Severe apicomplexan infection in the oyster Ostrea chilensis: a possible predisposing factor in bonamiosis

Abstract

Histological examination of 6455 oysters Ostrea chilensis from Foveaux Strait south of New Zealand over a 5 yr period showed > 85% contained apicomplexan zoites, irrespective of season. Zoites occurred around the haemolymph sinuses and the digestive diverticulae at all intensities of infection; occurrence in the sub-epithelium, Leydig tissue and gills/mantle increased with increasing intensity of infection. Many (> 35%) oysters were heavily infected, and most of them had severely damaged tissues. Heavy infections affected gametogenesis; 1% of lightly infected oysters had empty gonad follicles lacking germinal epithelium compared with 2% of moderately infected oysters and 9% of heavily infected oysters. Of oysters with empty gonad follicles, 75% were heavily infected with zoites. The parasite spread from the haemolymph sinuses and moved between Leydig cells, causing their dissociation and lysis. Some zoites were intracellular in Leydig cells. Lesions contained many haemocytes phagocytosing zoites, leading to haemocyte lysis and causing a haemocytosis. Fibrosis occurred to repair lesions in a few oysters. The zoites had a typical apical complex with 2 polar rings and 84 sub-pellicular microtubules. Prevalence and intensity of concurrent Bonamia exitiosus infection was related to the intensity of zoite infection, with only 3.8% of B. exitiosus infections occurring in the absence of zoites, 20.0% occurring in light zoite infections, 30.9% in moderate zoite infections, and 45.4% when oysters were heavily infected with zoites. The converse was not the case, as 75.3% of zoite infections occurred in the absence of B. exitiosus infection, including 51.1% of moderate to heavy zoite infections. There was a statistically significant association between intensities of B. exitiosus and of zoites (p < 0.0001). Zoites may increase the susceptibility of oysters to B. exitiosus by occupying and destroying haemocytes, and by destroying connective tissue cells and utilising host glycogen reserves. The parasite may be heteroxenous, with other stages in the terebellid polychaete Pseudopista rostrata. KEY WORDS: Apicomplexan · Zoites · Oysters · Ostrea chilensis · Bonamia exitiosus · Susceptibility · Gametogenesis Resale or republication not permitted without written consent of the publisherDis Aquat Org 51: [49][50][51][52][53][54][55][56][57][58][59][60] 2002 pinarum by Bower et al. (1992), scallops Argopecten irradians by Whyte et al. (1994), and oysters Crassostrea virginica by Meyers (1981). Azevedo (1989) described merozoites of Cryptosporidium sp. from the gills of clams Ruditapes decussatus. Morado et al. (1984) reported an unidentified coccidian from the kidneys of clams Protothaca staminea, and subsequently Desser & Bower (1997a,b) erected a new genus, Margolisiella, to accommodate the monoxenous parasite in P. staminea. They moved 4 monoxenous Pseudoklossia spp. into the new genus, thus separating them from heteroxenous Pseudoklossia spp. (Desser & Bower 1997b). Margolisiella tellinovum (Buchanan 1979, Desser ...

Partial 18S rRNA sequences of apicomplexan parasite ‘X’ (APX), associated with flat oysters Ostrea chilensis in New Zealand

“…The method has the potential to confirm the identity of APX-like cells seen previously by histology in NZ P. canaliculus and M. galloprovincialis. Additionally, despite the extensive histopathological examination of numerous flat oyster individuals, only the zoite stage of APX has ever been observed (Hine 2002), and the sister group of APX, T. gondii, is known for adopting several forms in its life cycle which spread through several hosts, suggesting the possibility that APX is heteroxenous. Future work should focus on validating the PCR methods developed in this current research and using the test to detect potential secondary host(s) of APX that will ultimately assist in managing APX in affected populations of flat oysters.…”

“…APX reported from the flat oyster Ostrea chilensis from Foveaux Strait, New Zealand (NZ), does not morphologically resemble any other apicomplexans previously identified in molluscs (Hine 2002). The only distinct APX developmental stages observed in specimens of flat oysters were identified as zoites (Hine 2002).…”

“…By contrast, the ubiquitous coccidian Nematopsis sp. is common in bivalves worldwide (Tuntiwaranuruk et al 2004, Uddin et al 2011, Ceuta & Boehs 2012, Sühnel et al 2016 but is rarely reported to cause any tissue reaction (Azevedo & Cachola 1992, Azevedo & Matos 1999, unlike apicomplexan 'X' (APX) re ported in this paper which can severely damage its host (Hine 2002).…”

“…The only distinct APX developmental stages observed in specimens of flat oysters were identified as zoites (Hine 2002). APX zoites appear to be associated with increased susceptibility of wild flat oysters to infection with the haplosporidian parasite Bonamia exitiosa (see Hine 2002).…”

“…APX zoites appear to be associated with increased susceptibility of wild flat oysters to infection with the haplosporidian parasite Bonamia exitiosa (see Hine 2002). The infection of flat oysters with B. exitiosa and APX from late 1985 to 1993 reduced the population of commercially harvestable oysters (> 58 mm shell diameter) in Foveaux Strait to 9% of its 1975 numbers (Hine 2002).…”