Lymnaea palustris and Lymnaea fuscus are potential but uncommon intermediate hosts of Fasciola hepatica in Sweden

Abstract

Background: Lymnaea palustris and L. fuscus are members of the European stagnicolines (Gastropoda: Lymnaeidae). The role of stagnicolines in transmission of Fasciola hepatica has been often proposed. To assess the possible relationship between these two stagnicolines and F. hepatica in Sweden, field monitoring in parallel with experimental infections of L. palustris and L. fuscus were conducted.

Methods: Stagnicoline snails were collected and identified on pastures grazed by either sheep or cattle on four farms suffering from fasciolosis in Sweden during 2011-2012. Field-collected L. palustris and L. fuscus were examined for F. hepatica DNA by PCR. In the laboratory, different age groups of L. palustris, L. fuscus and G. truncatula were each exposed to two F. hepatica miracidia and main infection characteristics were obtained.

Results: One field-collected L. palustris (out of n = 668) contained F. hepatica as determined by PCR. On the other hand, stagnicolines artificially exposed to F. hepatica miracidia resulted in successful infection with fully differentiated cercariae, but only in juvenile snails (size, 1-2 mm at exposure) and with a prevalence of 51% and 13% in L. palustris and L. fuscus, respectively. In contrast, 90% of juvenile (size, 1-2 mm) and 92% of preadult G. truncatula (size, ≥ 2-4 mm), respectively, were successfully infected. Delayed, reduced and/or no spontaneous cercarial shedding was observed in the two stagnicolines when compared to G. truncatula. However, at snail dissection most cercariae from L. fuscus and L. palustris were able to encyst similarly to those from G. truncatula.

Conclusion: Both L. fuscus and L. palustris can sustain larval development of F. hepatica but with an apparent level of age resistance. The finding of a single F. hepatica positive specimen of L. palustris, together with infection characteristics from the experimental infection, suggest that L. palustris is a more suitable snail vector of F. hepatica than L. fuscus. The reduced growth observed in both stagnicolines was contrary to the 'parasitic gigantism' theory. Overall, it seems that the epidemiological role of L. palustris in transmission of F. hepatica in Sweden is likely to be much lower than for G. truncatula.

Figure 1

Agarose gel electrophoresis of amplified ITS-2 and cox-1; evaluation of the specificity of our PCR method and the method previously described [23]: 1. adult Fasciola hepatica; 2. adult Fascioloides magna; 3. Lymnaea palustris experimentally infected with F. hepatica; 4. L. palustris experimentally infected with F. magna; 5. Galba truncatula naturally infected with Haplometra cylindracea. The first five (1–5) PCR products (ITS-2) were obtained according to our described ITS-2 gene amplification method (from the left side); five others (cox-1) were obtained according to the previously used PCR protocol with primers for mitochondrial cytochromoxidase 1 [23].

Figure 2

The effect of Fasciola hepatica infection on the size of Lymnaea fuscus, L. palustris and Galba truncatula. The data are expressed as mean shell sizes of groups 1–2 mm for L. fuscus and L. palustris (measured at day 95 PE) and groups 1–2 mm (white) and ≥2-4 mm (grey) for G. truncatula (measured at day 65 PE). The horizontal line inside each box represents the arithmetic mean of each group; whiskers are standard deviations. Abbreviations: IS, infected snails (cercarial shedding or no cercarial shedding); EUS, snails exposed but uninfected; Neg, negative controls.