-
Adult worms in humans reside in the veins in various locations: Schistosoma mansoni in the inferior mesenteric veins, Schistosoma japonicum in the superior mesenteric veins, and Schistosoma haematobium in the vesical veins (these locations are not absolute). The females (size 7-20 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 they are eliminated with feces or urine, respectively. Under optimal conditions, the eggs hatch and release miracidia, which swim and penetrate specific snail intermediate hosts. The stages in the snail include 2 generations of sporocysts and the production of cercariae. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and migrate through several tissues and stages to their residence in the veins. Human contact with water is thus necessary for infection by schistosomes. Various animals serve as reservoirs for S japonicum and Schistosoma mekongi. Image courtesy of the US Centers for Disease Control and Prevention.
-
These are small operculated eggs. Size is 27-35 μm X 11-20 μm. The operculum, at the smaller end of the egg, is convex and rests on a visible "shoulder." At the opposite (larger, abopercular) end, a small knob or hooklike protrusion is often visible (as here). The miracidium is visible inside the egg. Image courtesy of the US Centers for Disease Control and Prevention.
-
Wet mounts with iodine. The eggs are ellipsoidal. They have a small, barely distinct operculum (upper end of the eggs in panel A). The operculum can be opened (egg in panel B), for example, when slight pressure is applied to the coverslip. The eggs have a thin shell that is slightly thicker at the abopercular end. They are passed unembryonated. Size range is 120-150 μm X 63-90 μm. Image courtesy of the US Centers for Disease Control and Prevention.
-
Adult flukes size range is 20-75 mm by 8-20 mm. Image courtesy of the US Centers for Disease Control and Prevention.
-
Eggs are excreted unembryonated in the sputum, or, alternately, they are swallowed and passed with stool (1). In the external environment, the eggs become embryonated (2), and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues (3). Miracidia go through several developmental stages inside the snail (4): sporocysts (4a), rediae (4b), with the latter giving rise to many cercariae (4c), which emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, in which they encyst and become metacercariae. This is the infective stage for the mammalian host (5). Human infection with Paragonimus westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite (6). The metacercariae excyst in the duodenum (7), penetrate through the intestinal wall into the peritoneal cavity, and then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults (8) (7.5-12 mm X 4-6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this occurs, completion of the life cycle is not achieved because the eggs laid cannot exit these sites. Time from infection to oviposition is 65-90 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. Image courtesy of the US Centers for Disease Control and Prevention.
-
The average egg size is 85 μm by 53 μm (range, 68-118 μm X 39-67 μm). They are yellow-brown, ovoidal or elongate, have a thick shell, and are often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs of P westermani are excreted unembryonated. Image courtesy of the US Centers for Disease Control and Prevention.
Tables
Vector |
Geographical Area |
Type of Trematode |
Biomphalaria glabrata |
Brazil |
S mansoni |
Bulinus globosa |
Nigeria |
S haematobium |
Bulinus truncate |
Iran |
S haematobium |
Oncomelania hupensis nosophora |
Japan |
S japonicum |
Thiara granifera |
China |
P westermani; M yokogawai |
Semisulcospira libertine |
China |
P westermani; M yokogawai |
Polypylis hemisphaerula |
China |
F buski |
Parafossarulus manchouricus |
China |
C sinensis |
Bithynia leachi |
Germany |
O felineus |
Pirenella conica |
Egypt |
H heterophyes |
Lymnaea truncatula |
England |
F hepatica |
Trematode |
Definitive Host |
Intermediate Host 1st 2nd |
Source of Infection |
|
S haematobium |
Humans |
Freshwater snails (genus Bulinus) |
Absent |
Contact with water contaminated by cercariae |
S mansoni |
Humans, occasionally baboons and rodents |
Freshwater snails (genus Biomphalaria) |
Absent |
Penetration of skin by cercariae |
S japonicum |
Humans, dogs, pigs, cattle, mice, mustelids, and monkeys |
Amphibian snails (Oncomelania species) |
Absent |
Penetration of skin by cercariae |
S mekongi |
Humans and dogs |
Aquatic snails (Tricula aperta) |
Absent |
Penetration of skin by cercariae |
F hepatica |
Sheep, goats, cattle, and other herbivorous animals |
Amphibian snails (family Lymnaeidae) |
Aquatic vegetations and watercress |
Ingestion of aquatic plants and watercress infected with metacercariae |
C sinensis |
Humans, dogs, pigs, cats, rats, and several species of wild animals |
Freshwater snails (family Bulinidae) |
Freshwater fish (family Cyprinidae) |
Eating raw or partially cooked freshwater fish or dried, salted, or pickled fish infected with encysted metacercariae |
O felineus |
Humans and other fish-eating mammals |
Aquatic snails |
Freshwater fish |
Eating fish infected with metacercariae |
P westermani |
Humans, wolves, foxes, tigers, leopards, lions, cats, dogs, and monkeys |
Freshwater snails (family Pleuroceridae and Thiaridae) |
Freshwater crab or crayfish |
Ingestion of freshwater crabs or crayfish infected with metacercariae |
F buski |
Pigs and humans |
Planorbid snails of the genera Segmentina, Hippeutis, and Polypylis |
Freshwater plants such as water caltrops, water chestnut, water bamboo, water hyacinth, and lotus |
Ingestion of freshwater aquatic plants that harbor metacercariae |
|
S haematobium |
S mansoni |
S japonicum |
Adult |
|
|
|
Body surface of male |
Finely tuberculate |
Grossly tuberculate |
Nontuberculate (smooth) |
Testes |
4-6, in a cluster |
6-9, in a cluster |
7, in a linear series |
Position of ovary |
Posterior to middle of body |
Anterior to middle of body |
Posterior to middle of body |
Number of eggs in uterus |
20-30 |
1-4 |
50-300 |
Egg |
|
|
|
Size and shape |
110-170 μm long 40-70 μm wide Terminal spine |
114-175 μm long 45-68 μm wide Lateral spine |
70-100 μm long 50-65 μm wide Central spine |
Cercaria |
|
|
|
Cephalic glands |
2 pairs, oxyphilic |
2 pairs, basophilic |
4 pairs, oxyphilic |
