Outbreaks of illness
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
- toxins produced by algae
The main cause of such illnesses are human viruses, particularly
filter-feeding creatures, bivalve shellfish can acquire this virus
from polluted seawater. Outbreaks typically occur during the colder
Monitoring relevant harvesting waters for faecal contamination,
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.
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.
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