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Flashcards in 3 Microbiology: Helminth Parasites Deck (88):

Meaning of Helminth

means worm
refers to the parasitic and non-parasitic species belonging to the phyla PLATYHELMINTHES and NEMATODA


Species in Platyheminthes

flukes (trematodes)
tapeworms (cestodes)
and other flatworms


species in Nematoda

(heartworm, hookworm)


What are Helminths?

are multicellular eukaryotic parasites that usually possess digestive, circulatory, nervous, excretory, and reproductive systems.


Where are parasitic helminths found?

found extracellularly in infected host


Where can helminths live?

many live only in the intestinal tract of parasitized host
others may invade internal organs such as the: liver, lungs, blood, subcutaneous tissue, and brain


Helminths posses a protective external layer called?

a cuticle or tegument


Helminths may lack a?

a digestive system
nutrients can be passively absorbed from intestinal contents and surrounding fluids or by active ingestion of host tissue, fluids or both


Nutrients are stored as what in helminths?



In helminths respiration is predominantly?

(however, the larval forms may require oxygen)


What kind of reproductive systems do helminths have?

they have a complex repro system
majority of them are egg laying (oviparous)
a few are bear live young (viviparous)
can produce thousands of eggs, by which a suitable host is infected
adults are MACROscopic
eggs are MICROscopic


Helminth life cycle

have a complex life cycle involving various INTERMEDIATE HOSTS for completion of each larval (developmental) stage of the parasite and a DEFINITIVE HOST for the adult parasite


What is the definitive host?

the host that harbors the adult, sexually mature form of the parasite


What is the intermediate host?

the host that harbors the larval or asexual stage of the parasite (are not reproducing in host)


What is dioecious?

adult helminths may be dioecious
meaning that male repro organs are in one individual and the female repro organs are in another
and reproduction occurs when two adults of opposite sex are in the same host
(different sexes)


What is monoecious or hermaphroditic?

in some adult helminths
one parasite has both male and female repro organs.
two hermaphrodites may copulate and simultaneously fertilize each other or a hermaphrodite may fertilize itself.


Do helminths multiply within their hosts?

No they do not multiply within their hosts. (in numbers, just produce eggs or larva that need the next host)
the eggs produced by the female do no mature into adult worms in the host, instead, some of them are excreted to infect others.
so the number of worms in an animal or individual increases only though repeated exposure and so most animals and individuals carry low numbers of worms


(trematodes and cestodes)

are dorso-ventrally flattened
in most cases are hermaphroditic
are the most primitive of the helminths
either have no digestive tract or a rudimentary one
many have complicated life cycles that require an alternation of hosts (i.e indirect life cycles)
{life cycles: direct is definitive to definitive.
indirect is definitive to intermediate to definitive.}


Class Trematoda are what?



Class Trematoda (Flukes) have what type of bodies?

have flat, fleshy, leaf-shaped bodies


Flukes have two muscular suckers: what are they?

an oral sucker
a ventral sucker


what is the oral sucker?

the beginning of an incomplete digestive system


what is the ventral sucker?

is simply an organ of attachment


Describe a flukes digestive system

consists of lateral tubes that do not join to form an excretory opening
flukes can obtain food by absorbing it through their nonliving outer covering: cuticle (rudimentary digestive system)


Describe a flukes repro system

most are hermaphroditic with both male and female repro organs in a single body
SCHISTOSOMES are the only exception: they have cylindrical bodies (like roundworms) and separate male and female worms.


What is the the only exception to the hermaphroditic reproductive system in flukes?

Schistosomes, have separate male and female worms.
(they also have cylindrical bodies like roundworms too)


How are flukes named?

given common names according to the tissue of the definitive host in which the adult live
examples: intestinal fluke, liver fluke, blood fluke, and lung fluke


Describe a flukes life cycle?

complex life cycle that involve two or three sequential hosts (definitive and intermediate host(s)) for the completion of their life cycles)
without exception ALWAYS involves water and mollusks (snails and clams)
so each species of fluke is found in association with the specific species of snail required as its intermediate host.
in the intermediate host an asexual repro cycle takes place


What is an OPERCULUM?

a "lid" at the top of the shell of a fluke egg.
the lid opens to allow the larval worm to find its appropriate snail host
exception: the SCHISTOSOMES do not have an operculum, rather the egg shell splits to liberate the larva


What is an exception in flukes operculum?

Schistosomes, do not have an operculum, rather the egg shell splits to liberate the larva.


How are fluke eggs excreted?

eggs are usually excreted in feces of the definitive host
if the life cycle is to continue, the eggs must reach a body of water, usually freshwater


What is a miracidium?

a small, ciliated larva within the fluke egg


The rate of development of the miracidium in the egg is influenced by what?

the water temperature
below 10 degrees Celsius no development occurs
between 10 to 26 degrees celsius there is an increasing rate of development
at 26 degrees celsius development of the miracidium may take 2 to 4 weeks


How is the miracidium released from the egg?

the operculum at top of egg shell pops open, releasing the miracidium
the miracidium does not feed, /instead it swims about until it finds a snail of the right species for its first intermediate host.
if it fails to find a snail within 24 hrs, its energy reserves become depleted and it dies.


