Intestinal migration in the life-cycle of Pseudodiplorchis americanus (Monogenea)

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Abstract
SUMMARY: Adults of Pseudodiplorchis americanus occur in the host's urinary bladder, the typical habitat of polystomatid monogeneans in amphibians. However, larvae which invade the respiratory tract via the nostrils then migrate to the bladder through the stomach and intestine, a route which is without precedent amongst monogeneans. Parasite transmission is correlated with the spawning of the desert toad Scaphiopus couchii (June/July in Arizona, USA) and the gut migration, which takes as little as 5 min, occurs after 26 days post-infection (p.i.). Migration occurs during the host's feeding season, and therefore the parasites normally encounter maximum digestive activity along their path. Worms transferred experimentally from respiratory tract to the digestive fluids in the gut are killed within 60 s. However, natural migration is stimulated by an unidentified host factor which triggers protective adaptations, enabling parasite survival for up to 4 h in these lethal conditions. Without this essential stimulus, migration may be delayed for over 1 year and juveniles remaining in the respiratory tract undergo no development beyond that reached at 4 weeks p.i. Rapid reproductive development is resumed immediately after migration, leading to maturation around 1 month later and progressive accumulation of embryos in utero in preparation for transmission during the next summer's rains.