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Trematodes in the desert: The parasitization of
Scaphiopus couchii by Pseudodiplorchis
americanus
Jeffery Cote
A Toad and its parasite
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The Spadefoot toad (Scaphiopus couchii) is
found in the deserts of southwest North America.
 The parasite is the monogenean,
Pseudodiplorchis americanus (no common
name)
 One host system in this case.
Spadefoot toad
Surviving in the desert
 How
does an amphibian survive the
desert?
 How does a parasite, whose
ancestors evolved in an aquatic
environment, survive and infect its
host in the desert?
Toad coping mechanisms
 Toad
enters hibernation for 10 months
 Toads breed in temporary pools created
by intense summer storms.
 Usually a single breeding period, occurring
on one day after storm from 9pm to 4am.
Problems faced by the trematode
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The trematode’s larval stage requires water to infect the
toad.
Limited to a 7 hour time frame, with no second chances
(usually).
The larval stage enters the nostrils, though toad is not
entirely submerged.
Mature adults found in toad bladder, must survive the
stomach to get there.
Parasite lifecycle
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Mature adult dwells in bladder, produces eggs, eggs
leave the bladder and enter the water.
The oncomiracidia hatch instantly upon entering the
water and swim a toad’s nostrils.
The worms stay in the nostrils for 24 hrs. Lose cilia cells
within 1-2 hours of invasion.
Worms move to mouth and nearby cavities and begin to
feed on blood, this stage lasts 1 week.
Worms next migrate to the lungs and remain there for 12 weeks.
The juvenile worms go back up to the buccal cavity, then
migrate to the bladder.
Adult worms in bladder, simultaneously produce eggs
and grow.
Modified uterus
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The oncomiracidia developed to the infective stage in the
adult.
Adult worms have a modified uterus.
Trematode uses cytoplasmic connections to the lining of
the egg capsule, to mimic a placenta. Supplies nutrients
on tap to the larvae’s tegument.
Egg capsule is made of elastin.
Larvae size increases as it progresses through the
uterus.
Storage of eggs lasts 2 months, dead eggs recycled.
Benefits of modified uterus
 Resource
efficient
 Can store 100 eggs on average.
 Constant alert for infection
transmission possibility.
Parasite timing
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Parasite must be able to time egg release with host entry
into water.
Parasite relies on host sexual cues, host sexual
excitement.
Studies conducted on related parasite used
immunostaining of neurons associated with ootype
musculature.
Immunostains were strongly present during periods of
rapid egg production in parasite, triggered by toad
spawning.
Immunostains were absent after toads finished
spawning.
Super Larvae

Largest larvae of the Monogenea
 Oncomiracidia can swim for periods longer than
4 hrs.
 Oncomiracidia swim via cilia, which can resist
drying out for 1 hour, maintaining maximum
swimming capability.
 Average lifespan is 15 hours.
Mean abundance
Mean abundance cont.

Back to back storms create heightened infection
levels.
 1988 study, mean abundance of 44 worms per
toad after 2 consecutive storms.
 After 3rd storm, mean abundance of 81 worms
per toad, 100% infection level.
 Probability of successful infection by individual
larvae: 0.3
Larvae migration
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No known adaptations for passage in and out of lungs.
 Protection needed for crossing stomach.
 Worms artificially transplanted from lungs to digestive
tract, all died within 2 min. Tegument stripped away..
 Cue for migration to bladder is also cue to produce
protection.
 Juvenile worms develop two types of vesicles, unaltered
until migration. Vesicles discharged as worm moves
through gut.
Temperature
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Parasite reproduction fastest at 25 degrees
Celsius and inhibited at 16 degrees or lower.
 Temperatures only favourable for 4.5-5 months
out of the year.
 Older worms continue to grow and produce
eggs, no maturation period.
 Trend continues with worm age.
Environmental restraint summary
Questions?