The digenean parasite, Plagiorchis elegans can establish infections in the incompatible snail, Biomphalaria glabrata , a vector of human schistosomiasis. Although embryonic development is arrested at the sporocyst stage, infection with a single parasite egg reduced reproductive success of this incompatible host to 64%. Heavier doses reduced this to 45%. Biomphalaria glabrata quickly acquired large numbers of parasites by ad libitum browsing on egg-contaminated substrates. Age of the host at exposure affected subsequent reproductive success and survival. Snails exposed as young (3mm), produced 54% fewer eggs, and suffered relatively high mortality. Adults (9mm) were affected only marginally. Plagiorchis elegans shares its ability to establish truncated infections in incompatible hosts with at least one other plagiorchiid. Haematoloechus medioplexus castrated the snail Stagnicola elodes, but not B. glabrata. Findings are discussed in the context of using incompatible digenean parasites as agents in the biological control of snails and snail-borne diseases, and ecological consequences of these infections.

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No description available.

Plagiorchis elegans is a typical digenean parasite that cycles through aquatic molluscs and insects as intermediate hosts. During emergence of P. elegans cercariae, infected snails moved to the top of the water column where they remained immobile for 2-3h. Consequently, the cercariae formed a dense cloud which dispersed slowly. The infectivity of cercariae was $<$20% upon emergence and peaked at 76% 4-6h later. This delay in reaching maximum infectivity may be an adaptation to prevent superinfection and the associated mortality of insect hosts. Cercariae transformed into metacercariae after penetrating Aedes aegypti larvae, the experimental insect host. Overall development of metacercariae, and excystment of infective metacercariae in vitro, was temperature dependent. However, there was an initial 8-hour period of obligatory host-parasite contact that was temperature independent. This may represent a period of major nutrient acquisition since young metacercariae were more active metabolically than older metacercariae, as measured by the in vitro uptake of $ sp3$H-glucosamine and $ sp3$H-leucine. Mosquitoes may have mechanisms to reduce losses of larvae to parasites. Oviposition by adult A. aegypti was reduced in waters that had previously contained P. elegans-infected larvae. We propose that this selective oviposition was due to the production of an oviposition deterrent compound produced by parasitized larvae that serves to reduce oviposition in sites detrimental to larval development.

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Cercariae of the digenean Plagiorchis noblei penetrate and kill a variety of aquatic larval dipterans. The present study determined whether these parasites can infect blackfly larvae, and established what biotic and abiotic factors affect parasite acquisition and the survival of infected hosts in the laboratory. Four species of blackfly larvae, Prosimulium mixtum, Simulium vittatum, S. decorum, and Stegopterna mutata, were exposed to cercariae in flowing water. Prevalence and intensity of infection of all species varied directly with exposure intensity and decreased with increasing water velocity, prevalence increased with larval size. Infection levels were higher for P. mixtum. The production of silk strands by the blackfly larvae and their persistence at low water velocities may facilitate parasite acquisition. Mortality among infected larvae of all species was three times that of controls. The data suggest that exposure to P. noblei cercariae adversely affected the survival of blackfly larvae.

Exposure of fourth instar Aedes aegypti larvae to graded concentrations of the entomopathogenic cercariae of the digenean Plagiorchis elegans, induced fundamental changes in the partitioning of time among a number of major behavioural activities. In the absence of the parasite, swimming, breathing and feeding remained stable over time. Grooming was rarely observed. In the presence of cercariae, larvae increased grooming 26-fold. Concomitantly, feeding was reduced to 33% of control values. Times spent swimming and breathing remained unchanged. The increase in grooming activity was attributable to the attachment of cercariae to the surface of the mosquito larvae, but was independent of infection. Grooming reduced cercarial penetration. Seventy-five percent of cercariae were lost to predation by larvae, but their caloric value was insignificant and failed to compensate for the loss of feeding time due to grooming. The findings are discussed in the context of their impact on larval growth, development and survival.

The host/parasite association between the pulmonate snail Stagnicola elodes and the digenean Plagiorchis elegans was examined with a view to identifying some of the biotic and abiotic factors that govern parasite egg development, infectivity and survival, the dynamics of cercarial production, as well as host susceptibility to infection, growth, reproduction and mortality. Plagiorchis elegans eggs passed with the feces of the hamster (Mesocricetus auratus) experimental definitive host were unembryonated. Between 0 and 30 C, the rate of embryonation continued to increase with temperature, whereas infectivity and survival were optimal between 10 to 20 C. Light adversely affected embryonation. Parasite acquisition increased with the intensity of exposure and sexual maturity. Infection invariably enhanced pre-patent host growth in a dose-dependent manner. Cercarial production increased with time and reached a plateau. Snails infected prior to sexual maturity released more cercariae than mature individuals. Heavily infected snails tended to die prematurely regardless of age at infection and parasite dose, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails infected when immature never reproduced, whereas reproductive snails ceased egg production within days of infection. Similar effects were observed in the incompatible snail host Biomphalaria glabrata, although no cercariae were ever produced. Both enhanced growth and parasitic castration are attributable to the mother sporocyst.

Cercariae of the digenean Plagiorchis elegans cause high levels of morbidity and mortality among larvae of Aedes aegypti . The impact of this parasite on the larval population dynamics of the experimental host as mediated through intraspecific competition, egg hatchability and ovipositional preference was assessed by calculating the probability of pre-imagos to develop from one stage to the next, or to die within consecutive 24 h intervals. Attractiveness of the water to ovipositing females in a dynamic larval population was not affected by exposure to the parasite, but varied significantly over time, regardless of food abundance. In optimally fed populations, these changes were positively correlated with pupal production, but were not affected by early instar development. Most of the entomopathogenic effects of the parasite were expressed in the pupal stage. Thus, exposure to the parasite significantly reduced adult emergence, but did not greatly impair pre-imago development. Nonetheless, exposure to various levels of the parasite significantly increased mortality of all larval stages. Suboptimally fed larval populations displayed severely impaired development and produced few adults. Exposure to P. elegans increased adult production slightly, suggesting depensatory mortality. In nutritionally stressed populations, no correlation was found between biomass and ovipositional preference, but attractiveness of the water was significantly increased by the removal of individuals by pupation or mortality. Egg hatchability was not significantly affected by population structure, but varied with the nutrient content of the water. This study provides new insight into the use of parasites as agents in the biological control of mosquitoes.

No description available.