Plagiorchis noblei infections impair the survival and development of fourth instar Aedes aegypti larvae. Mortality during the larval and pupal stages reached 92%, and 60% of the emerging adults were malformed. The metacercariae interfere with pupation and the emergence of adults. Larvae and pupae that fail to transform to the next developmental stage within the normal time characteristically persist for extended periods, but invariably die without transforming. Whereas 82% of the control larvae gave rise to functional adults, only 4% of infected larvae managed to do so. Such effects may facilitate the transmission of the parasite.
Infection with the metacercariae of the digenean Plagiorchis noblei had no effect on the susceptibility of Aedes aegypti larvae to predation by guppies (Poecilia reticulata). In contrast, infected larvae were rendered more susceptible to being taken by meadow voles (Microtus pennsylvanicus) than were uninfected controls. Since voles, which will feed at the edge of water, are capable of supporting the adult fluke, transmission of the parasite to the definitive host may be facilitated.
Third-instar mosquito larvae (Aedes aegypti (L.)) infected with the metacercariae of the digenean Plagiorchis noblei Park exhibited significant behavioral changes. Larvae infected with more than three metacercariae were less active, spent more time suspended from the surface of the water, and spent less time in the bottom half of the water column than did uninfected control larvae. In contrast, larvae infected with one or two metacercariae were more active and spent less time suspended from the surface of the water. Such lightly infected larvae spent more time in the bottom half of the water column than the more heavily infected larvae. Since cercariae of P. noblei generally settle to the bottom of the water column, lightly infected larvae thus may be rendered more susceptible to superinfection. In contrast, heavily infected larvae may be removed from further exposure. Furthermore, heavily infected larvae may be more susceptible to surface-feeding predators. These behavioral changes of the insect host may facilitate the transmission of P. noblei both to the second intermediate and definitive host.
The effects of distributional patterns of the host on the acquisition of Plagiorchis noblei cercariae by Aedes aegypti larvae were determined. Mosquito larvae that were allowed to disperse were more susceptible to infection than confined larvae. Because these mosquito larvae are known to aggregate in light and disperse in darkness, they are more likely to acquire P. noblei infections at night. The timing of cercarial emergence in relation to the distributional patterns of the mosquito host is discussed.
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