Some species of the family Lymnaeidae (Mollusca: Gastropoda) act as intermediate hosts in the life cycle of Fasciola hepatica. In Europe and in Africa, the main snail host is Galba truncatula. However, most of the other European species can also sustain the larval development of this parasite. This is the case with Omphiscola glabra if miracidial exposure occurs within the first 2 weeks of the snail's life (Boray, Reference Boray1978) or if it is co-infected simultaneously with F. hepatica and Paramphistomum daubneyi (Abrous et al., Reference Abrous, Rondelaud, Dreyfuss and Cabaret1998, Reference Abrous, Rondelaud and Dreyfuss2000; Dreyfuss et al., Reference Dreyfuss, Vignoles and Rondelaud2003).

Besides lymnaeids, several freshwater pulmonates were also reported in the literature for their role as intermediate hosts. Successful infections of Physa cubensis with F. hepatica were reported by Viguera & Moreno (1938), but this result has never been verified in subsequent experiments (de Barros et al., Reference de Barros, Pile, de Vasconcellos, dos Santos and Lessa2002). Cercaria-carrying rediae of F. hepatica have been found in experimental infections of juvenile Bulinus truncatus (Barthe & Rondelaud, Reference Barthe and Rondelaud1986). Using experimental co-infections of Anisus spirorbis ( = Planorbis leucostoma) with F. hepatica and Paramphistomum daubneyi, Abrous et al. (Reference Abrous, Rondelaud, Dreyfuss and Cabaret1998) reported successful infections of this snail with F. hepatica. Infection of A. spirorbis alone with F. hepatica was also shown in the field (Abrous et al., Reference Abrous, Rondelaud and Dreyfuss2000). The aim of the present note is to report natural infection of B. truncatus with F. hepatica in the region of Sejnane (North Tunisia), as this district is known to be a zone of endemic fasciolosis ( Jemli et al., 1991).

Unlike south-western Tunisia, for which G. truncatula has been reported as the single intermediate host of F. hepatica (Hammami & Ayadi, Reference Hammami and Ayadi1999; Hammami et al., Reference Hammami, Hamed and Ayadi2007), the snail host of this parasite had not been identified in the north. Parasitological investigations were thus carried out every month from April 2007 to March 2008 in a private farm located near the Ghar Ettin Lake in the Sejnane region (south-west of Bizerte town). The mean monthly temperature of this area was 16°C while the total rainfall recorded from April 2007 up to March 2008 was 704 mm. The ruminants living in this farm (mainly cattle) were frequently infected with F. hepatica and this natural infection was confirmed in 2007 by coproscopy (cattle: 50%; sheep: 47.8%) and by haemagglutination or electrosyneresis (cattle: 50%, sheep: 66.6%, goats: 27.7%). The sampling site (area, 1200 m²) was a permanent pond fed only with run-off. The clayey soil was covered by 1-m deep stagnant water (Ca²⁺: 134 mg/l; Cl⁻ : 73.9 mg/l) and the vegetation bordering this pond, with Apium nodiflorum and Sonchus maritimus as dominant species, was grazed by cattle and sheep. According to the farmer, cattle, sheep and goats were autochthonous and did not come from any exchange with other areas of Tunisia.

Bulinus truncatus was the single freshwater snail species living in the pond and no lymnaeid was found in the other meadows of this farm. A total of 163 snails, measuring more than 3 mm in height, were collected from this pond. After their collection, snails were transported to the lab and dissected under a stereomicroscope to detect any parasitic infection and identify species based on the morphology of larval forms. The identification of F. hepatica rediae was made using the criteria reported by Ollerenshaw & Graham (Reference Ollerenshaw and Graham1986) and those by Dar et al. (Reference Dar, Vignoles, Dreyfuss and Rondelaud2003), while cercariae of F. hepatica were recognized according to their morphology (Andrews, Reference Andrews and Dalton1999).

