The role of phenoloxidase suppression in QX disease outbreaks among Sydney rock oysters (Saccostrea glomerata)

The paramyxean protozoan, Marteilia sydneyi, is the etiological agent of QX disease in Sydney rock oysters (Saccostrea glomerata). QX disease affects the farming of oysters in Queensland and on five rivers in New South Wales (NSW), Australia. Disease outbreaks occur during summer months (January to April) and are associated with high mortality rates (up to 98%), which limit the growing out period for oyster production. This study investigated the relationship between oyster host defense systems and QX disease. Oysters from the same brood stock were harvested from QX prone and QX free growing areas over the course of the 2000–2001 outbreak. A variety of parameters—infection intensity in the digestive diverticulum, total hemolymph protein content, condition index and phenoloxidase (PO) activity—were measured in these oysters. Phenoloxidase activity, which is often associated with host defense, was significantly suppressed in oysters from the QX prone area when compared to those from the QX free river. There was also a strict negative correlation between phenoloxidase activity and the intensity of parasitic infection ( p = 0.0038). In contrast, neither total protein content nor condition indexes differed between oysters from the QX prone and QX free rivers. These data suggest that inhibition of prophenoloxidase cascade may facilitate lethal infection by M. sydneyi. D 2003 Elsevier Science B.V. All rights reserved.