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Using a range of parameters, the ability of rats (Rattus norvegicus) to successfully transmit Echinostoma friedi to the next host was examined under experimental conditions. The concept of Experimental Transmission Success (TM), defined as the number of hosts that become successfully infected after exposure to a number of infective stages produced by a previous host per unit of inoculation at which this latter host was exposed, was introduced. Using data for the egg output and miracidium hatching and infectivity, the TM permits us to estimate the ability of a particular defintive host species to successfully transmit a parasite species. This concept may be also useful to compare the transmission fitness of a parasite in different definitive host species. Moreover, variations of the Experimental Transmission Success over the course of the infection were calculated by the use of the Weekly Experimental Transmission Success (TMW). Overall, considering the complete duration of the experiment, the TM of E. friedi using rats as definitive hosts was 0.68 infected snails/metacercaria. However, positive values of the TMW were only obtained from 2 to 4 wk post-infection, with a maximum during the third wk post-infection. When comparing the TM values of E. friedi in rats with those calculated in hamsters on the basis of previously published data, E. friedi appears to be more appropriate to move through this portion of its life cycle when using hamsters (Mesocricetus auratus) as the final host than rats. Toledo Navarro, Rafael, Rafael.Toledo@uv.es ; Carpena Hernandez, Ines, Ines.Carpena@uv.es ; Espert Fernandez, Ana M., Ana.M.Espert@uv.es ; Sotillo Gallego, Javier, Javier.Sotillo@uv.es ; Esteban Sanchis, Jose Guillermo, Jguillermo.Esteban@uv.es

The population structure and habitat selection of Anisakis simplex in 35 harbor porpoises off Denmark are described. The nematodes were collected from the stomach and duodenal ampulla and were categorized as third-stage larvae, fourth-stage larvae, subadults, and adults. The porpoises harbored 8,043 specimens of A. simplex. The proportion of adults and subadults increased with infrapopulation size. The number of development stages across infrapopulations covaried significantly (Kendall's test of concordance). Concordance was higher in hosts with the highest intensities than in those with low and medium intensities. All stages occurred mainly in the forestomach, but this trend was stronger for the adults. Adult and subadult sex ratios did not depart significantly from 1:1. Our data suggested that recruitment and duration of each stage were the main factors accounting for infrapopulation structure. The preference of A. simplex for the forestomach conformed with previous studies, but the narrower distribution of adults relative to other stages might indicate a strategy to enhance mating opportunities. Information on sex ratios of A. simplex is scarce and contradictory. We suggest that the discrepancies might partly reflect differences in categorization criteria and statistical methods. Herreras Belled, Maria Victoria, Maria.V.Herreras@uv.es ; Balbuena Diaz-Pines, Juan Antonio, J.A.Balbuena@uv.es ; Aznar Avendaño, Francisco Javier, Francisco.Aznar@uv.es ; Raga Esteve, Juan Antonio, Toni.Raga@uv.es