Of the 163 B. truncatus collected throughout the study period, only F. hepatica was noted and no other infection or co-infection with another digenean was found. The presence of F. hepatica larval forms was noted in 65 snails (prevalence, 39.9%), and table 1 gives, for each month, the number of infected snails and the monthly prevalence of this natural infection. Two peaks, the first in June and the second in October, were observed. Free cercariae of F. hepatica, encysting or not after snail dissection, were found in most snails (data not shown). Shells of these infected bulinids ranged from 3 to 12 mm in height (data not shown).

Table 1 The prevalence of natural infection with F. hepatica in B. truncatus from April 2007 to March 2008.

The presence of B. truncatus as a natural intermediate host of F. hepatica in northern Tunisia is surprising as this species is usually known in North Africa to be the snail host of Schistosoma sp. (see review by Brown, Reference Brown1994), of Paramphistomum microbothrium (Dinnik, Reference Dinnik1965) or of Echinostoma deserticum (Kechemir et al., Reference Kechemir, Jourdane and Mas-Coma2002). The natural infection of this bulinid may be explained by accepting the hypothesis proposed by Kendall (Reference Kendall1965, Reference Kendall1970) for European lymnaeids other than G. truncatula. According to this author, natural infections of several lymnaeids with F. hepatica occurred in countries where G. truncatula was absent. However, the height of infected B. truncatus (3–12 mm) suggests that infection of this bulinid with F. hepatica would be a long-established process and that adaptation between both partners would be complete. These last assumptions are supported by the great infection rate (39.9%) noted in the present study, even if snail samples only concerned preadult and adult bulinids.

Two other results warrant comment. First, the two species of plants: Apium nodiflorum and Sonchus maritimus, found in the pond from the Sejnane region were also found in Gafsa oases, central Tunisia (Hammami et al., Reference Hammami, Hamed and Ayadi2007) and were suspected by these authors to be at the origin of local contamination of ruminants with F. hepatica. Indeed, the first species has been reported as a metacercaria-carrying plant in the natural watercress beds of the Limousin region, central France (Dreyfuss et al., Reference Dreyfuss, Vignoles and Rondelaud2003). A larger survey on both plants in the Tunisian areas where fasciolosis occurs would be useful to determine their role in the local transmission of the disease. Second, the infection rate of B. truncatus (39.9%) found in the present study was greater than those noted for G. truncatula in Gafsa oases (19.2%: Hammami et al., Reference Hammami, Hamed and Ayadi2007) and in Tozeur oases (26%: Hammami & Ayadi, 1999). As this percentage was much higher than those reported by Ollerenshaw (Reference Ollerenshaw1971) for G. truncatula in the UK (usually less than 2%) and by Mage et al. (Reference Mage, Bourgne, Toullieu, Rondelaud and Dreyfuss2002) for the same species in central France (5.1%), the most valid explanation would be to relate this high value found in bulinids to the small number of snails (163 only) dissected in the present study.

The natural infection of B. truncatus with F. hepatica confirms the report by Barthe & Rondelaud (Reference Barthe and Rondelaud1986) and indicates that this bulinid is a potential intermediate host of this digenean. However, the mechanism by which B. truncatus became infected with F. hepatica in the natural environment remains to be shown. Three hypotheses were proposed to explain the larval development of F. hepatica in snails other than G. truncatula. The first involved the immaturity of the defence system in young snails at exposure, so that larval forms can develop (Boray, Reference Boray1978). The second hypothesis might be an effect of facilitation in snails simultaneously co-infected by two digeneans, the first species penetrating the mollusc and favouring the development of the second parasite (Augot et al., Reference Augot, Abrous, Rondelaud and Dreyfuss1996). The last assumption may be the consequence of a particular aptitude that this bulinid population would have by sustaining frequent natural infections with another digenean (probably a species with two sporocyst stages in its life cycle), as demonstrated by Vignoles et al. (Reference Vignoles, Rondelaud and Dreyfuss2007). According to these authors, populations of G. truncatula, known for their natural infections with a plagiorchiid, are better intermediate hosts for metacercarial production of F. hepatica. To verify these hypotheses, further experiments are necessary, subjecting young bulinids from this population and other bulinid communities living in the Sejnane region to single- or multiple-miracidium infections with F. hepatica, or to experimental co-infections with this digenean and another parasite such as Paramphistomum sp.