III. Life-cycle of Trematodes (Chapter 15 pp. 228-234), BLY 459 2011
A.
At least 2, usually 3, sometimes 4 hosts
B.
Final hosts are vertebrates
1.
Usually fish and birds
2.
Humans can be hosts
C.
Adults
1.
Typically in digestive tract (= lumen) of host (There are important
exceptions that infect humans.)
2.
Limited to mucosal and epithelial tissues
D.
Eggs
1.
Pass out in feces
2.
Can be eaten by first intermediate
3.
Can hatch into miracidia (plural)
Slide: Egg of Schistosoma mansoni
E.
MIRACIDIUM
1.
Free living larval stage
2.
Most are swimmers
3.
Find and penetrate tissues of the first intermediate host (always a
mollusk)
4.
Covered with cilia
Slide: Miracidium of Fasciola hepatica
F.
SPOROCYST
1.
Sac in intermediate host snail
2.
Depending on species, they undergo asexualreproduction to
produce . . .
a.
Daughter sporocysts
b.
Rediae (plural)
c.
Cercariae
G.
REDIA
1.
Characteristics
a.
Mouth
b.
Sucker
c.
Rudimentary gut
d.
Motile
2.
Some have been observed to consume the larval stages of other
trematodes in the snail host (= interspecific competition)
3.
Reproduce by sequential polyembryony to produce (depending
upon density) . . .
a.
Daughter rediae
b.
Cercariae (plural)
Slide: Redia of Fasciola hepatica
H.
CERCARIA
1.
Produced asexually by both daughter sporocysts and rediae
2.
Characteristics
a.
Two suckers
b.
Tail
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3.
Free living stage that leaves snail to either . . .
a.
Penetrate skin of definitive host
b.
Encyst in a second intermediate host to become
metacercariae (plural)
Slide: Cercaria of Fasciola hepatica
Slide: Cercaria of Schistosoma mansoni
I.
METACERCARIA
1.
Found in/on almost anything that is eaten by a vertebrate
a.
Fish
b.
Crabs
c.
Snails
d.
Hydromedusae
e.
Plants (Encyst upon, not in)
2.
Second intermediate host eaten by vertebrate definitive host
3.
Metacercariae excyst in digestive juices of definitive host
Slide: Metacercariae in grass shrimp & crab meat
Slide: Metacercarial cysts of the trematode, Microphallus turgidus, in the abdomen of a
grass shrimp (Palaemonetes pugio)
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