What does the miracidium do once it finds a snail?

it bores into the snail's tissues.
glands at the pointed anterior end of the miracidium produce lytic enzymes that allow the penetration of the soft tissues of the snail.
following penetration, the ciliated coat is lost and the miracidium undergoes a metamorphosis to form a long, tubular larva called a SPOROCYST.


what is a sporocyst?

is an undifferentiated mass of germ cells


Once a SPOROCYST is formed how does development continue?

the sporocyst migrates to the gonad or digestive gland (often referred to as the "liver") of the snail where it continues to form masses of germ cells
each germinal cell undergoes repeated divisions to become a germinal ball.
a germinal ball then develops into a REDIA
****a sporocyst may give rise to 5 to 8 rediae***
these rediae are released into the tissues of the snail when the sporocyst wall ruptures


What is a Redia (or rediae)?

a more differentiated larva possessing a mouth and a rudimentary digestive tract.


What is the CERCARIA and how is it developed?

germ cell within each redia develop into second generation rediae
each germinal ball formed from the second generation rediae develops into the final larval stage

the redia has a birth pore through which second generation redia or cercariae produced inside escape


Describe a CERCARIUM (cercaria)

resembles the adult worm possessing suckers and a rudimentary digestive and excretory system.
also possesses a tail for locomotion after leaving the snail
must reach the definitive host in order to complete its life cycle
(go to host through water ingestion or through skin)


If only one intermediate host is required, how does the cercaria enter the definitive host?

the cercaria can ACTIVELY penetrate the skin of the definitive host (e.g as in schistosomiasis)
or may enter the definitive host PASSIVELY when the host drinks water in which the cercaria is swimming
or may first lose its tail and enclose itself in a protective cyst on aquatic vegetation and is eaten with the vegetation by the definitive host (metacercaria)


what is a metacercaria?

is cercaria that lost its tail and enclosed itself in a protective cyst.
(gets to definite host by vegetation, or through another intermediate host (such as crawfish) that then the definite host digests)


If more than one intermediate host is required, how does the cercaria enter the definitive host?

after the cercaria leaves the first intermediate host, it enters the second intermediate host and ENCYSTS inside it until it is eaten by the definitive host


What happens once the metacercarial cyst is in the definitive host?

the metacercarial cyst is digested in the host's small intestine and the liberated immature fluke penetrates the intestinal wall and migrates to its predilection site where it eventually matures into an adult fluke


what is a marita?

is a liberated immature fluke


See figure and description of "Life History of Fasciola hepatica (Liver Fluke)" page 116 of Helminth notes!!!!!



What is in the class Cestoda?




the habitat of the adult cestode is the intestinal tract
DEFINITIVE HOST is the animal in which the larval stage develops into an adult worm
INTERMEDIATE HOSTS are the animals which the eggs develop into the larval stage


Describe a tapeworms digestive system

tapeworms do not have a digestive system
food is absorbed from the small intestine through the soft body wall of the worm


Describe what tapeworms look like.

adults vary in size from few millimeters to several meters in length
are flat, ribbonlike
and are divided into segments called PROGLOTTIDS


What are stobila?

chain of proglottids


Describe tapeworms repro system

with male and female repro organs present in each mature proglottid


What is the SCOLEX? and describe it

the SCOLEX is the head at the anterior end usually has two or four muscular cup-shaped SUCKERS and CROWN OF HOOKLETS for attaching to the intestinal mucosa of the definitive host
posterior to the scolex is a short unsegmented portion called the NECK
the scolex is the "germinal center" from which the other segments of the tapeworm arise


Proglottids are continually produced by?

continually produced by the neck region of the scolex, as long as the scolex is attached and alive
the segments near the scolex are young and sexually immature
they increase and size and the development of their internal parts progresses as they are pushed farther and farther away from the scolex by the new segments


The middle segments are called what? and why?

because the reproductive organs have become mature and functional
the male genital organs develop first, followed by the female repro organs


What happens in the TERMINAL PROGLOTTIDS?

in the terminal proglottids, following fertilization of the eggs, the repro organs degenerate, leaving only the uterus, which is now full of thousands of fertilized eggs
the proglottids are now called GRAVID PROGLOTTIDS



the gravid segments are passed out of the host, either singly or in chains
the eggs are released by disintegration of the gravid segments or by expression of the eggs through a genital pore in the uterus


Embryonic development takes place where?

in most cases it occurs in the uterus
the eggs, when they are laid, contain spherical or ovoid embryo


what is an oncosphere?

a spherical or ovoid embryo contained in the egg
the first-stage larva and is infective for the intermediate host
consists of a hexacanth embryo (possesses three pairs of hooks) surrounded by two embryonic membranes


See figure on page 118 of notes!!!



