Cyathocotyle bushiensis and Sphaeridiotrema globulus were previously implicated in the annual mortality of wild ducks in southern Quebec. Of 21 snail species collected from nine aquatic communities in southern Quebec and southeastern Ontario, May to October 1989 and 1990, only Bithynia tentaculata was heavily and consistently infected with C. bushiensis metacercariae. Although infection levels varied spatially, a consistent concave seasonal pattern of prevalence and abundance was found which may be due to overwintering metacercariae and the almost simultaneous onset of cercarial transmission and recruitment of new snails in mid-summer. Even though S. globulus metacercariae were found in 18 of 21 snail species, B. tentaculata is believed to contribute most to transmission to ducks. The variety of seasonal patterns and large scale spatial heterogeneity in the prevalence and abundance of S. globulus suggested that cercariae may be from more than one source. Some of the spatial variation in metacercarial levels may be due to small scale processes; extensive spatial heterogeneity at a scale of 10 m was detected in S. globulus cercarial transmission to sentinel snails. This small scale heterogeneity may be due to B. tentaculata having small home ranges as suggested by mark-recapture studies on movement. In aquaria, even though smaller B. tentaculata were more active than larger ones and intraspecific microhabitat preferences were found, preliminary results from ducks (Anas discors) feeding on snail communities in pools suggested that snails are ingested in proportion to their presence. Therefore, the importance of a snail species or size class in the transmission of the two parasites to ducks may depend only on its density and metacercarial abundance. Finally, another mode of transmission is proposed for S. globulus: ducks may become infected by consuming empty snail shells containing viable cysts.

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.

No description available.

In this thesis, the efficiency of orally applied praziquantel (PQ) in two concentrations (Pramik 1 and Pramik 2) and mebendazole (MB) were tested on metacercaria of eye flukes (Diplostomum spathaceum, Rudolphi, 1819) in the grass carp (Ctenopharyngodon idella, Valenciennes, 1844). Naturally infected fish (average fish weight was 1.6 g and standard deviation was 0.1) were divided into four groups, for each tested group (Pramik 1, Pramik 2 and MB) belonged two aquariums of 100 liters each with 25 fish and for a control group belonged three aquariums of 100 liters each with 25 fish. The fish in the individual groups were fed with medicated feed containing praziquantel (Pramik 1 - 2.5 g PQ. kg-1 of feed, Pramik 2 - 1.25 g PQ. kg-1 of feed) and medicated feed mixture KP1 enriched with mebendazole (2.5 g MB. kg-1 of feed). The control group was fed with KP1 without drug addition. During the test the parasitological examinations of the ophthalmic lens were performed. The prevalence and intensity of infection were recorded. In the group of fish fed with medicated mixture containing MB, no statistically significant differences of the intensity of the infection indicating the MB potential in the fight against this fish infection were reported during the test. In the case of tested groups of fish fed with medicated feed mixture containing PQ (Pramik 1, Pramik 2), a statistically significant difference in the intensity of the infection was proved compared with the control group already after the first examination (estimated dose of consumed PQ. kg-1 of weight of fish = 500 and 250 mg). Near complete elimination of the eye flukes occurred in the group fed with feed mixture Pramik 1 after the expected consumption of 900 mg PQ. kg-1 of weight of fish. Statistically significant differences in the intensity of the infection in PQ treated fish groups compared to control groups indicate the usability of medicated feed containing PQ in the fight against fish diplostomosis.