A Case Study of Protozoan Parasite Gregarine Nematopsis spp. (Apicomplexa: Sporozoa) Infestation in Mangrove Oyster Crassostrea belcheri Imported from Thailand

摘要

One hundred mangrove oysters (weight: 248.41 ± 28.41g and length: 10.69 ± 0.73cm) from Thailand meant for human consumption were examined for parasite infestation. Gross observation showed that the infestation comprised mostly cliona (43%) followed by mud (1%) and water blister (1%). Histopathological study revealed high prevalence of gregarine Nematopsis sp oocysts (99%), metaplasia (92%), inclusion cells (15%), brown cells (14%), oedema (5%), ceriod (8%) and abscess (1%). A large number of phagocytes with gregarine oocysts inside the gill and connective tissues were observed. Most of the oocysts were located within parasitophorous vacuoles and phagocytes. The oocysts appeared ellipsoidal at 73.55μm wide and 140.56μm long with a thick outer surface. The clusters of oocysts consisted of a single vermiform sporozoite engulfed by phagocytes. The number of oocysts in each phagocyte varied ranging from 1 to 9 with an average of 4.9. The findings revealed that gregarine oocysts did not cause much pathological damage as there was no irregular arrangement and disruption of gill filament or loss of cilia. Similar histopathology was also observed in four molluscs (i.e. Anadara granosa, Perna viridis and Arcuatula arcuatula and Paphia undulate) as reported in Thailand, and three cultivated molluscs (A. granosa, P. viridis and C. iredalei) as reported in Malaysia. The present study provides significant baseline information on the health profile of the important molluscan species, particularly on Nematopsis spp. infestation in oysters.