Laboratory 8 cestodes

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PLATYHELMINTHES
“Cestodes”
Phylum Platyhelminthes
Class Cestoda
Order Pseudophyllidea
Diphyllobothrium latum (broad fish tapeworm)
Diphyllobothrium (Spirometra) mansonoides (sparganosis)
Order Cyclophyllidae
Family Taeniidae
Taenia saginata (beef tapeworm)
Taenia solium
(pork tapeworm)
Taenia pisiformis
Echinococcus granulosus (hydatid disease)
Echinococcus multilocularis
Family Hymenolepididae
Hymenolepsis nana
Hymenolepsis diminuta
Family Diplepididae
Dipylidium caninum (cucumber tapeworm)
All tapeworms infecting man belong to the orders: Cyclophyllidae & Pseudophyllidae
CESTODE FORM and FUNCTION: (lack digestive, respiratory, circulatory systems)
1. SCOLEX
-the “head” region of the tapeworm
-Equipped with some sort of holdfast organ – some variations include:
1) suckers
2) hooks
3) grooves
-the types of sucker-like organs on scolices of cestodes that infect man:
1) Acetabula (sing. acetabulum)
-circular or oval cup-shaped with muscular wall
-usual pattern: 4 equally spaced around scolex
2) Bothria
-“Slit-like” shallow pits or long grooves
-Usually 2 in number
-Lateral or dorsoventral in location on scolex
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-Rostellum
-protruding dome-shaped area on anterior end of scolex
- “armed” - hooks are arranged on it This is the rule.
-“unarmed” - no hooks
(presence and absence of rostellum and shape and arrangement of hooks
are identification keys in cestodes)
2. NECK
-region between scolex and strobila segments
-contains stem cells that give rise to new proglottids
3. STROBILA
-long chain of proglottids (segments)
depending upon species, from a few to thousands of proglottids
-each has sets of reproductive organs of both sexes
-“genitalium” in each set
Strobilization
-growth of strobila
-from anterior end as new proglottids are added
- proglottids closest to scolex – most immature
-as move further to posterior end, sexual maturity occurs
“oldest and most mature proglottids at posterior
Gravid Proglottids
-“gravid” = filled with eggs
-proglottids in strobila “cross-fertilize” (rarely self-copulate within same
proglottid)
How proglottids are shed: depends upon species of cestode
a) Intact gravid proglottid – Taenia spp.
detaches and shed intact in feces
b) Proglottid disintegrates as shed and eggs go out with feces – Hymenolepsis sp
c) Eggs shed from attached proglottid through uterine pore – Diphyllobothrium sp.
senile (empty) proglottids detach in a short chain which is shed in feces
“Senile” means an empty, formerly gravid proglottid
– has shed all the eggs in the uterus and is no longer producing any
more eggs
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4. TEGUMENT (old term for this is “cuticle”)
-Absorption: all nutrients absorbed thru tegument
-no GI tract in cestodes
-covered in microvilli called “microtriches” even on the suckers – increases
the absorptive area of tegument
-Excretion of wastes
-Osmoregulation
5. MUSCULAR SYSTEM:
-each proglottid has two layers of muscle
longitudinal
transverse
-enables each segment to move independently
6. NERVOUS SYSTEM:
-well developed in cestodes
-sensory: tactoreceptive
Chemoreceptive
7. REPRODUCTION
-Dioecious – rare!! (does not occur in species that infect man or his domestic animals)
-MONECIOUS is the Rule in cestodes
-hermaphroditic
-1 or two compete sets of both male and female reproductive organs per
segment (proglottid)
-fertilization occurs when repro systems mature (as proglottids proceed
toward posterior part of strobila)
-self-mating within the same proglottid is the exception, not the rule.
sperm are transferred between mature proglottids that lie next to
each other in intestine of definitive host
-gravid proglottids – contain 100’s to 1000’s of eggs
-CESTODE EGGS – three different types of shells (you will see two types in lab)
1. Dipylidium type
- thin shell
- operculated in Diphyllobothrium latum (look like trematode eggs)
-in egg packets for Dipylidium caninum
-seen also in Hymenolepsis nana and H. diminuta
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2. Taenia type – thick striated shell
-hexacanth embryo within
-Taenia sp., Echinococcus sp
LIFE CYCLES:
-Most require minimum of two hosts for completion of cycle
There is always an exception: Hymenolepsis nana
-- can complete life cycle in same host.
