Description of microbiological risks

Sources of microbiological contaminations and consequences for human health.

Microbes, or germs, are single-celled organisms invisible to the naked eye. They are present everywhere in the environment, including inside our body, and are essential for life.

Microbiological contamination refers to an excessive quantity of germs (bacteria, viruses, vibrios and parasites) present in the water or seafood, and potentially dangerous to human health.

  • Generalities : 

Some pathogenic germs are naturally found in marine waters (for example vibrios), but microbiological contamination mainly refers to pathogenic germs (bacteria, viruses, parasites) coming from human or animal feces.

The main pathogens involved in fecal contaminations are: Salmonella, Shigella, some Escherichia coli, campylobacter, novovirus, HAV (hepatitis A virus), Cryptosporidium, Gardia,...

Pathogens are many, their search in laboratory is difficult and they are rare in the wild; therefore, a single parameter is used as a microbiological contamination tracer: the enterobacteria Escherichia coli.

E. coli is present in feces only, but in a large quantity. Although this bacteria is most of the time harmless, it is used as a bioindicator for fecal contamination of bivalve shellfish, and so its presence indicates the probability that other pathogens may be present as well.

  • Sources of contamination:

Harvested areas lie on the foreshore, between low and high tide marks, and are therefore affected by both natural and anthropogenic (i.e linked to human activities) factors.

Sources of contamination are many:

     -Collective waste treatment (Sewage treatment plants, sanitation network, pumping stations...)

     -Independent waste treatment (or on-site sanitation: septik tanks, infiltration)

     -Agriculture (dung and liquide manure spreadings, pasture, pits)

     -Agribusiness (especially slaughterhouses)

     -Water sports (pleasure crafts)

     -Wild animals (in minor proportion)


Sources of microbiological contaminations


Shellfish contaminations depend on these sources as well as on other factors:

- Time of the year: in the summer, touristic activities increase the anthropogenic pressures on the shore (waste rejections rise up with the number of inhabitants).

- Pluviometry: the rain drains soils to rivers and streams, and it can saturate the sewage treatment facilities in case of heavy raining.

- Gomorphology of watersheds: the inclination of slopes around streams, rivers and the coast, geology, size of waterway networks (streams, rivers)...

- Shellfish physiology and habitat: the different shellfish species do not filter the same amount of water. Burrowing or near-seabed shellfish would tend to be more contaminated than others.

- Germs mortality: microbes coming from feces are unable to develop in the wild and will survive only a short period of time when released in nature. The length of this survival depends on several factors: temperature, salinity, light, turbidity... For example, E. coli can survive from 2 hours to a few days, when novoviruses may last from a week to a month.

  • Effects on human health:

The consumption of infected bivalve shellfish may lead to the outbreak of symptoms similar to those of gastroenteritis: headaches, diarrhoeas, nausea and vomiting. In more serious cases, infectious diseases such as hepatitis A or salmonella (typhoid fever...) may develop.