What is a health hazard and which types of contamination affect water and shellfish?
First of all it is important to point out that recreative harvesters and consumers buying shellfish are not equal before health hazards.
Professionally-retailed bivalve shellfish go through SPECIFIC INSPECTIONS (on-site, in professional farms and in retail shops) AND benefit from PURIFICATION PROCESSES. These actions GUARANTEE THE SANITARY SECURITY OF RETAILED SHELLFISH.
A health hazard is the probability of health troubles occurring after having been exposed to a source of contamination. Therefore, a hazard depends on both the type of infectious agent (level of toxicity, ingested quantity...) and the person infected (elderly people, immunocompromised host...). In the marine environment, these risks are associated to several types of contamination:
- Microbiological contamination:
It is an excessive quantity of germs (bacteria, viruses, vibrios and parasites) present in water or seafood, and potentially dangerous to human health. It is more deeply described in the Description of microbiological risks article.
- Phycotoxin contamination:
The term "phytoplankton" refers to vegetable plankton (aka microalguae or microphyte). It is the largest group of organisms present in marine ecosystems, some of them being potentially toxic. As a matter of a fact, some phytoplankton species are able to secrete harmful phycotoxins.
Ifremer has set up a monitoring program for phytoplankton and phycotoxins named REPHY (for REseau de surveillance du PHYtoplancton et des PHYcotoxines). Its purposes are both sanitary and environmental, and it has allowed, since its creation in 1984, to identify three main groups of potentially toxic species that might occur on our shore:
- Dinophysis is a microphyte belonging to the dinoflagellates family. It produces Diarrheic Shellfish Poisoning (DSP). Gastrointestinal symptoms (such as diarrhoea, vomiting, abdominal pains, however no fever) appear 30 minutes to 12 hours after having ingested contaminated bivalve shellfish.
- Alexandrium is also a microphyte belonging to the dinoflagellates family and secreting Paralytic Shellfish Poisoning (PSP). Symptoms appear 5 to 30 minutes after ingestion and vary, depending on the level of contamination, from minor troubles (buccal, finger and toe tips parasthesia, headaches, vomiting) to mild troubles (speech impairment, lack of coordination...) or even severe troubles (diplopia and respiratory paralysis).
- Pseudo-nitzschia is a microalguae belonging to the diatoms family and secreting Amnesic Shellfish Poisoning (ASP). Gastrointestinal symptoms (nausea, vomiting, abdominal pains) occur within 24 hours, then clinical neurological signs (such as memory loss, desorientation) appear within 48 hours.
These toxins are all HEAT-STABLE: they resist to cooking. The REPHY monitoring program is carried out by coastal Ifremer laboratories along the French shore.
There is a participative science project named PHENOMER inviting you to communicate any phytoplankton blooms you might witness. Blooms may be noticeable by the way they colour waters, when the phytoplankton concentration is so high that it becomes visible to the naked eye.
- Chemical contamination:
The presence of chemical compounds in sea water may lead exposed creatures to a poisoning risk. Most of these pollutants are harmful when reaching a certain concentration level: heavy metals, hydrocarbons, pesticides, persistant organic pollutants... Sources of contamination are many but the main ones are still industrial and agricultural effluents, and gas emissions. The effects on health vary from a compound to another but a long-term exposure to some pollutants may disturb the endocrinal system, degrade fertility and cause cancer. Moreover, these pollutants can bioconcentrate in animal adipose tissues when going up the food chain (=biomagnification).
Marine bivalve shellfish filter important quantities of water for their physiological needs (feeding, breathing), and therefore accumulates surrounding substances and microorganisms in their tissues.
Although bivalve molluscs coming from professional shellfish farms are subject to specific controls and may be purified, it is not the case for recreationally-harvested shelffish.
This is why it is essential to respect regulations and to inquire about the sanitary quality of the areas you are planning on harvesting.
In France, there is an inventory of infection cases caused by nutrition. These cases are named Collective Food Poisoning (CFP). When a CFP occurs, food types and pathogens responsible are sought. From 1996 to 2010, 5% of CFPs were related to the consumption of bivalve shellfish (561 cases out of 11 261, (Vaillant & al., mai 2012 ,Bulletin Epidémiologique n°50, p.42-46)).
Although bivalve shellfish are easily subject to various types of environment contaminations, this number allows us to put the health hazards they represent into perspective when compared to other types of food poisoning. It is however necessary to keep in mind that:
- This number is probably overestimated as many cases are too quickly associated to shellfish poisoning.
- CFP, whatever causes them, are under-declared as they may present quickly-disappearing or relatively minor symptoms.