Intro to Trematodes Flashcards
What is a trematode?
phylum platyhelminthes (flatworms - flukes) with bilateral symmetry (acoelomate) class trematoda subclass digenea (at least 2 hosts in life cycle). always snail in 1st IH
What is important about the egg?
- exists host to external environment 2. contains embryo (miracidium) 2. operculum - “escape hatch” that allows miracidium to hatch
What causes hatching?
condition different from internal environment (e.g. light, temperature, osmolarity, pH)
Describe the structure of a miracidium.
- 100 um, non feeding, 2. cephalic ganglion (brain) 3. cilia 4. sensory endings: photoreceptors, georeceptors, chemoreceptors 5. glands 6. germinal cells
What is primary miracidial behavior?
find and infect snail within 24 hours
What are the 2 components of miracidial behavior?
- host finding 2. infection of snail
How does a miracidium find a host via host finding behavior?
- environmental cues (+ or - to light, gravity, temperature) 2. similar to responses of snail hosts so parasite can go to same location 3. host cues (miraxone)
What is miraxone?
substances of nail origin (amino acids, fatty acids, NH3)
What is evidence that miracidium have host finding behaviors?
attraction measured by rate of change of direction (RCD) & swimming speed: 1. far from snail: low RCD & fast swimming -> moves to other spaces 2. close to snail: high RCD & slow swimming -> stays close to host
What happens when a miracidium infects a snail?
respond to components of snail mucus: 1. wrong cues -> leave 2. correct cues -> penetrate snail 3. once inside -> metamorphosis
What are the characteristics of a sporocyst?
- loses cilia 2. forms new tegument (microvilli - increased surface area allows for absorption) 3. no mouth/digestive system 4 germinal sac - embryo -> daughter (secondary) sporocysts -> rediae -> cercariae
What does redia do?
exit sporocyst & produce either daughter rediae or cercariae
What is the effect of sporocysts & rediae on snail hosts?
parasitic castration: rediae move to reprodutive structures & feed on tissue & destory it, hormone manipulation
What is parasitic castration?
snail not allocating resources to reproduction: resources go to growth
What occurs during hormone manipulation?
schistosomin is snail peptide hormones involved in normal reproduction -> trematode larvae stimulate over production -> inhibits snail reproduction
What behavior do cercariae exhibit?
exit snail via glands into external environment (1-3 days) swim or craw to encyst/infect host
What is true about the metacercaria stage?
transmission stage (A & B strategies), inside DH it responds to complex stimuli to excyst and develop into adult worm
Describe the structure of a metacercaria
no tail. multilayered cyst wall produced by cystogenous glands, A & B are thin & simple
What is polyembryony?
asexual reproduction that results in multiple offspring from one embryo
Where does species A encyst?
on plants: mucus from post acetabular glands & sheds tail
How do species B & C find a suitable host?
via spatial location (gravity, light, temperature, host cues) & temporal location (circadian release - cercariae released from snail only at certain times of day that correlate to when the host is present)
What happens when species B & C infect a host?
post-acetabular glands (mucus -> attachment), sheds tail, preacetabular glands produce enzymes -> allows for penetration
What happens when species B & C is inside a host?
species B has cystogenous glands -> metacercaria in 2nd IH. species C inside DH - migrates in body & develops into adult worm
What do bird schistasomes respond to?
respond to bird cues (ex: movement, warmth, shadow, chemical cues)
