1. Trematodes in the desert: The parasitization of Scaphiopus couchii by Pseudodiplorchis americanus
Jeffery Cote

2. A Toad and its parasite
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.

3. Spadefoot toad

4. 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?

5. 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.

6. Problems faced by the trematode
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.

7. Parasite lifecycle
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 1-2 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.

8. Modified uterus
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.

10. Benefits of modified uterus
Resource efficient
Can store 100 eggs on average.
Constant alert for infection transmission possibility.

11. Parasite timing
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.

12. 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.

13. Mean abundance

14. 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

15. Larvae migration 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.

16. Temperature
Parasite reproduction fastest at 25 degrees Celsius and inhibited at 16 degrees or lower.
Temperatures only favourable for months out of the year.
Older worms continue to grow and produce eggs, no maturation period.
Trend continues with worm age.

17. Environmental restraint summary

18. Questions?