-A retrospective study was carried out over a 10-to 12-year period to analyse the changes in prevalences of natural fasciolosis and paramphistomosis among cattle and snails in central France, and to determine the causes which had induced these changes. The prevalences of natural fasciolosis in cattle increased from 1990 to 1993 (13.6% to 25.2%) and diminished afterwards up to 1999 (at 12.6%). Those of natural paramphistomosis showed a progressive increase between 1990 and 1999 (from 5.2 to 44.7%). The prevalences of natural infections and the numbers of free rediae counted in the snails (Lymnaea truncatula) infected with F. hepatica did not show any significant variations over time. By contrast, the prevalences of natural paramphistomosis in snails significantly increased from 1989 to 1996 and remained afterwards in the same range of values (3.7-5.3%), while the number of free rediae significantly increased up to 2000 (from a mean of 6.5 to 13.8 rediae per infected snail, respectively). Three hypotheses may explain the increase of paramphistomosis in cattle and snails: a better quality of diagnosis for the detection of P. daubneyi eggs in veterinary analysis laboratories, the use of specific molecules in the treatment of cattle fasciolosis since 1993, and the lack of an effective treatment up to now against cattle paramphistomosis. Since the objective of most farmers in central France is to obtain the highest antiparasitic efficiency with a single treatment of cattle per year, it is reasonable to assume that the prevalence of bovine paramphistomosis will continue to increase in the future. Résumé -Fasciola hepatica et Paramphistomum daubneyi : changements dans les prévalences des infestations naturelles chez les bovins et chez Lymnaea truncatula dans le centre de la France au cours des 12 dernières années. Une étude rétrospective a été réalisée sur une période de 10 à 12 années pour analyser les modifications qui se sont produites dans les prévalences de la fasciolose et de la paramphistomose naturelles chez les bovins et les limnées du centre de la France, et pour déterminer les causes qui ont provoqué ces changements. Chez les bovins, la prévalence de la fasciolose naturelle s'est accrue entre 1990 et 1993 (de 13,6 à 25,2 %) avant de diminuer par la suite jusqu'en 1999 (à 12,6 %). Celle de la paramphistomose naturelle a augmenté de manière progressive entre 1990 et 1999 (de 5,2 à 44,7 %). La prévalence de l'infestation naturelle avec F. hepatica et le nombre de rédies libres chez Lymnaea truncatula n'ont pas montré de variations significatives dans le temps. Par contre, la prévalence de l'infestation naturelle avec Paramphistomum daubneyi chez les limnées a présenté une augmentation significative entre 1989 et 1996 avant de rester ensuite dans la même gamme de valeurs (3,7-5,3 %), tandis que le nombre de rédies libres a augmenté jusqu'en 2000 (de 6,5 à 13,8 rédies en moyenne par mollusque infesté). Trois hypothèses peuvent expliquer cet accroissement de la paramphistomose chez les bovins et les mollus...

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Experimental infections of Galba truncatula with Fasciola hepatica, Fascioloides magna, or Paramphistomum daubneyi were carried out at 20 degrees C to determine if the use of 14-cm petri dishes for breeding lettuce-fed snails enhanced the characteristics of snail infections. Compared to infected snails raised in boxes up to day 30 post-exposure and later in individual 35-mm dishes, the survival of G. truncatula kept in 14-cm dishes and the shell height of cercariae-shedding snails during the first 45 days were higher, whatever the digenean species is. The consequence of such enhanced characteristics was a greater production of metacercariae in the case of F. hepatica (1.7 to 5.6 times higher) and P. daubneyi (2.3 times). In contrast, metacercariae of F. magna were few in number, whatever the method of snail breeding is, and this might be explained by a still incomplete adaptation between the parasite of Czech origin and the French population of G. truncatula. The use of these 14-cm dishes reduced the time necessary for snail maintenance and metacercaria collection, thus allowing a decrease in the cost price of these larvae for commercial production.

An update on the redial generations of Fasciola hepatica was carried out to highlight the different developmental patterns of rediae, the effects of some factors on these generations, and the consequences of such developmental patterns on cercarial productivity. The development of generations is dependent on the behaviour of the first mother redia of the first generation. If this redia remains alive throughout snail infection, it produces most second-generation rediae. In contrast, if it dies during the first weeks, daughter redia formation is ensured by a substitute redia (the second mother redia of the first generation, or the first redia of the second generation). Environmental and biotic factors do not modify the succession of redial generations, but most act by limiting the numbers of rediae, either in all generations, or in the second and/or third generations. An abnormal development of rediae reduces the number of cercariae and most are formed by the second cohort of the first generation. By contrast, most cercariae are produced by the first cohort of the second generation when redial development is normal. The mother rediae described by previous authors might correspond to the first generation and the second cohort of the second generation, while daughter rediae would be the second cohort of the second generation and the first cohort of the third generation. Under certain circumstances, daughter redia formation is ensured by the first two mother rediae or all first-generation rediae, thus demonstrating that the first mother redia is not the only larva to ensure daughter redia formation.

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