Download Introduction to Parasitic Helminths Parasitic Helminths

9/9/2014
Introduction to Parasitic Helminths
Lecture 4
Medical Parasitology Course (MLAB 362)
Dr. Mohamed A. El-Sakhawy
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Parasitic Helminths

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
EndoCestodes
Arthropods ---------------- Ecto2
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Taxonomic classification of helminths
Sub
kingdom
Metazoa
Phylum
Class
Ascaris (roundworm)
Trichuris (whipworm)
Ancylostoma (hookworm)
Necator (hookworm)
Enterobius (pinworm or
threadworm)
Strongyloides
Nematodes
Round worms; appear round in
cross section, they have body
cavities, a straight alimentary canal
and an anus
Platyhelminthes
Cestodes
Flat worms; dorsoventrally
flattened, no body cavity and, if
present, the alimentary canal is
blind ending
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
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
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Genus – examples
Taenia (tapeworm)
Fasciolopsis (liver fluke)
Schistosoma (not leaf shaped!)
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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Schistosoma
Life Cycle of Blood Fluke
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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.
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Ascaris lumbricoides
Head with three lips
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Ascaris
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Hookworms
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