Allopatric combination of Fasciola hepatica and Lymnaea truncatula is moreefficient than sympatric ones

Parasites are capable of rapid evolutionary changes relative to their hosts , due to short life cycle, short generation time, and high fecundity. The d irection of the evolution of parasite virulence can be studied in cross-tra nsfer experiments, combining hosts and parasites from different localities, and comparing the outcome of established (sympatric and potentially locall y adapted) and novel (allopatric) combinations of hosts and parasites. We a imed to compare the compatibility with snails hosts, the infectivity of met acercariae in rabbits and rats, and the fitness among different combination s (French-FF and Spanish-SS sympatries and allopatry-FS). The first isolate of Fasciola hepatica and its corresponding intermediate host, Lymnaea trun catula originated from Lugo's northwestern Spain. The second isolate of par asite and snail was collected in the Limoges area in central France. The Sp anish snails were more susceptible to their sympatric trematode than the Fr ench snails. The Spanish flukes were more infective to intermediate hosts ( snails) than the French flukes, but subsequent definitive hosts (rats or ra bbits) infections remained similar. The estimated fitness was low in sympat ric infections and highly similar (from 4.7 to 5.3). The fitness similarity corresponds, however, to different variations in life-history traits that could represent different strategies among the host-parasite local combinat ions. The infection rate in snails, metacercarial productivity, metacercari al infectivity, and the estimated fitness were better for allopatric combin ation (FS). The susceptibility data showed a higher efficiency of flukes in the allopatric snail population than in their local snail population. Howe ver; our results were obtained after one generation and from a single isola te and it remains to be determined if all allopatric fluke-snail isolates m ay present a better fitness. Nevertheless our results indicate that introdu ction of liver fluke-infected cattle should be monitored carefully, as it c ould result in the introduction of more efficient parasites. (C) 2000 Austr alian Society for parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.