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The effects of praziquantel on various larval and adult parasites from freshwater and marine snails and fish.

In addition to human and veterinary medicine, the defaunation of helminths in hosts is important in studies of behavioral ecology, life cycles and invertebrate immunology. Ecological studies have investigated the effects of larval parasites on the behavior of marine and estuarine snails (James, 1968, Journal of Natural History 2: 21-37; Sousa, 1984, Journal of Experimental Biology and Ecology 73: 273-296) and crustacean hosts (Holmes and Bethel, 1972. In Behavioral aspects of parasite transmission, E. U. Canning and C. A. Wright (eds.). Linnean Society, London, pp. 123-150). Researchers often want to experimentally complete helminth life cycles or make comparisons between infected and uninfected hosts but are unable to do so because of the lack of uninfected hosts. It would be useful to challenge previously infected snails with subsequent infections to study invertebrate "immunological memory" (Manning, 1980. Phylogeny of immunological memory. Elsevier/NorthHolland Biomedical Press, New York, 318 p.). Praziquantel (Droncit?, Bayvert Div., Cutter Lab) has been successfully used in veterinary and medical experiments against cestodes and trematodes in mammals and larval Digenea in fish (for review see Heckmann, 1983, Final Report, Idaho Department Fish and Game, 83 p.; Heckmann, 1984, American Fisheries Society, Fish Health Section 12: 7). The purpose of this study was to test the effectiveness of this drug on larval digenetic trematodes in freshwater and estuarine snails, larval cestodes in marine copepods and adult cestodes in elasmobranchs. The freshwater snails Physa sp. and Lymnaea sp., infected with larval Diplostomum spathaceum (Strigeoidae), were collected from the upper Salmon River Hatchery, Stanley, Idaho, in August 1983. All snails were exposed to artificial light to test for cercarial release. Physa sp. and Lymnaea sp. were 100% and 25% infected, respectively. The test and control groups had an equal number of each species of snails. There were 2 trails. The trails had 10 and 12 specimens of each species, respectively. The test groups were placed in aquaria containing 76.5 L of fresh water to which had been added 0.1 ml of injectable praziquantel (56.8 mg/ml active ingredient; final concentration 5.68 mg/ml). Approximately onethird of the snails were opened at 12, 24, and 48 hr to determine the condition of the sporocysts in the viscera. The viscera were examined histologically by standard light microscopic techniques. All tested snails contained degenerating sporocysts and dead furcocercous cercariae. The untreated snails had normal appearing sporocysts and larvae. To determine the effects of praziquantel at test concentrations on free living cercariae, a drop of praziquantel from the test aquarium was placed on a microscope slide containing cercariae from the control snails. All cercariae died upon exposure to the test solution. This experiment was repeated twice. The estuarine snails used in this experiment were Batillaria attramentaria and Cerithidea california. Batillaria attramentaria were collected from Elkhorn Slough in Monterey Bay, California. All specimens greater than 30 mm were infected with 1 species of larval trematode which had a sporocyst and non-furcocercous cercaria. Cerithidea california were collected in June 1984 in Goleta Slough, Santa Barbara, California. Half the specimens greater than 30 mm were infected by 1 or more species of larvae. Eleven species of larval digeneans have been identified from C. california in Goleta Slough (Yoshino, 1975, Veliger 18: 156-161). All snails used had shed cercariae when exposed to artificial light for 30 min. Fifty snails of each species were placed in each of 3 75.6-L aquaria. The salinity was 20-22 ppt. Test tanks (0.44 mg/L and 0.89 mg/L total concentration) contained either 1 or 2 praziquantel tablets (34 mg/tablet active ingredient) and 1 tank served as a control. The tablet was powdered in a mortar and pestle in which an aliquot sample of aquarium water had been added. This was repeated until all the powder went into solution. Ten snails were opened to check the condition