Describe the tapeworm life cycle

have a complex life cycle requiring one or more intermediate hosts


What happens once ingested by the intermediate host?

the egg hatches releasing the hexacanth embryo which penetrates into the intestinal wall in order to reach a suitable part of the body for its further development
here it grows into a CYST (a fluid-filled sac)


What happens in the CYST?

further development occurs in the cyst resulting in the formation of a BLADDERWORM (cysticercus)
the bladderworm is passively transferred to the final host when the latter ingests the infected intermediate host


What does the bladderworm do once it arrives in the intestine?

he bladderworm evaginates its head and attaches it to the mucosa
the bladder is discarded and the scolex begins producing proglottids


See figure and description of "Life History of Taenia pisiformis (Dog Tapeworm)" on page 119 in notes!!!




are free-living (in soil and water)
or are parasitic (on plants and animals)
unsegmented worms, usually cylindrical and elongate in shape and tapered at the ends



a large body cavity that contains fluid under pressure that may vary up to one half atmosphere above that of the surrounding medium


Describe roundworms digestive system

have a complete digestive system with a mouth, an intestine, and an anus



inelastic fibers are in the body cuticle and are arranged in such a way that an increase in internal pressure causes an increase in length of the nematode with minimal change in diameter.


Describe roundworms repro system

most are dioecious (male and female organs are on separate individuals)
Males are smaller than females of their species and have one or two hardened COPULATORY SPICULES on their posterior ends.


What are COPULATORY SPICULES used for?

are used to dilate the vulva of the female and guide sperms to the female’s genital pore.


How are roundworm infections divided?

are divided into the intestinal roundworms and the blood and tissue roundworms.


Roundworms life cycle

Various types of life cycles are found among the nematodes
depending to some extent on the degree of adaptation to a parasitic existence that has been reached.


Describe round worms life cycle WITHOUT an intermediate host, in which eggs DO HATCH in the open and larvae are free-living for a time.

infective larvae are active
Entry into the host is through the mouth with food and water
but some infective larvae of some species can penetrate the host’s skin as well as entering through its mouth (eg, Ancylostoma caninum [hookworm]).


Descibe a round worms life WITHOUT an intermediate host, in which eggs develop in the open but DO NOT HATCH there.

infective larvae are passive inside the egg.
Entry into the host occurs via ingestion [eg, Ascaridae].


Describe roundworms life cycle WITH an intermediate host present, in which eggs DO HATCH and when they DO NOT HATCH.

eggs DO hatch:
the larvae enter the intermediate host after a limited free existence.
Intermediate host is eaten by the definitive host.
eggs DO NOT hatch:
are ingested by the intermediate host.
Intermediate host is eaten by the definitive host.



filariae are parasites of lymph, blood, subcutaneous and connective tissues.
transmitted by mosquitoes or biting flies.


Describe the Filarioidae

worms are viviparous and produce larval worms called MICROFILARIAE
The larvae enter the blood of the host, from which they are taken up by a blood-sucking intermediate host, inside which the infective larvae develop.

When the intermediate host sucks the blood of the definitive host, the infective larvae break out of the proboscis of the intermediate host and penetrate into the definitive host through its skin.


Describe the migration of roundworms once they enter the final host

many nematodes migrate through the body before settling down in their normal habitat, resulting in mechanical damage to various organs and tissues.
the presence of extraintestinal larvae in the host are at times more serious than that of adult worms in the intestine.


See the figure an description of "Life History of **Dirofilaria immitis (Canine Heartworm)***" on page 122 in notes!!!



The pathogenic consequences of helminthic infections are related to what?

the size, movement, and longevity of the parasites
effects are usually very varied and in many cases represent a combination of several entities.


Example of a pathogenic consequence of helminthic infections related to host's tissues

Removal of the host’s tissues and fluids by blood-sucking activities of certain nematodes [eg, hookworms].
Excessive loss of blood can result in the death of the host


Example of a pathogenic consequence of helminthic infections related to nutrition

Competition for food. Sometimes the parasite may compete with the host for food.
Competition for vitamin B12 by Diphyllobothrium latum may result in anemia in the host.
The parasite may indirectly be the cause of decreased food utilization by the host; it may cause a reduced appetite with a concomitant reduction of food intake, or an increased passage of food through the digestive tract.


Example of a pathogenic consequence of helminthic infections related to migration through tissues

Migration through body tissues such as the skin, lungs, liver, intestines, eyes, and central nervous system can damage the tissues directly.


Example of a pathogenic consequence of helminthic infections related to mechanical tissue damage

Blockage of internal organs.
Large adult organisms can physically block the intestinal canal to produce necrosis and rupture [eg, ascarids], as well as the bile ducts.
Blockage of the bile ducts can result in ICTERUS [jaundice].
Blockage of lymph flow leading to edema and elephantiasis, is associated with the presence of adult filaria in the lymphatic system.
Blockage of blood vessels can produce tissue infarction.
Tissue damage occurs when various organs of attachment [eg, head-spines, suckers, etc] are inserted into the tissues as anchors.


Fasciola hepatica (fluke)

only uses intermediate host, Lamnaeid snail.


Taenia pisiformis (tapeworm)

intermediate host is cottontail rabbit.
(most common tapeworms though the intermediate host are fleas)


Dirofilaria immitis (heartworm)

intermediate host (mosquito)

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