Trematode infection among freshwater gastropods in Tessaout Amont irrigation system, Morocco Hammou Laamrani 1 , Eline Boelee 2 † & Henry Madsen 3 * 1 Ministry of Education, BP 881, Tabriquet, Salé, Morocco 2 Department of Parasitology, Faculty of Medicine, Leiden University, the Netherlands 3 Danish Bilharziasis Laboratory, Jaegersborg Allé 1 D, Charlottenlund, Denmark Received 14 November 2002. Accepted 27 July 2004 In the summer of 1994 and the spring of 1995, a snail survey aimed at determining relationships between the distribution of molluscan fauna, various environmental factors and trematode infection was carried out in the Tessaout Amont irrigation system, Morocco. The molluscan fauna consisted of nine species belonging to seven families (including one bivalve). Cercariae belonging to nine families were collected. Ancylus fluviatilis and Mercuria confusa showed the highest prevalence of infection (1% and 2%, respectively) but were not listed among infected snails reported from other regions of the country. Melanopsis praemorsa and Bulinus truncatus were shedding cercariae belonging to four and three families, respectively. Amphistome and echinostome cercariae were found for the first time in Morocco. No case of mixed-species infections was found. Key words: freshwater snails, snail control. INTRODUCTION The majority of studies on trematode infections in snails in Morocco have focused on those of medical and veterinary importance, while little information on other trematode species is avail- able. Environmental changes following irrigation development are likely to modify the pattern of distribution of snails and trematodes. Therefore, larval digeneans shed by snails could be used to assess environmental impact (Lafferty 1997). Tessaout Amont irrigation scheme in central Morocco constitutes an illustration of the health impact of irrigation development. Schistosomiasis haematobium was not known in the area before the establishment of the irrigation system in the early seventies (Laamrani et al. 2000). A few years later, the first cases were detected. Information on snail distribution and infection, ecology and behaviour could strengthen the efficiency of control measures against snails of medical and veterinary impor- tance. The relevance of such studies was particu- larly emphasized in the control of schistosomiasis transmission (Madsen 1992; Sturrock 1993). A combination of different methods of control, when possible, is recommended as well (World Health Organization 1993). Possible competitive interactions between larval stages of native trematodes have been suggested as candidates for biological control of schistosomiasis (Combes 1982). Several field experiments on the use of sterilizing trematodes to control snail intermedi- ate hosts have been conducted and reviewed by Combes (1983). The present investigation aimed at providing information on distribution patterns of molluscan species in relation to habitat parameters with emphasis on the importance of trematodes. The purpose of the present note is to list cercariae shed by different snail species, and to provide baseline data for more comprehensive studies on the diversity of parasite fauna which has not been thoroughly investigated so far in Morocco. MATERIALS & METHODS The irrigation system The Tessaout Amont irrigation system, on the Haouz plain in Central Morocco (Fig. 1), consists of the Moulay Youssef dam, two smaller dams, Timnoutine and Bouachiba, and an extensive network of canals. Principal, primary, secondary and tertiary canals are concrete-lined. Primary canals (total length = 58.5 km) are embedded and have a trapezoidal cross-section. Secondary (170 km) and tertiary (720 km) canals are semi- circular conduits of different size elevated above ground level. Quaternary canals and remaining so-called ‘ seguias’ or traditional canals are earth- lined field canals. An extensive network of drain- African Zoology 40(1): 77–82 (April 2005) *Author for correspondence. E-mail: hm@bilharziasis.dk † Present address: International Water Management Institute, P.O. Box 2075, Colombo, Sri Lanka.