• Observing and modelling
• Marine Climate Change Centre (MC3)
• Ecosystems and biodiversity
• Assessing human impacts
• Animal health and food safety
  • Aquatic animal disease
  • Food safety
    • Algal toxins surveillance
    • Classification and microbiological monitoring
    • Microbiology and human health
    • Outbreaks of illness
    • Purification plants
    • Reference Laboratory functions
    • Sanitary surveys
    • Shellfisheries water quality
  • Environment and animal health
• Fisheries information

Related links

• Health Protection Agency
• Food Standards Agency
• Human health and climate change
• EURL - bivalve molluscs
• NRL - bivalve molluscs
• Cefas shellfish testing service
• New testing services coming soon!

Outbreaks of illness

People can sometimes fall ill after eating bivalve molluscs (such as oysters and mussels). The most frequent symptoms reported are diarrhoea and vomiting (gastroenteritis). Generally, there are three main causes:

• viruses
• toxins produced by algae
• bacteria.

Viruses

The main cause of such illnesses are human viruses, particularly norovirus. As filter-feeding creatures, bivalve shellfish can acquire this virus from polluted seawater. Outbreaks typically occur during the colder winter months.

Monitoring relevant harvesting waters for faecal contamination, classifying production sites, and purifying shellfish at "depuration" plants help to minimise risks of illness outbreaks. However, outbreaks may still occur under particularly adverse environmental conditions such as following heavy rain - which may cause sewage spills.

Algal toxins

Less frequently, gastroenteric illness may be caused by algal toxins. Naturally occurring algae may bloom under favourable environmental conditions. During such times some species may produce toxins, which can accumulate in filter-feeding shellfish. Algal blooms typically occur during the spring or summer, and this is when the risk of illness is more likely to occur.

Incidences of such illness are rare in the UK because comprehensive monitoring programmes are in place to protect shellfish consumers. However, illnesses do occasionally occur when consumers eat imported shellfish or shellfish taken from beds that have not been properly monitored.

Bacteria

Illness may also occasionally be associated with bacterial causes of food poisoning, such as Salmonella spp. or naturally occurring marine vibrios (rod-shaped bacteria). Marine vibrios proliferate in warm conditions, so the risk from such infections in seafood is linked to seawater temperatures in production areas. Although shellfish-associated vibrio infections are currently infrequently reported in the UK, climate change may increase the risk.

Following a suspected outbreak

Local authority environmental health officers are responsible for investigating incidents of human illness associated with shellfish consumption, and controlling the situation following an outbreak. If you have a concern you should contact your local environmental health officer in the first instance.

We assist local authorities, the Food Standards Agency and the Health Protection Agency to investigate such events, particularly those associated with bivalve shellfish. Generally, following a request from a local environmental health officer and with the agreement of the Food Standards Agency, we will test shellfish to help determine the causative agent. The results of such tests then help to inform potential risk-management activities, such as bed closures or product recalls.

For further information about our work in this area contact SHS@cefas.co.uk.