الفهرس 
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Abstract
During a survey of cercarial infection of fresh water snails in El-Fayoum governorate, a total of 3489 snails were examined during October 2009 to February 2010. The rate and seasonality of the infection were recorded for each cercaria species with full morphological description.
The overall incidence of infection was 12.4% (435/3489). Cleopatra bulimoides have the highest incidence of infection (23.75%) followed by Melanoides tuberalata (14.9%), then, Biomphalaria Alexandrina (14.8%), Bulinus truncatus (3.07%) and the lowest in Lanistus carinatus (2.9%) while Physa actua and Lymnaea natalensis snails were found free from any infection during the period of study.
Regarding the seasonality, the highest incidence was reported during autumn (13.7%) followed by spring (13.2%) then summer (11.8%) while the lowest incidence was recorded during winter (11.6%).
During the present study, nineteen morphologically distinguishable species of cercariae belonging five major groups were recorded. These groups of cercariae were Xiphidiocercariae represented by 8 species; Furcocercous cercariae represented by 7 species; Parapleurolophocercous cercariae represented by 2 species; Echinostome cercariae represented by one species and Gymnocephalus cercariae represented by one species. The types of cercariae and their incidence in each snail were as following:
1- Cleopatra bulimoides snails: eight species of cercariae were recorded with incidence (23.75%), including Cercaria vivax I (7.5%), Cercaria vivax II (3%), Cardicola sp. cercaria (1.3%), which are belonging to Furcocecous cercariae type; Xiphidiocercaria sp. I (1.9%), Xiphidiocercaria sp. II (1.6%) and Xiphidiocercaria sp. III (5.3%) which are belonging to Xiphidiocercaria type; Philipophothalmus sp. cercaria (1.25%) which is belonging to Gymnocephalus cercaria type and Echinostom sp. cercaria (6.7%), which is belonging to Echinostome cercaria type.
2- Melanoides tuberculata snails: Four species of cercariae were recorded with incidence (14.9%) including Haplorchis pumilio cercaria (5.4%) and Stictodora sp. cercaria (4.6%) which are belonging to parapleurolorophocercous cercaria type, Xiphidiocercaria sp. I (1.9%) and Xiphidio cercaria sp. II (1.6%) which are belonging to xiphidiocercaria type.
3- Biomphalaria alexandrina snails: Three species of cercaria were recorded with incidence (14.8%), including Longifurcate – pharyngeate distome cercaria type I (6%), and Logifurcate pharyngeate distome cercaria type II (6.2%) and Schistosoma mansoni cercaria ( 2.7%) which are belonging to furcocercous cercaria type.
4- Bulinus truncatus snails: Two species of cercaria were recorded with incidence (3.07%), including Schistosoma haematobium cercaria (2.1%) which is belonging to furcocercous cercaria type and xiphidiocercaria sp. (0.9%) which is belonging to xiphidiocercaria type.
5- Lanistes carinatus snails: Two species of cercaria were recorded with incidence (2.9%) including Xiphidiocercaria sp. I (1.5%) and Xiphidiocercaria sp. I (1.5)% which are belonging to xiphidiocercaria type.
It is worthy to mention that in the present work three species of cercariae were recorded for the first time in Egypt. These species are:
1- Cercaria vivax II from Cleopatra bulimoides snails
2- Echinostome sp. cercaria from Cleopatra bulimoides snails
3- Longifurcate pharyngeate cercaria type II from Biomphalaria alexandrina snails.
Attempts were carried out to investigate some biological characters for these cercariae in addition to its encystement ability. The following conditions were recommended:
1- Cercariae encysted without need to second intermediate host as Philophthalmus sp. cercaria.
2- Cecariae required second intermediate host:
a- Snails can act as second intermediate host as in case of Xiphidiocercaria sp. from Bulinus truncatus needs to the same snail for encystement.
b- Tilapia nilotica can act as second intermediate host as in case of Ceraria vivax I which need juveniles of tilapia to encysted in its muscles.
During the present study, polymerase chain reaction (PCR) has been used to detect the infection by Schistosoma mansoni in Biomphalaria alexandrina snails. The PCR results showed positive snails (field collected snails that shed cercaria in the lab) gave positive reading at 710 bp similar to that of the positive control. One pool of negative snails (did not give cercariae in the lab when crushed) gave no reading in PCR in contrary another pool of negative snails gave positive reading at 710 bp.