Histopathological study for evaluation of trematode larval infection in the carpet shell clam, Tapes decussatus, from three Egyptian clam fisheries

A histopathological evaluation of the carpet shell clam, Tapes decussatus collected from three Egyptian clam fisheries [El-Max (Alexandria), Timsah Lake (Ismailia) and Ezbet El-Borg (Damietta)] was conducted over a twelve months period (December 2010–November 2011). Total of 2160 clams (shell length = 17–51.5 mm) were examined for infection with trematode larvae. Smear preparations and histopathological examination (n = 30 and 30/site/month, respectively) showed maximum prevalence (% clams infected) of 81% and 65% in Timsah Lake in May and April, respectively. Maximum infection intensity (% area of transverse tissue section occupied by parasites, n = 60 slides examined per site) was observed in the May samples of Timsah (78%), followed by El-Max (45%). Infection prevalence and intensity differences among sites were insignificant but differed significantly with season (P < 0.05) with maximum value in spring. Infection prevalence and intensity were significantly higher in females than in males (31.4% and 16.9%, respectively) and were synchronized with the spawning season. Digenean larvae caused a wide range of damage to clam gonads. Castration occurred in 15.1% of clams due to heavy infection and led to reproductive failure during spawning trials. Infection intensity increased with clam size (more in clams ⩾31.9 mm SL with ripe gonads).