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Dr Abdul Ghaffar |
BACTERIOLOGY | IMMUNOLOGY | MYCOLOGY | PARASITOLOGY | VIROLOGY | |||||||||||||||||||||||||||||||||||
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Reading: Murray et al. (3rd
ed.), pp 661-669 |
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TEACHING OBJECTIVES |
The most significant trematodes from a
clinical point of view are blood flukes, Schistosoma mansoni, S. japonicum and
S. hematobium. Other trematodes of significance are intestinal fluke,
Fasciolopsis buski, liver fluke, Clonorchis sinensis and lung fluke,
Paragonimus westermani.
Schistosomiasis (Bilharziasis) The three species of Schistosoma have different geographic distributions. S. hematobium is prevalent in Africa, S. mansoni is found in Africa and America and S. japonicum is common in the far east. Epidemiology Morphology Life cycle Symptoms Pathology and Immunology Diagnosis Treatment and control
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WEB RESOURCES VIDEO
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Figure 1ASchistosomes. WHO
Figure 1CIntestinal schistosomiasis: eggs in the wall of the gut. WHO
Figure 1EEggs of Schistosoma haematobium (A). In this species, the eggs are large and have a prominent terminal spine at the posterior end. Length 112-170 µm. In (B), a greater magnification shows the miracidium inside the egg. CDC
Figure 1GSchistosoma haematobium eggs in section of bladder (H&E) © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
B
Figure 1IEgg of Schistosoma japonicum (A). The egg is typically oval or subspherical, and has a vestigial spine, which is better shown in (B). Schistosoma japonicum eggs are smaller (68 - 100 µm by 45 - 80 µm) than those of the other species. CDC
Figure 1KSchistosoma japonicum adult male and female © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
B
Figure 1MEggs of Schistosoma mansoni in a patient from Egypt. These eggs are large (length 114 - 180 µm) and have a characteristic shape, with a prominent lateral spine near the posterior end. The anterior end is tapered and slightly curved. When the eggs are excreted, they contain a mature miracidium (visible especially in A). CDC
Figure 1OSchistosoma mansoni adult male and female, in copulo © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
Figure 1QSchistosoma mansoni miracidium © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
Figure 1SSchistosoma mansoni cercaria © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission |
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Figure 1TLife cycle of schistosomes Eggs are
eliminated with feces or urine
Human contact with
water is thus necessary for infection by schistosomes. Various
animals, such as dogs, cats, rodents, pigs, hourse and goats, serve as
reservoirs for S. japonicum, and dogs for S. mekongi. |
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Figure 2AThe abdomen of an 11-year-old boy with intestinal schistosomiasis with the size and extent of the liver and spleen marked. Both are well below the midline, indicating the severity of infection. The disease has caused a stunting of the boy's growth, he is only 120cms tall and weighs 22 kg. WHO/TDR/Crump
Figure 2CA 13-year-old boy with schistosomiasis (bilharziasis). Hepatosplenomegaly, ascites, muscle atrophy, pyrexia, anaemia and haemorrhage from the gastrointestinal tract. WHO/TDR/Vogel
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Epidemiology Morphology Life cycle Symptoms Pathology Diagnosis Treatment and control
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Figure 3ALiver fluke, a trematode liver parasite - helminth (Fasciola spp.) Mouth and pharynx of the adult liver fluke. Humans are infected by ingestion of uncooked aquatic vegetation on which the metacercariae stage is encysted. Metacercariae excyst in the duodenum and migrate through the intestinal wall in to the peritoneal cavity. The larvae enter the liver by penetrating the capsule and wander through the liver parenchyma for up to 9 weeks. Most damage is done in the liver parenchyma by physical irritation and metabolic by products. © Dennis Kunkel Microscopy, Inc. Used with permission
Figure 3CFasciolopsis buski egg © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission |
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 Figure
4Life cycle of Fasciolopsis buski
Immature eggs are discharged into the intestine and stool
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Epidemiology Morphology Life cycle Symptoms Diagnosis Treatment and control
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Figure 5AClonorchis sinensis egg © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
Figure 5CClonorchis sinensis adults in section of liver (H&E) © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission |
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Figure 6Embryonated eggs are discharged in the biliary ducts and in the stool  .