-- Invertebrate intermediate host is optional for H. nana
-very few complete life cycles of cestodes are known
-Life span: adults in intestine -- depends upon species involved
from few days to years and years (T saginata – 30 years!!)
ONCOSPHERE: the hexacanth larva in the egg
METACESTODE: juvenile (larval) form of tapeworm found in intermediate hosts
General Cycle:
egg Æoncosphere Æ metacestode Æ adult worm
(in egg)
(in intermediate
(in definitive host)
host)
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PSEUDOPHYLLIDIAN LIFE CYCLE:
egg Æ oncosphere = coracidium Æ procercoid Æ plerocercoid Æ adult tapeworm,
Coracidium:
-egg must get into water to hatch
-ciliated free-swimming oncosphere hatches out
-can survive only short time (must be eaten by intermediate host)
-has hooks which help it penetrate through copepod gut
Procercoid: in 1st intermediate host
-1st intermediate host usually arthropod, e.g. copepod
-a differentiating metacestode formed when coracidium enters 1st intermediate host
-loses cilia and penetrates thru gut into coelom and into tissue (becomes parenteral
which means in tissue outside of the gut)
-still has the onchospheral hooks seen in the coracidium
---now in a structure called the cercomer
---are used to help attach and penetrate through 1st host tissues
Plerocercoid: in 2nd intermediate host
- 2nd host (usually a fish) eats first intermediate host
-procercoid penetrates through gut wall, migrates into muscle and becomes
plerocercoid -develops a scolex
-this is the infective stage for definitive host
Sparganum : term for plerocercoid of Diphyllobothrium (Spirometra) mansonoides
What is difference between procercoid and plerocercoid?
Plerocercoid develops a scolex and has some strobila formation
What happens if bigger 2nd intermediate host eats a smaller infected 2nd intermediate host?
(big fish eats infected smaller fish)
Plerocercoid invades new 2nd intermediate host muscle tissue and remains as a
plerocercoid. Cannot develop further until in definitive host.
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CYCLOPHYLLIDIAN LIFE CYCLE:
Egg Æ oncosphere Æ
metacestode Æ
adult
Eggs
-eggs contain oncosphere when passed in feces of definitive host
-can remain viable in soil for weeks to months
-egg must be eaten by intermediate host for hatching to occur
-oncosphere uses hooks to penetrate thru gut wall and into tissues to transform into
metacestode form
4 Basic Metacestode (larval) Forms in Cyclophyllidians that you will see in lab:
Remember: there is only one metacestode type per species.
1. Cystercoid
-solid cyst ~ .03 mm or 300 μm in size with a single, inverted scolex within
-intermediate host is an invertebrate
- e.g. Dipylidium sp
Hymenolepsis sp
2. Cysticercus (“bladder worm”)
-fluid filled cyst (hence the term ‘bladder” worm) ~ 10 mm in size with scolex
inverted and invaginated
-intermediate host is a vertebrate
-e.g. Taenia sp
3. Coenurus
-fluid filled cyst ~ 20 – 100 mm with several inverted scolices each on a stalk
-intermediate host usually a vertebrate
-e.g. Taenia multiceps and T. serialis
4. Hydatid Cyst
-intermediate host is a vertebrate
-grow slowly but can get very large containing quarts of fluid and thousands and
thousands of scolices
-the most complex of the metacestode forms
-cyst is lined with germinal epithelium which gives rise to:
1) individual scolices (protoscolices)
2) brood capsules (each with many protoscolices within)
-“hydatid sand” – a granular deposit in hydatid cysts consisting of liberated
brood capsules and free scolices
-asexual reproduction and is often referred to as “budding” from the cyst
-e.g. Echinococcus sp
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Two types of hydatid cysts:
1) Unilocular: described above and seen in E granulosus infections
2) Multilocular: buds to outside - buds off daughter cysts externally – highly
invasive like an aggressive metastatic tumor: invasion of tissues of
infected organs;
-seen in E. multilocularis infections
Adult tapeworms in definitive host:
-metacestode form excysts in gut
-growth can be rapid
-Hymenolepsis diminuta can increase its size over a million times greater
than initial size at infection within 2 weeks
-once mature size for species is attained, growth then becomes the production of
proglottids to replace the gravid ones that are shed
-no real pathology associated with adults in lumen of intestine attaching to gut
lining
-absorb nutrition from nutrients in lumen
Note: D. latum has such a high requirement of vitamin B12 that it can cause
anemia in susceptible hosts (not everyone)
PSEUDOPHYLLIDIANS:
Order: Pseudophyllidae
-Scolex with bothria
-Proglottids remain attached and shed eggs through pore
shed proglottids are no longer productive
-Genital pores are central
-eggs are operculated (look like trematode eggs)
-1st intermediate host is arthropod (e.g. copepod)
-2nd intermediate host is fish
-Definitive host can be a human
Diphyllobothrium latum
Diphyllobothrium mansonoides (formerly Spirometra mansonoides)
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Diphyllobothrium latum
-worldwide distribution: Scandinavia, Japan, Russia, China, South America,
west coast and Great Lakes region of North America
Procercoid stage
1st intermediate host; copepod
- eggs must be deposited in fresh water for cycle to continue.
