Host-dependent variations in the seasonal prevalence and intensity of heterophyid encysted metacercariae (Digenea: Heterophyidea) in brackish water fish in Egypt.

The prevalence of heterophyid (Digenea: Heterophyidae) encysted metacercariae (EMC) in its second intermediate host, the fish Mugil spp. and Tilapia spp. was studied in a subtropical permanent Lake in northeastern Egypt. Seasonal changes in the occurrence of the EMC in different fish hosts were monitored in a longitudinal field survey lasting 12 months from June 2006 to May 2007. This study tested two hypotheses; (i) prevalence and intensity of heterophyid EMC fluctuate seasonally throughout the year and (ii) variation in the prevalence and intensity of heterophyid EMC is host-dependent. A total of 832 fish specimens comprising 5 species collected from Manzala Lake, northeastern Egypt were examined by artificial gastric juice digestion for EMC. All five species of brackish water fish examined were found to harbor the EMC of the family Heterophyidae in their muscles. The overall infection prevalence of EMC over 12 months was 23.2%. The adult flukes recovered from puppies experimentally infected with morphologically different metacercariae from different fish species were compatible with six species belong to five genera of Heterophyidae, namely, Heterophyes heterophyes, Heterophyes aequalis, Pygidiopsis genata, Phagicola sp., Haplorchis sp. and Stictodora sp. EMC of H. heterophyes were most abundant, detected in 56% of the total fish examined. P. genata was ranked second, followed by Phagicola sp., H. aequalis, Haplorchis sp., and Stictodora sp., respectively. Seasonal differences in infection were observed for all heterophyid species studied in all fish species examined. Heterophyid infections reached peak prevalences during the summer season 38.2% followed by spring 26.6% and autumn 19.3% seasons, whereas the lowest prevalence was recorded in the winter 8.7%. Intensity of heterophyid EMC followed the same seasonal pattern, being high during summer months and low in winter months. All fish species were infected with all the heterophyid digeneans, but with different prevalence. Possible reasons for these findings are discussed with reference to host, climatic and biotic factors.