Eggs are ingested by a suitable snail intermediate host
 ;
there are more than 100 species of snails that can serve as intermediate
hosts. Each egg releases a miracidia
 ,
which go through several developmental stages (sporocysts
 ,
rediae  , and cercariae
 ).
The cercariae are released from the snail and after a short period of
free-swimming time in water, they come in contact and penetrate the
flesh of freshwater fish, where they encyst as metacercariae
 .
Infection of humans occurs by ingestion of undercooked, salted, pickled,
or smoked freshwater fish  .
After ingestion, the metacercariae excyst in the duodenum
and ascend the biliary tract through the ampulla of Vater
 .
Maturation takes approximately 1 month. The adult flukes
(measuring 10 to 25 mm by 3 to 5 mm) reside in small and medium sized
biliary ducts. In addition to humans, carnivorous animals can
serve as reservoir hosts.
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Epidemiology Morphology Life cycle Lung fluke infects man (and domestic carnivores) when crabmeat infested with encysted metacercaria is consumed. The metacercaria reach the small intestine, exit their shell and bore their way, as young flukes, through the intestinal wall, through the thoracic diaphragm and penetrate the lung. There, they become enclosed in 1 to 2 cm cysts and reach maturity. The eggs are found in the sputum or, if swallowed, in the feces, 2 to 3 months after infection. The eggs, when introduced in fresh water produce a miracidia which penetrates the suitable snail. In the snail they develop into cercaria which break out in water and penetrate gills, muscle or viscera of fresh water crabs and become encysted in flesh as metacercaria (figure 8). Symptoms Diagnosis Treatment and control
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Figure 7AParagonimus westermani egg © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
Figure 7CParagonimus westermani adult in section of lung (H&E) © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission |
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Figure 8Paragonimus westermani (Lung Fluke) Life Cycle The eggs are excreted unembryonated in the sputum, or alternately they are swallowed and passed with stool  .
In the external environment, the eggs become embryonated
,
and miracidia hatch and seek the first intermediate host, a snail, and
penetrate its soft tissues .
Miracidia go through several developmental stages inside the snail
:
sporocysts  , rediae
 ,
with the latter giving rise to many cercariae
 ,
which emerge from the snail. The cercariae invade the second
intermediate host, a crustacean such as a crab or crayfish, where they
encyst and become metacercariae. This is the infective stage for
the mammalian host  .
Human infection with P. westermani occurs by eating inadequately
cooked or pickled crab or crayfish that harbor metacercariae of the
parasite  . The
metacercariae excyst in the duodenum
 ,
penetrate through the intestinal wall into the peritoneal cavity, then
through the abdominal wall and diaphragm into the lungs, where they
become encapsulated and develop into adults
(7.5 to 12 mm by 4 to 6 mm). The worms can also reach other organs
and tissues, such as the brain and striated muscles, respectively.
However, when this takes place completion of the life cycles is not
achieved, because the eggs laid cannot exit these sites. Time from
infection to oviposition is 65 to 90 days. Infections may persist for 20 years in humans. Animals such as pigs,
dogs, and a variety of feline species can also harbor P. westermani. |
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This page
copyright 2007, The Board of Trustees of the University of South Carolina |
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, and
shed their forked tail, becoming schistosomulae
.
The schistosomulae migrate through several tissues and stages to their
residence in the veins (
,
).
Adult worms in humans reside in the mesenteric venules in various
locations, which at times seem to be specific for each species
.
For instance, S. japonicum is more frequently found in the
superior mesenteric veins draining the small intestine
,
and S. mansoni occurs more often in the superior mesenteric veins
draining the large intestine
.
However, both species can occupy either location, and they are capable
of moving between sites, so it is not possible to state unequivocally
that one species only occurs in one location. S. haematobium
most often occurs in the venous plexus of bladder
,
but it can also be found in the rectal venules. The females (size
7 to 20 mm; males slightly smaller) deposit eggs in the small venules of
the portal and perivesical systems. The eggs are moved
progressively toward the lumen of the intestine (S. mansoni and S.
japonicum) and of the bladder and ureters (S. haematobium),
and are eliminated with feces or urine, respectively
.
The adults have a life span of about one year.