- coracidium will hatch and must be eaten in short time to continue cycle
2nd intermediate host: freshwater fish Plerocercoid stage
usually a minnow that eats the copepod.
(can be repeated many times until eaten by fish large enough to be eaten by
human or other definitive host)
-definitive hosts: “Fish-eating carnivores”
pinnipeds, mustelids, feline, canine, ursine, humans
-transmission to Definitive Host: eating raw or undercooked fish containing
plerocercoids
-carried to small intestine and attach to wall of gut by bothria
-Little or no pathology in humans unless affinity for B12 causes anemia in certain
susceptible individuals
- Adult parasite requires high amounts of vitamin B12 - must be obtained from
definitive host
--certain susceptible individuals (<2% of infected human population) can
develop a megaloblastic anemia from the worm exhausting their supply
of B12 (These individuals have a genetic defect: cannot produce
enough intrinsic factor which they need to absorb B12 in the diet. )
-Large worm – up to 10 meters long (lots of worm there!)
--usually only one per host (thank goodness)
-Prolific reproducer: female can produce a million eggs per day
-- proglottids are shed only when no longer producing eggs
-DX – usually finding long string of exhausted (senile) proglottids in feces (Yuk!)
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Morphology:
Adults:
- proglottids wider than long
- centrally located genital pore through which eggs pass
Eggs:
-are fertilized when passed in feces but are unembryonated
-eggs are operculated with thin shell (look like a trematode egg such as F. hepatica)
Prevent: DO NOT EAT RAW FRESHWATER FISH or RAW SALMON!
Diphyllobothrium mansonoides aka Spirometra mansonoides
-causes “sparganosis” in man (plerocercoid called a “sparganum” )
-Definitive host: feline and canine (NOT humans)
-1st intermediate host - copepod or other small crustacean
-2nd intermediate host – wide range of vertebrates including man.
Plerocercoids have capacity to develop in almost any animal and almost
indefinite plerocercoid transmission
can occur before being ingested by definitive host.
Examples include: frogs and other amphibians, snakes, birds, chickens
mammals such as pigs and man can also be intermediate or paratenic host
-Man as 2nd intermediate host:
stage in man as host is always a plerocercoid
nd
--as 2 IH, man can become infected by either procercoid or plerocercoid:
---procercoid Æ get infected by drinking unfiltered or unboiled water with
infected copepods in it
---plerocercoid Æ can get infection by eating raw or undercooked 2nd
intermediate host flesh
(Chinese practice of eating raw snake as a cure for certain ills, for example)
--- plerocercoid Æ can also get infected by following an Eastern Asian practice
of using frog (or snake ) poultices on wounds
Why would they do this? certain secretions from frog skins contain
peptides that have bactericidal activity!
Problem arises if frog ate an intermediate host and has plerocercoids
Parasites, stimulated by warmth of human host, will migrate out of skin
and into wound site
Patient now has plerocercoids – will cause a painful edematous
nodule around each plerocercoid - “sparganosis”
If plerocercoid goes to eye, can damage retina and cause vision problems or
get to brain via migration up optic nerve.
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Prevention: Don’t eat raw or undercooked meat, especially not raw snake and
don’t wear frog skins, even briefly!!
D. latum operculated egg
(small knob on non-operculated end
called abopercular knob)
D. latum slides: should be able to identify this species by scolex and mature proglottids
Uterus is darkly staining and rosette shaped in middle of proglottid
Ova resemble trematode eggs.
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D. mansonoides slides are labeled Spirometra mansonoides
CYCLOPHYLLIDEA
-Most tapeworms of mammals and birds are found in this category.
