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

Breeding for QX disease resistance negatively selects one form of the defensive enzyme, phenoloxidase, in Sydney rock oysters

“…Phenoloxidase enzyme activity provided a complementary measure of the onset of QX disease. Previous accounts of the QX infective cycle have shown that phenoloxidase activities decrease immediately before M. sydneyi infections are established [4,6]. In this study, a five-fold decrease in phenoloxidase activity was observed between January 30 and February 12, 2004, immediately prior to the detection of M. sydneyi in the gills of oysters.…”

“…This tissue was homogenised in 1 ml of filtered seawater (FSW; 0.45 mM filter) in a Dounce homogeniser. The homogenates were diluted 1:10 in FSW and M. sydneyi sporonts were stained with KOVA (Hycor Biomedical Inc., California, USA) so that their frequency could be calculated using a haemocytometer according to the method of Peters and Raftos [6]. The identification of M. sydneyi was based on previous descriptions [22,23].…”

“…Peters and Raftos [6] have shown that M. sydneyi infects oysters after their immune system, notably the phenoloxidase cascade, has been suppressed. Phenoloxidase is a bifunctional enzyme that is critical to host defence and wound repair in many invertebrates [6e10].…”

“…Melanin and a number of intermediate metabolites in the phenoloxidase pathway have immunological functions including antimicrobial activity. Peters and Raftos [6] concluded that suppression of phenoloxidase activity decreases the ability of oysters to control M. sydneyi, leading to the development of QX as an opportunistic disease. That conclusion is supported by recent evidence, which shows that M. sydneyi is present in many growing areas where QX disease outbreaks do not occur (R. Adlard, unpublished data).…”

“…There are currently no farming practices that can control QX and the disease continues to spread [6]. Hence, the development of disease resistant oysters has been made a priority by the NSW Department of Primary Industries (DPI).…”