In October 2006 unexplained high mortalities of P. maxima oysters were reported. Many farms are affected and the cause is not known. The disease is termed Oyster Oedema Disease (OOD) and it is likely that an infectious agent is involved. The disease spread rapidly and there are no known control measures, no knowledge of a causative agent, no understanding of how widespread the disease was, and no way to test for it. Molecular and electron microscope research into the mortalities resulted in the discovery of two undescribed Chlamydiales-like organisms (CLO’s) associated with affected shell, and their involvement in OOD is unknown but it is considered unlikely that they are by themselves the major pathogen. However, it is possible that these organisms are contributing to the onset of OOD and further work is required to eliminate or confirm these organisms as a factor in OOD. A report detailing the OOD mortality event (AusVet Report on OOD, 2007) concludes that it is likely that more than one factor contributes to the onset of OOD. Therefore one or both of these CLO’s may play an important role in OOD. Preliminary research in WA suggests a link between OOD-affected animals and the presence of at least one of these CLO’s. The discovery of CLO’s in oysters is a new finding.

Mortalities continue to be reported and OOD still represents a high risk to surviving and young oysters. This proposal seeks to continue the work started at the Department of Fisheries WA in 2007 with the discovery of two CLO’s in P. maxima oysters. There is a need to further characterise these two organisms in relation to each other and to similar organisms worldwide, determine if they play a role in the onset of the fatal OOD, and represent a risk to surviving pearl oysters.