-Largest are found in family Taeniidae and intermediate host usually a mammal
Order: Cyclophyllidae
-scolex with 4 acetabula
-no rostellum: T saginata
-rostellum with hooks (armed rostellum)
-genital pores lateral
-eggs with thick, striated shells which will contain a hexacanth embryo
-intermediate host usually a vertebrate
-Taenia sp., Dipylidium sp., Hymenolepsis sp., Echinococcus sp.
Taenia saginata
“Beef” tapeworm
- Distribution: anywhere that beef is consumed in diet and there is a lack of sanitation
(one infected person decides to defecate in a pasture, can infect whole herd of
cattle!)
- no rostellum
-Scolex with 4 acetabula (suckers)
-proglottids: immature and mature slightly wider than long
gravid – much longer than wide
-gravid proglottids are shed intact and are motile! (will crawl around)
as shed proglottids dry up, will rupture releasing eggs
-eggs have characteristic thick, striated shell with hexacanth oncosphere within
(oncosphere has 3 pairs of hooks = six hooks = hexacanth!)
Intermediate host: Cattle (herbivore)
- eggs will hatch only in digestive system of the herbivore
Definitive host: man
egg with hexacanth Æ eaten by cattle Æ hatch in Small intestine Æ hexacanth uses
hooks and penetrates
gut wall
enters venules (bloodstream) Æ leaves capillary into striated skeletal muscle tissue
Æ develops into cysticercus
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“measly beef” is term used to describe muscle that has cysticerci in it.
Man eats undercooked “measly beef” Æ wall of cysticercous is digested away leaving
scolex
Scolex attaches to small intestine wall (Niche in definitive host)
2-12 weeks later gravid proglottids are being shed in feces
Gravid proglottids may actively migrate out anus at night
Cooking will prevent disease. Freezing at -5oC for at least 1 week will also kill the parasite.
Clinical signs definitive host: usually none
Level of parasitism in gut: more than one adult may be present
so flexible and relatively fragile, that obstruction of gut does not occur.
Rx: niclosamide or praziquantel
cattle vaccine available but not affordable in regions where parasite is endemic.
typical Taenia sp egg
cannot differentiate
between species with
egg
Note thick, striated shell
typical of Taenia eggs
Gravid proglottid of T. saginata
longer than wide
extensively branched uterus
(15-20 lateral branches/side)
1 lateral genital pore
Can distinguish between species
with scolex and gravid proglottids
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Taenia solium
“Pork” tapeworm
Distribution: SE Asia, Mexico, South and Central America, Eastern Europe, Micronesia,
Philippines
-intermediate host – pig and humans
-definitive host - man
Scolex:
-4 suckers
-armed rostellum (2 circles of hooks)
-mature proglottids: wider than long
-gravid proglottids longer than wide with lateral genital pore
(can distinguish from T saginata – fewer lateral branches in uterus)
-eggs look like any Taenia sp eggs
-adults can grow to 6 meters long in intestine
LIFE CYCLE:
eggs with
Æ
hexacanth
oncosphere
pigs eat Æ hatch and hexacanth
penetrates gut wall
Man eats undercooked pork
Æ migrates to muscle Æ forms a
cysticercus
Æ digests away cyst Æ scolex attaches to small intestine
5-12 week , T. solium producing gravid proglottids
Intestinal infections: generally asymptomatic in humans
no significant inflammatory response in intestine
parasite does not ingest host tissues
(like T. saginata, T. solium absorbs predigested nutrients in lumen of
SI)
Dx: proglottids not eggs! Eggs will not distinguish between T. saginata and T. solium
Care must be taken in handling samples – eggs of T. solium are infective
for humans also!
Interestingly enough, infected pigs do not become reinfected. They develop immunity to the
oncosphere.
T solium gravid proglottid
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DANGER WITH T SOLIUM – Man as Intermediate Host
-most dangerous adult tapeworm in humans because humans can be IH too!
1. Autoinfection can cause human “cysticercosis”
2. Reinfection by ingestion of eggs can cause cysticercosis
Autoinfection:
- Proglottids can migrate retrograde to stomach. Digestive enzymes stimulate hatching
of eggs. Now get 50,000 eggs hatching!! Massive infection!
- Gravid proglottid ruptures in intestine before passed in feces (not close to digestive
enzymes so infection not likely to be massive like above)
Autoinfection is the most pathogenic way of becoming intermediate host.
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Reinfection:
- Cysticercus is infective stage but man can be infected by eggs also!
-ingest eggs from environment (remember, eggs come from human feces!!)
Eggs contaminate the environment and can get in food. Patients have eggs under
fingernails, between toes, between fingers etc. despite personal hygiene.
This puts them and the rest of the household at risk.
egg hatches in GI tract Æhexacanth oncosphere released Æ penetrates through gut wall
Æ into bloodstream and then to virtually every organ/tissue in body
-Cysticerci can develop in any, every organ in body.
-Most common site: subcutaneous tissues
-Second most common site: eye followed by brain,
muscles, heart, liver, lungs and peritoneal cavity
-Pathology depends upon location of cysticercus. Host response
is to form capsule around cyst. Cysts will grow for about 3
weeks and then stop growing. Their size creates a space
occupying lesion in the affected tissue. See photo at right.
-In eye, where location is protected from host immune response,
the cysts grow larger. If form in retina, can cause irreparable
damage to vision.
-in brain ventricle, can cause hydrocephalus (again protected
from host immune response)
If there is one thing that can be considered at least a little good news: only about 25% of
people harboring adults will develop cysticercosis. Host immune response helps prevent it.
-Within 2 years, cysticerci die and become calcified in tissue. While alive, parasites
apparently produce substances that can down modulate the host immune responses to
them. When the parasites die, this no longer occurs and the dying parasites release
antigens that create an acute, local inflammatory response.
DYING PARASITE = PATHOLOGY. This can be very severe and clinically very significant
If cysticerci are located in the brain, the clinical condition is called “neurocysticercosis.”
Brain lesions can result in necrosis of brain tissue, encephalitis, meningitis, blindness,
paralysis, disorientation and other symptoms. Multiple brain cysts can be fatal. The
sudden onset of epilepsy in an adult with no family history should alert the physician to the
possibility of neurocysticercosis.
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Diagnosis: can be difficult, especially for neurocysticercosis: CAT scans, MRI, antigen
capture ELISA using CSF fluid; can biopsy cyst if subcutaneous and identify by microscopy
Taenia multiceps and Taenia serialis are two tapes with carnivores as definitive hosts
that will develop into coenurus in tissues of man if accidentally infected with eggs
from dog feces. Intermediate hosts: sheep for T. multiceps and rabbit for T. serialis.
Taenia pisiformis
-definitive hosts dogs and cats
-intermediate hosts rodents and rabbits
larval predilection site for cysticercus formation: peritoneum of rodent/rabbit
-next to Dipylidium caninum, is most common tapeworm of dogs
-not infective for humans
-scolex has 4 suckers and double row of hooks on rostellum
-segments are more rectangular in shape and genitalia (male and female) are
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single in each segment
-genital pores occur in irregular alternating sequence on either lateral margin
Echinococcus granulosus
-smallest tapeworm adults only 5mm long
-intermediate host: usually grazing herbivore
-definitive host: dog and other carnivores
- if infected, humans become intermediate host with the hydatid cysts
-sylvatic echinococcosis – no threat to man
-carnivore-herbivore relationship and cycle in wild animals
-Domestic cycle is threat to man dog and sheep!
-humans accidentally infected with eggs from dog feces
-Found in areas where sheep are raised: Europe, Africa, New Zealand, SW USA
Asia (Russia), South America, Canadian arctic (Inuits)
The northern variety in Canada involves caribou and moose as intermediate
hosts instead of sheep
-scolex 4 suckers, armed rostellum with hooks
-adult is composed of: scolex, neck and 3 proglottids (only last one is gravid)
-definitive host: infection can number into thousands of adults
-gravid proglottid disintegrates and eggs pass out with feces
Life cycle:
eggs with Æ ingested Æ hatches in Æ hexacanth Æ enters bloodstream Æ goes to
hexacanth
by
small
penetrates
organs
oncosphere intermediate intestine
gut wall
host
Within each organ, each larva secretes a hyaline membrane to surround itself.
The inner side will differentiate into a germinal epithelium
This germinal epithelium will give rise to scolices (“protoscolices”) and small cysts
called brood capsules (each with 20-30 protoscolices within it).
This entire structure is now a “hydatid cyst” which is fluid-filled and can become quite
large. Some of the largest have contained around 15 quarts of fluid!
This is a “unilocular” hydatid cyst.
Fluid within hydatid cyst is under pressure and trauma to cyst can rupture it.
Fluid is very antigenic and can cause anaphylactic shock if released from ruptured cyst. It
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also contains thousands to millions of scolices, each of which can establish a new second
generation hydatid cyst!!! (Why surgical excision has great risk).
Liver is most common site for hydatid cyst.
They can establish in lungs, brain and even bone marrow.
They can remain viable for 20 years – very long lived and continue to grow.
Intact cyst creates a space occupying lesion. In brain, may see CNS signs as a result of the
presence of the cyst. In limited confines of bone marrow, has been known to cause
necrosis of bone.
How do you diagnose these? DO NOT BIOPSY HYDATID CYSTS EVER!!!
radiographs, CAT scans, MRI
case history – dogs/sheep or travel to endemic region
Prevention: Keep sheep dogs dewormed. Yomesan (niclosamide)will kill adults.
Don’t feed raw infected meat to dogs!!
Echinococcus multilocularis
-definitive host: wild carnivore
-intermediate host: rodent such as field mouse, vole
- Humans can be intermediate hosts in this parasite’s life cycle.
Humans accidentally become infected. Trappers and fur handlers are individuals at risk
from eggs on fur. Camping, watch the unwashed berry eating!! Don’t set up a camp site
where there is a lot of animal droppings Æ eggs in the soil.
This one is really nasty! E. multilocularis forms the multilocular or alveolar hydatid
cyst. This cyst buds new daughter cysts to outside of cyst, not the inside. This disease
has a long, slow incubation period but the organism creates an aggressive invasion of
tissue in affected organ. It is progressive and destructive to organ tissue. It has a
metastatic-like spread. This means that it grows like a tumor, extending new daughter cysts
into the surrounding tissues.
When humans are intermediate hosts, the parasite does not produce protoscolices.
Therefore there is no hydatid sand in the fluid. The alveolar hydatid cysts are not filled with
fluid under pressure because they do not grow large. They bud outward into the
surrounding host tissue.
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Echinococcus egg (typical Taenia egg)
Family Hymenolepididae
Hymenolepsis nana
“dwarf” tapeworm
-old name: Vampirolepsis nana
-cosmopolitan parasite and most common tapeworm of humans
-scolex with retractable armed rostellum (circle of hooks)
-segments wider than long (very wide proglottids with large testes – only 3-4 proglottids)
-genital pores unilateral
-gravid segment disintegrates and eggs pass out with feces
-intermediate host is optional! can have direct life cycle with definitive host
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-when intermediate host is used - usually grain beetle
-forms cysticercoid in beetle
-cysticercoid is released when grain beetle is eaten, usually accidentally with
uncooked cereal (bugs got in the box to eat the cereal grain)
-definitive hosts: rodents and humans
Humans can serve as both intermediate and definitive hosts when you ingest H. nana eggs.
- eggs ingested Æ hatch in duodenum Æ oncosphere penetrates Æ forms cysticercoid
gut wall next to lymph
vessels in villi
1 week later
develops to adult
emerges from villi tissue into gut lumen
You are serving as both intermediate and definitive hosts when you ingest eggs of H. nana.
Pathology rare and usually only in massive infections resulting from autoinfection!
-with children, may see diarrhea with heavy infections
Dx; eggs in feces eggs are 30 to 50um (sizeable)
- with thin outer membrane and thick inner membrane
- has polar filaments on the inner membrane
Prevent: get rid of rodents
Hymenolepsis diminuta
-definitive host is Rat and occasionally humans
-larger than H. nana
-unarmed rostellum
-intermediate host: grain beetle (and about 90 other arthropod species)
-egg larger than H. nana egg and has no polar filaments within the egg
Dx: eggs in feces
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H. nana egg
H diminuta egg
Family: Dilepididae
Dipylidium caninum
“cucumber tapeworm”
scolex with 4 suckers, armed retractable rostellum
-definitive host: dog and occasionally man
-intermediate host: flea
-Larval stage in flea: cysticercoid
Life Cycle:
egg with Æ ingested by flea Æ cysticercoid Æ flea accidentally ingested
hexacanth
by dog or child playing
oncosphere
with dog
cysticercoid Æ scolex in small intestine Æ adult
proglottids produced and released
by 25 DPI (days post infection) gravid
-entire proglottid shed and is motile – will crawl around
-owners often get upset when see “rice grains’ crawling around dog’s rear end
- of if they are infected, proglottids in clothing or on bed.
-when disintegrated, released packet of eggs
-gravid proglottid long with double genital pores
-resembles a cucumber seed in shape, hence the
common name given this parasite
-flea larvae love to chow down on these egg packets.
- for definitive host, it’s the flea, not the proglottid!!
-No clinical disease
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