sucking organs, a segmented body but

no alimentary canal

Each body segment is hermaphrodite

Taenia (tapeworm)

Trematodes Non-segmented, usually leaf-shaped,

with two suckers but no distinct head

They have an alimentary canal and

are usually hermaphrodite and leaf

shaped

Schistosomes are the exception. They

are thread-like, and have separate

sexes

Fasciolopsis (liver fluke)

Schistosoma (not leaf shaped!)

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Parasitic helminths spend most or all of their lives

in host and usually have the following

specializations:

May lack a digestive system. Absorb nutrients from

host’s food, body fluids, or tissues.

Have a reduced nervous system.

Means of locomotion is reduced or absent.

Complex reproductive system. Individuals produce

many eggs that can infect another host.

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Two main groups (phyla)

I. Platyhelminths (Flatworms)

II.Nematoda (Roundworms)

Life Cycle

Extremely complex

Intermediate hosts harbor larval (developmental) stage.

Definitive host harbors adult stage.

Sexual reproduction strategies:

Dioecious: Male and female reproductive organs are

found in separate individuals.

Monoecious (Hermaphroditic): One animal has both

male and female sex organs. Most hermaphrodites

copulate with other animals, a few copulate with

themselves.

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I. Platyhelminths (Flatworms)

Flattened from front to back.

Include:

1. Trematodes or Flukes

Leaf shaped bodies

Suckers for attachment and sucking fluids from host.

Some can absorb nutrients through their cuticle.

Named for host tissues in which adult lives. Blood Fluke (Schistosoma spp.): Cause

schistosomiasis which affects over 200 million people worldwide.

Cause damage to blood vessels, liver, and many other organs.

Live in waters contaminated with feces, burrow through skin of human and enter the circulatory system.

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Life Cycle of Blood Fluke Schistosoma

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2. Cestodes or Tapeworms

Long flat bodies

Intestinal parasites

Lack a digestive system, absorb food through cuticle.

Body Organization:

Head or scolex has suckers for attachment.

Body is made up of segments called proglottids.

Each proglottid has both male and female reproductive

organs.

Proglottids farthest from head are mature and contain

many fertilized eggs.

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Cestode (Tapeworm) Body Structure: Scolex and Proglottids

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Parasitic human tapeworms:

Beef Tapeworm (Taenia saginata): Human is definitive

host. Can reach up to 6 meters in length, scolex is 2

mm long with hundreds of proglottids. Infection

occurs by ingestion of contaminated, undercooked

beef.

Pork Tapeworm (Taenia solium): Human is definitive

host. Infection can occur from eating infected

undercooked pork.

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II. Nematodes (Roundworms)

Cylindrical body tapered at each end.

Have a complete digestive system: mouth, intestine, and

anus.

Body is covered by tough cuticle that resists drying and

crushing.

Most species are dioecious: separate males and females.

Males are smaller than females.

Over 90,000 known species. Most are free-living. Only

about 50 are human parasites.

Life cycle of parasitic nematodes is simpler than that of

flatworms.

Infections can be caused by eggs or larvae.

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Infectious eggs Ascaris (Ascaris lumbricoides): Large nematode,

up to 30 cm. Live in small intestines of humans, horses, and pigs. Eggs can survive in soil for long time.

Infectious larvae Adult Hookworm (Necator americanus): Live in

small intestine of humans, eggs are excreted in feces. Enter host by penetrating skin. Enters bloodstream, travels to lungs. Avoided by wearing shoes.

Ascaris lumbricoides

Head with three lips

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Ascaris

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Hookworms

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