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Cases of fasciolosis in ruminants have been recorded in several French farms in the absence of Lymnaea truncatula, which is considered the only snail intermediate host in western Europe. These farms harbored other species of freshwater snails in large numbers (Lymnaea glabra, Physa acuta, or Planorbis leucostoma) and, in many cases, had cattle or sheep infected by another trematode (Paramphistomum daubneyi). These other freshwater snails may serve as intermediate hosts for F. hepatica due to a coexisting infection with P. daubneyi. We have demonstrated that L. glabra, either infected with F. hepatica alone or coinfected by P. daubneyi, was capable of developing a F. hepatica infection. A broader range of L. glabra size classes (up to 10 mm in height) were susceptible to infection if simultaneously infected with P. daubneyi. Planorbis leucostoma can only serve as an intermediate host for F. hepatica, if infected with P. daubneyi. Lastly, P. acuta smaller than 4 mm cannot serve as an intermediate host. These results may explain, in part, the maintenance of low-level F. hepatica infections in the absence of the normal intermediate host, L. truncatula.

The development of redial burden and cercarial shedding were studied in two groups of Lymnaea truncatula subjected to successive cross-exposures to one miracidium of Paramphistomum daubneyi and one of Fasciola hepatica per snail, or vice versa. The results were compared with those obtained in controls subjected to two unimiracidial exposures to the same trematode species. The infection rate was 61% in the group cross-exposed to P. daubneyi/F. hepatica and 37% in that cross-exposed to F. hepatica/P. daubneyi; it was 37% in the control group exposed to F. hepatica and 21% in that exposed to P. doubneyi. Snails harboring larval forms of both trematodes were few in number in cross-exposed groups and the redial burden was low, with one trematode dominating over the other. Free cercariae of F. hepatica and those of P. daubneyi were significantly more numerous at day 35 in the group cross-exposed to P. daubneyi/F. hepatica than in the controls or the other cross-exposed group. Mixed cercarial sheddings were obtained from 40% of snails with emission in the group cross-exposed to P. daubneyi/F. hepatica and from 21% of those in the F. hepatica/P. daubneyi group. The numbers of P. daubneyi metacercariae were significantly greater in the group cross-exposed to P. daubneyi/F. hepatica than in the other cross-exposed group, whereas no significant difference in mean numbers was noted for the F. hepatica cysts. Repartition of metacercariae over the patent period was clearly more irregular for P. daubneyi than for the other trematode.

Natural infections of three freshwater snails withFasciola hepaticaand/orParamphistomum daubneyiwere studied during two periods in 1996 and 1997 (June–July and September–October) on 18 farms located in the departments of Vienne and Haute Vienne (central France), and known for low prevalences ofF. hepaticainfections in ruminants. A total of 1573Lymnaea glabraand 1421L. truncatula6 mm high or more were collected in the meadows of 13 farms and dissected under laboratory conditions. Snails with single or concurrent infections ofF. hepaticaand/orP. daubneyiwere found for eachLymnaeaspecies. InL. truncatula, global prevalences of natural infections withF. hepatica(3.8% in June–July, and 3.6% in September–October) were significantly greater than those recorded forP. daubneyi(1.1% and 0.8%, respectively). InL. glabra, global prevalences ofF. hepaticainfections (0.4% in each investigation period) were significantly lower than those found inL. truncatula, whereas there were no significant differences between prevalences ofP. daubneyiinfections. A total of 2721Planorbis leucostomameasuring at least 4 mm in diameter were collected in the meadows of the other five farms on whichL. truncatulawas absent. In these planorbids, global prevalences of natural infections withF. hepaticawere 0.4% in each period of investigation. Contrary to fasciolosis, snail infections withP. daubneyiwere not noted on all farms of the Vienne and Haute-Vienne departments. Natural single or concurrent infections withF. hepaticaand/orP. daubneyiinL. glabraand a natural infection ofP. leucostomawithF. hepaticawere found in swampy meadows on acid soil.