A comprehensive health survey of pearl oysters Pinctada maxima was undertaken across northern Australian marine waters in a collaborative project between fisheries organisations and pearl producers in Northern Territory (NT), Queensland (Qld) and Western Australia (WA). The majority of animals examined in the study represented mature animals from the wild or from pearl culture farms from NT, Qld and WA (4502 animals). The study also reports on 22 batches of 150 spat, examined after spending a minimum of 6 weeks in open water sites in WA as part of the regulatory controls in place controlling oyster movements in the State. A low number of mature and immature animals examined for disease investigations and following placement in sea cages/panels in NT were also included in the study.

The study established the occurrence, prevalence and distribution of a taxonomically diverse range of microbial, protozoan and metazoan agents associated with pearl oysters in Australian waters and, within the limits of the study, ascribed pathogenic significance to these agents. In some cases, the prevalence and distribution of agents identified in earlier studies were established. The majority of animals examined were free from infectious agents which may adversely impact upon oyster growth and pearl production. A proportion of oysters carried agents which were not considered significant pathogens. A number of microbial, protozoan and metazoan agents were identified in the shell matrix or in the tissues of the oyster which were considered to have potential to adversely impact upon the breeding, rearing and production of pearl oysters in Australian tropical waters.

Pathogenic or potentially pathogenic agents identified in mature P. maxima from clinically normal populations in the study included a papova‐like virus of the palp associated with epithelial hypertrophy and cilia loss, viral‐like inclusion bodies in the digestive gland associated with tubular degeneration, enigmatic protozoan‐like bodies associated with severe degenerative and inflammatory lesions in the digestive gland of mature oysters and a copepod associated with oesophageal occlusion and epithelial erosion. The shell matrix was also a target for potentially pathogenic boring bivalves, invasive sponges and mudworms, resulting in shell denaturation and blistering.

In the first 6 weeks of exposure of juvenile oysters to the marine environment in WA, a Haplosporidian sp. with high morbidity was detected, together with a heart apicomplexan, palp virus, rickettsiales‐like agent in the digestive gland, viral‐like inclusion bodies in the digestive gland, a copepod in the digestive gland, Ancistrocomid‐like ciliates in the alimentary tract and gills.

Sequential examination of batches of juvenile oysters up to 23 weeks after placement in the sea in panels in the NT demonstrated progressive colonisation by a range of unidentified protozoan and metazoan organisms.

Examination of diseased mature and juvenile oysters in NT associated mortalities with Vibrio spp., an enigmatic protozoan‐like agent and abnormal environmental parameters.

A number of agents showed marked differences in distribution between states and between regions within states. The establishment of a restricted geographic distribution of potentially pathogenic agents in Australian P. maxima provides a basis on which rational quarantine may be implemented to avoid introduction of deleterious agents or pests when considering translocations or introductions of oyster stocks from different regions within Australia.

The study established normal histological criteria for P. maxima and defined a range of host responses to injury. These studies provide a basis on which the normal structure of the pearl oyster may be differentiated from the structure altered by disease, thus establishing criteria for disease diagnosis in pearl oysters. The normal histological criteria and histopathological changes associated with infectious and non‐infectious conditions found in the study are to form the basis of an FRDC atlas of pearl oyster morphology and pathology.

The study included a comprehensive review of infectious and non‐infectious agents, conditions and disease states of pearl oysters Pinctada spp. This review provides an international comparative basis on which to diagnose infections and disease states in Australian oysters and also provides an international perspective if introductions from elsewhere are contemplated.

All findings of the study have been collated on a relational database which can be utilised to determine the prevalence, occurrence and distribution of all agents and conditions identified and by which correlations between variable factors and specific agents or conditions can be made. It is intended that the database be made available to interested parties.

The study provides essential baseline data on disease occurrence and prevalence and a basis for the diagnosis of infectious and non‐infectious diseases of P. maxima. Avenues for further investigation of infectious agents are suggested.

Keywords: pearl oysters, Pinctada maxima, health survey, populations, pathogenic significance, regulatory controls, serious diseases, detection