Oyster Disease Aquaculture

Primary reference(s)

OIE, 2019. Manual of Diagnostic Tests for Aquatic Animals. World Organisation for Animal Health (OIE). Accessed 19 October 2020.

Additional scientific description

Aquatic food makes an important contribution to human health and development as an essential source of high-quality proteins, vitamins and micronutrients. Global aquaculture production is now a USD 157 billion industry, with USD 20.5 billion of this total representing the production of molluscs for food in the marine environment. The largest share of this represents culture of bivalves such as oysters, clams and scallops. Mollusc culture is a well-developed industry in many countries, notably in the northern hemisphere, such as the USA, Canada, Japan, Korea, France, Spain and the Netherlands (Carnegie et al., 2016).

Oysters are subject to a number of diseases which can impact the local population and reduce harvests in a commercial setup.

The agents causing oyster diseases do not pose any direct human health implications. However, oysters could potentially pose a health concern for humans in cases where they contain high levels of Vibrio spp. (V. parahaemolyticus, V. vulnificus, and choleragenic, V. cholera) and are consumed raw, or where the oysters are produced in an area containing biotoxin or heavy metal contamination (e.g., lead) (WHO, 2005). Bivalve molluscs are filter feeders; therefore they are susceptible to picking up and accumulating toxins, and chemical or bacteriological contaminants from their environment. By filtering a great quantity of water, they may bioaccumulate a high number of microorganisms in their tissues that can be considered infectious for humans and higher vertebrates (Zannella et al., 2017). The causative pathogens live in aquatic environments in both tropical and temperate zones. High temperatures and salinities favour the proliferation of some of the pathogens.

Epizootic diseases have been shown to cause catastrophic mortality among marine bivalves, which can severely affect fisheries and aquaculture activities worldwide. Some of these diseases have become a serious constraint on the development and sustainability of shellfish farming and fishing. Currently, the main cause of epizootic outbreaks is thought to be the transfer of infectious agents through the transport of live shellfish.

Severe outbreaks have resulted in significant economic destruction, with some outbreaks essentially causing the collapse of entire industries. For instance, epizootics of protistan parasites Haplosporidium nelsoni in the 1950s to 1960s and Perkinsus marinus in the 1980s to 1990s devastated the planting industry for oyster Crassostrea virginica in the US mid-Atlantic (Carnegie et al., 2016).

Illness in humans is linked to the consumption of raw oysters. Metrics and numeric limits None identified.

Metrics and numeric limits

None identified.

Examples of drivers, outcomes and risk management

As a consequence of globalisation, urbanisation and global change, the number of emerging diseases of molluscs is increasing. These diseases have a significant impact on aquatic animals resulting from (i) a change of known pathogenic agent or its spread to a new geographical area or species; or (ii) a newly recognised or suspected pathogenic agent. Different scenarios can lead to emergence of diseases, but the cause is often the introduction of infected animals.

The World Organisation for Animal Health (OIE) Aquatic Animal Health Code has set out standards and recommendations to improve the safety of international trade in aquatic animals, including marine bivalve molluscs (OIE, 2019b). The aim is to prevent the pathogen’s introduction into an importing country, so as to avoid the onset of disease outbreak rather than to eradicate the pathogen, which would be more difficult and expensive owing to the high-density production systems used in commercial hatcheries and nurseries and the continuous stock movements around the world (Zannella et al., 2017).

The OIE member countries are required to report the detection of listed and emerging diseases (Carnegie et al., 2016).

There is currently no available vaccine, treatment or chemical control agent for the OIE listed diseases of oysters, therefore the surveillance and control plan of these diseases based on their prevention have a key role. For this reason, it is important to implement high levels of on-farm and live-holding facility biosecurity and to restrict stock movements (Zannella et al., 2017).

Maintaining the biosecurity of international aquaculture industries requires control of pathogens of concern. This can include rapid detection in animals proposed for sale or transfer to prevent pathogen introduction to new areas and maintain disease-free facilities and regions, and focused management of pathogens where they do occur, including the use of pathogenresistant broodstocks to reduce the magnitude of seasonal epizootics (Carnegie et al., 2016).