The Helminths
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•  Helminths (parasitic worms)
•  Eukaryotic
•  Multicellular animals
•  Chemoheterotrophic
•  Kingdom: Animalia
•  Phylum: Platyhelminthes (flatworms)
•  Class: Trematodes (flukes)
•  Class: Cestodes (tapeworms)
•  Phylum: Nematodes (roundworms)
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
We will focus on parasitic helminths
• Characteristics of parasitic animals
• They may have a reduced digestive system
• They may have a reduced nervous system
• They may have a reduced nervous system
• The generally have very complex life cycles
• Parasitic helminths go through many developmental
stages in (often) several hosts
• Each stage is referred to as a LARVAL stage
• Dioecious - each adult is only one sex (most NEMATODES!)
• Monoecious - each adult is BOTH sexes (true hermaphrodite most PLATYHELMINTHS! )
• The generally have very complex life cycles:
• The DEFINITIVE HOST harbors the adult
INTERMEDIATE
HOSTS
harbor larval stages
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Education, Inc., publishing as Benjamin
Cummings
Kingdom Animalia/Phylum Platyhelminthes
•  Platyhelminthes - "the flatworms”
•  Trematodes, or "flukes", attach to host tissue
and suck tissue fluids for nutrition.
•  Cestodes, or "tapeworms", generally exist in
the G-I tract and eat a lot of their host's food!
Disgustingly, they can get quite large (greater
than 20 ft.).
•  Generally monoecious
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Phylum: Platyhelminthes/Class:Trematoda
Trematodes or “flukes”
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Asian liver fluke
Humans as Definitive Host: the lung fluke
(Paragonimus westermani)
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Figure 12.26
Schistosomiasis: a dangerous disease caused by a fluke
•  Symptoms are caused by eggs shed into the
bloodstream from the adult in a human host
•  Female lives in a groove on the male’s body
•  Spread by feces and urine that contaminate
water supplies
•  250 million people affected!
•  The eggs can infiltrate and lodge in tissues
•  Inflammatory reactions cause tissue damage
called granulomas
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Schistosomiasis
• S. haemotobium
Granulomas in urinary
bladder wall
• S. japonicum
Granulomas in intestinal wall East Asia
• S. mansoni
Granulomas in intestinal wall African, Middle East,
South American,
Caribbean
• Swimmer’s itch
Cutaneous allergic reaction
to cercariae
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Africa, Middle East
U.S. parasite of wildfowl
Schistosomiasis
Egg and granuloma
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Male and female schistosomes.
Schistosomiasis
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Phylum: Platyhelminthes/Class:Cestoda
Cestodes or
“tapeworms”
Scolex (point
of attachment)
•  They are
intestinal
parasties
•  Have no
digestive
system
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Mature
proglottids
(segments)
Humans as Definitive Host (e.g., Beef tapeworm)
•  Taenia spp.
Taenia solium
•  Transmitted as larvae in
undercooked meat (beef) ova do not infect humans
•  Cysticerci may develop into
adults in humans, living in the
colon
•  Diagnosed by observing
proglottids and eggs in feces
•  Worm may live in the human
host for 25 years and grow to
18 feet
•  Strangely, there are often few
symptoms
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Humans as Intermediate Host (Dog tapeworm)
(Echinococcus granulosis)
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We are a
dead end in
the cycle
Hydatid Disease (when larvae migrate to liver, lungs, brain) •  Echinococcus
granulosus
•  Definitive host:
Dogs, wolves
•  Intermediate host:
Sheep and other
herbivores;
Humans
•  Transmitted by
ingesting E.
granulosis eggs
•  Treatment is
surgical
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Kingdom Animalia: Phylum Nematoda
•  Nematoda - the "roundworms” - Generally not
parasites, and are very ubiquitous in soil and
water.
•  "Pinworms" and "hookworms" are the exceptions.
•  Humans can be infected by either eggs or larvae all are dioecious!
•  Example: Pinworm Enterobius vermicularis
•  Spends its entire life in a human host (G-I
system), we are infected by the eggs (we are
definitive host)
•  Host is infected by ingestion of eggs - often
contaminated clothing or bedding
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Nematodes: Eggs Infective for Humans
Eggs are deposited on perianal skin
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Hookworms
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Hookworms
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Kingdom Animalia: Phylum Nematoda
•  Example: Hookworm Trichinella spiralis
•  Causes trichinellosis (trichinosis), acquired
by eating undercooked pork or game animals
•  Larvae migrate to muscles and encyst
•  Symptoms include fever, swelling, muscle
pain, and gastroenteritis
•  Can cause death if heart muscle is affected
•  Few cases in U.S., about 100/year, few
deaths
•  The disease is self-limiting
•  One pork chop can contain 10,000 larvae!
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Trichinellosis
Garbage,
including
undercooked
or raw pork
1 Adult Trichinella spiralis develop,
invade intestinal wall of pig, and
produce larvae that invade muscles.
Capsule
2
5 Meanwhile,
other animals
are infected by
eating infected
meat that has
been dumped.
Section showing T.
spiralis larvae encysted
in pig’s muscle tissue
(capsule is 0.25 to 0.5 in
length).
Section of T. spiralis
Undercooked pork
3 Human eats
undercooked
pork containing
cysts.
4 In human intestine, cyst
walls are removed, and
T. spiralis adults
develop. Adults
produce larvae that
encyst in muscles.
T. spiralis adult
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
A very common hookworm: Ascariasis
•  Ascaris lumbricoides
•  Often diagnosed
when an adult worm
(about a foot long)
emerges from the
anus, mouth, or nose
(I know…)
•  Lives in human
intestines
•  Transmitted by
ingesting Ascaris
eggs
•  Not usually severe
symptoms
Infects over 1 billion people worldwide
Proper sanitation helps elimate risk
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Helminthic Diseases of the Digestive System
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Summary of genera
•  Flukes
•  Clonorchis, Paragonimus, Schistosomia
•  Tapeworms
•  Taenia, Echinococcus
•  Pinworms
•  Enterobius
•  Hookworms
•  Ascaris, Trichinella
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings