INFORMATIONAL HANDOUT:
INTRODUCTION: In the Domain EUKARYA, there are several groups (phyla) of organisms that can cause problems
for humans. We will discuss the parasitic protists (single celled eukaryotes) and fungi in another section. Almost all
multicellular animals that are parasitic of humans fall into a group we commonly call the helminth worms. It is a group
name used to refer to members of 2 phyla: Platyhelminthes (the flatworms) and Nematoda (the roundworms).
Please keep in mind that there are many, many beneficial worms – even in these 2 groups! The free-living soil and
water nematodes, for example are of incalculable importance in maintaining the Earth ecosystem. So – keep this in
mind as we explore the horrors of the world of parasitic helminth worms. Please also note that microbiologists who
study parasitic worms and protozoans are called PARASITOLOGISTS.
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PARASITIC HELMINTHS are different from their ‘free-living counterparts:
1. They usually lack a digestive system – getting nutrients by absorption from host.
2. Their nervous system is reduced since they do not need to hunt prey.
3. Their means of locomotion is reduced or lacking since they’re immersed in suitable habitat, & don’t need to seek it.
4. Their reproductive system and life cycle is highly developed and complex.
Adults may be dioecious (male and female separate individuals) or hermaphroditic (male and female organs in one
individual). Most start life as an egg, develop into a larva, then mature into an adult – and each stage may be carried
out in a different host or location.
A. ROUNDWORMS (Also called Nematodes) Phylum Nematoda
Examples: Pinworm, Intestinal roundworm (Ascaris), Trichinella, Hookworm (Necator and Ancylostoma)
images below are from Graphic Images of Parasites, Ohio State University, and we abide by Fair Use copyright guidelines in our borrowing of
them. http://www.biosci.ohio-state.edu/~parasite/ascaris.html
Nematodes - What They Are and A Few Interesting Facts: Nematodes are tiny roundworms that are common in
soils everywhere, from the freezing Arctic to dry, hot deserts. They are particularly abundant in grassland
ecosystems. To give you an idea of exactly how common nematodes are, consider this: one cubic foot of soil can
contain millions. Nematodes can be most easily classified according to their feeding habits. Some graze on bacteria
and fungi. Some like plant roots; others prey on other tiny animals. Some aren't fussy at all and will eat any of the
above mentioned food items. Nematodes can't move through the soil unless a film of moisture surrounds the soil
particles. Under hot, dry conditions, nematodes can become dormant, allowing them to survive long periods of
drought. As soon as water becomes available, they quickly spring back to life.
Why They Are Important
Among the thousands of species that have been identified, many are considered beneficial because they boost the
nutritional status of the soil. Nematodes feed on decaying plant material, along with organisms that assist in the
decomposition of organic matter (bacteria and fungi). This helps disperse both the organic matter and the
decomposers in the soil. Increased organic matter concentration and decomposition boost nitrogen and phosphorus
levels. Nematodes aren’t all good guys. Some are parasites of animals (including humans ) and plants. But because
some nematodes prey on other animals, they can be useful for control of pest insects. Nematodes are also being
investigated for their potential as biological controls for noxious weeds.
Biology 215, Ruth A. Gyure
Western CT State University
1. INTESTINAL ROUND WORM OF HUMANS: Ascaris lumbricoides
http://www.biosci.ohio-state.edu/~parasite/images.html
Ascaris lumbricoides is one of the largest and most common parasites found in humans. Adult females, longer than
males, can measure up to 18 inches long. It is estimated that 25% of the world's population is infected with this
nematode. The adult worms live in the small intestine and eggs are passed in the feces. A single female can
produce up to 200,000 eggs daily! Humans are infected when they ingest developed eggs (with larvae inside, about
2 weeks old) passed in the feces. The eggs hatch in the small intestine, the juvenile penetrates the small intestine
and enters the circulatory system, and eventually the young worm enters the lungs. Here it leaves the circulatory
system, enters and migrates up the air passages into the pharynx where it is swallowed, and once in the small
intestine the juvenile grows into an adult worm. Ascaris infections can cause many serious problems as worms
migrate throughout the body, causing blockages in GI tract and liver, and hemorrhage or fluid buildup in lungs.
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1. A large mass of Ascaris lumbricoides that was passed from the intestinal tract. Ruler at bottom of image is 4 cm (about 1.5 inches) in length.
2. Scanning electron micrograph of anterior end of Ascaris showing 3 prominent "lips." (Original image from http://www.soton.ac.uk/~djab/.)
3. Ascaris lumbricoides, fertilized egg. Note egg is covered with a thick shell that appears lumpy (bumpy). approximate size = 65 µm in length.
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2. TRICHINOSIS, caused by Trichinella spiralis
http://www.biosci.ohio-state.edu/~parasite/trichinella.html
Trichinella spiralis, the trichina worm, can be found in many species of carnivores and omnivores. Animals are
infected when they eat developed larvae (juveniles) in raw or undercooked meat. The larvae mature into adults in
the small intestine. In a few weeks, female worms give birth to larvae. (The males die after fertilizing the females,
and the females die after producing larvae.) and larvae enter blood stream of the host and, ending up in the host's
muscles where they mature. The next host eats the muscle and so on. In the muscles the larvae cause a severe host
reaction that results in soreness and tenderness. Although this parasite probably only rarely causes fatalities in
humans, it can cause extreme discomfort. We know Trichinosis as a parasite that humans can contract from eating
raw or undercooked pork. Through an aggressive program of meat inspection and education, the incidence of
trichinosis in pigs in the United States has been lowered to less than 1%, so it is unlikely (but not impossible) that
pork products purchased in your local supermarket will contain Trichinella larvae.
Trichinella spiralis larvae in muscle section.
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Biology 215, Ruth A. Gyure
Western CT State University
3. PINWORMS, Enterobius vermicularis
http://www.biosci.ohio-state.edu/~parasite/enterobius.html
It’s estimated that pinworms infect more than 400,000,000 people throughout the world (10% of humans), and in
many areas of the world (e.g., North America and Europe) it is the most common nematode parasite of humans. It is
#1 parasite world-wide, infecting more than 1,000,000,000 people total (25% of humans). Adult pinworms (5-10mm
ong) live in the large intestines. After copulation the males die. When the female is ready to lay eggs she crawls out
of the anus and deposits the eggs on the perianal skin. A single female can produce more than 10,000 eggs. After
laying her eggs, the female also dies. When ingested by another person (highly contagious - spread by contact with
infected person’s hands, fingernails, bed linen, etc.) the eggs hatch in the small intestine, and the juvenile worms
grow into adult, sexually mature worms in about a month. Pinworms infections can be asymptomatic or result in mild
gastrointestinal upsets. Scratching of the perianal skin to relieve the itching can lead to bacterial infections that result
in more itching, etc. This cycle can result in a situation where the infected person becomes very uncomfortable.
Children infected with pinworms often undergo behavioral changes, including restlessness, irritability, and insomnia.
In women, the adult pinworms can enter the vagina and cause additional irritation.
Adult pinworm (anterior end is on the left). Original
image from Oklahoma State U., College of Veterinary
Medicine,
http://www.cvm.okstate.edu/~users/jcfox/htdocs/clinpa
ra/clinpara.htm
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4. HOOKWORMS, Necator americanus and Ancylostoma duodenale
http://www.biosci.ohio-state.edu/~parasite/hookworms.html
Adult hookworms live in small intestine of humans. Eggs are excreted in feces, hatch out in soil, and the larvae feed
there on bacteria. Larvae enter host by penetration of skin – then they migrate to the lungs via blood and lymph.
There, it is coughed up in sputum, swallowed and passed to small intestine. It attaches there by hooks on it’s mouth
and enjoys your delicious and nutritious blood. The worms eventually mate and lay thousands more eggs. Human
hookworms infect an estimated 800,000,000 persons worldwide. Attachment of hookworms to the host's small
intestine causes hemorrhages, and the hookworms feed on the host's blood. Hookworm disease can have
devastating effects on humans, particularly children, due to the loss of excessive amounts of blood.
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1. Ancylostoma braziliense, a common hookworm of cats. The oral opening contains one large (and one smaller) cutting "tooth" one each side.
2. Another image of the anterior end of Ancylostoma caninum. The cutting "teeth" (6 total, 3 on each side) can be seen clearly.
3. Anterior end of Necator americanus, one of two species of hookworms infecting humans. The oral opening has cutting "plates" as opposed to
"teeth." The muscular esophagus is labeled in this image (*).
Biology 215, Ruth A. Gyure
Western CT State University
4. Scanning electron micrograph of the oral opening of Ancylostoma duodenale, another species of human hookworm.
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B. FLATWORMS: Phylum Platyhelminthes
Examples: Tapeworm (Taenia sp.), Blood fluke (Schistosoma), Lung fluke (Paragonimus)
1. TAPEWORM (Taenia spp. ) Also called cestodes, Class Cestoda
http://www.biosci.ohio-state.edu/~parasite/taenia.html
There are 2 Taenia species for which humans serve as the definitive host: Taenia saginata (now often called
Taeniarhynchus saginatus), the beef tapeworm; and T. solium, the pork tapeworm. Several species of Taenia also
infect dogs and cats and veterinarians encounter them in pets’ feces. All species of Taenia have similar life cycles.
The hermaphroditic adult tapeworm lives in the definitive host's small intestine. Proglottids, which contain eggs,
break off the posterior end of the tapeworm, and are either passed intact in the host's feces, or they dissolve and only
eggs are passed in the feces. The intermediate host ingests the eggs, and a cysts develops inside. The definitive
host is infected when it eats an intermediate host infected with cysts. As adults, tapeworms rarely cause problems; in
exceptional cases the tapeworms might physically block the intestinal tract, due to their large size, or proglottids
might become lodged in the appendix and result in appendicitis. The proglottids of Taenia are large and muscular
and occasionally single proglottids or long chains of proglottids might crawl out of the anus of an infected human.
Not fun!!! Infection with cysts, as in the intermediate host is not at all common for humans – but if it occurs can cause
serious problems.
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1. A stained whole mount of the scolex (holdfast) of Taenia solium, the pork tapeworm; measures ~ mm across. The four suckers are numbered.
Note the presence of an armed (hooked) rostellum (*); the scolex of Taenia saginata, the beef tapeworm, does not have an armed rostellum.
2. Anterior region of Taenia solium showing the scolex and neck region.
3. A gravid proglottid of Taenia solium. (gravid means ‘bearing eggs’).
4. Mature proglottids of Taenia sp.; each proglottid measures about 4 mm in length.
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2. BLOOD FLUKES (Schistosoma sp. ) Often called ‘flukes,’ or ‘trematodes,’ Class Trematoda
http://www.biosci.ohio-state.edu/~parasite/schistosoma.html
Schistosomes differ from tapeworms in that the sexes are separate and they reside in the blood vessels of the
definitive host. There are a number of species of schistosomes that can infect humans, but most human infections
are caused by one of three species distributed throughout Africa, southeast Asia, northern S. America and parts of
Caribbean. It is estimated that approximately 200,000,000 million people are infected with schistosomes, resulting in
1,000,000 deaths each year. Male and female worms (about 10 mm) live in the veins of the abdominal cavity. Here
they mate and females produce eggs. Adult worms can live 20-30 years and, depending on the species, and each
female can produce several hundred eggs each day. The eggs escape from the body by penetrating the walls of the
veins and small intestine or urinary bladder, and they are passed in the feces or urine. They hatch in water, the first
intermediate host (a snail) is infected, and cercariae are liberated from the snails. Humans come in contact with
them and the cercariae penetrate their skin. This occurs when the humans swim, bath, wash clothes, etc., in rivers
and streams. After the cercariae penetrate the skin the immature worms enter the circulatory system and migrate to
the veins of the abdominal cavity, and in about six weeks they reach sexual maturity. Eggs penetrate the walls of the
veins and the small intestine or urinary bladder, and cause significant damage to the tissues which can hemorrhage
and leak blood in urine or feces. As infection progresses, tissues become inflamed, fibrotic, and unable to function
normally. Some eggs escape and are swept up in the circulatory system and deposited in the host's liver. The liver
Biology 215, Ruth A. Gyure
Western CT State University
responds to the presence of the eggs by encapsulating them in a fibrous granuloma. Damage to small intestine (or
urinary bladder) and liver accumulate and result in chronic, disabling disease that is often fatal.
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1. Schistosoma mansoni eggs. Note the lateral spine on each egg.
2. and 3. Schistosoma sp. in copula. The female worm lies within a ventral groove of the male called the gynecophoral canal, an obvious
adaptation to these worms living in a "turbulent" environment (the host's blood stream).
3. LUNG FLUKE (Paragonium westermani )
This diagram can be found in your text on page 359.
It shows typical fluke life cycle.
4. Sheep liver fluke (you'll see in class)
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OTHER FLATWORMS – Class Turbellaria (Planaria!)
The planaria are flatworms found in freshwater environments around the world. These interesting creatures live in shallow pools, streams, swampy
places, ponds and lakes. They do prefer cool, running water, so warm stagnant ponds are not the place to look for them. They secrete slime and
use cilia on their ventral side to move along surfaces looking for food. Planarians suck food into a pharynx which protrudes from the middle of the
ventral side. These interesting animals exhibit a variety of behaviors as they move toward and way from various environmental stimuli.
Image source: http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html
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TEXT REFERENCE: Tortora CHAPTER 12, 25, 23:
ACKNOWLEDGMENT AND OTHER INFO:
Atlas of Medical Parasitology http://www.cdfound.to.it/HTML/intpar1.htm
Images of Parasites (Ohio State University) http://www.biosci.ohio-state.edu/~parasite/ascaris.html
Nematode NewsFlash: http://nematode.unl.edu/cobbyak.htm
Biology 215, Ruth A. Gyure
Western CT State University