When harvested in a clean environment and handled hygienically until consumption, fish is very safe. Unfortunately, unhygienic practices, insufficient refrigeration and sub-standard manufacturing practices have been at the origin of many outbreaks of fish-borne illnesses.
Fish-borne illnesses can be broadly divided into fish-borne intoxications and fish-borne infections (see table below). In the first case, the causative agent is a toxic compound that contaminates the fish or is produced by a biological agent in the fish. If the agent is biological, intoxication will occur even if the agent is dead as long as it has previously produced enough toxins to precipitate the illness symptoms. In the case of infections, the causative agent (bacteria, viruses or parasites) must be ingested alive, and then it invades the intestinal mucous membrane or other organs (infection) or produce enterotoxins (toxi-infection).
It is worth noting that, apart from Vibrio species, L. monocytogenes and Cl. botulinum that are part of the indigenous fish flora, all the other micro-organisms that cause the majority of the fish-borne illnesses are caused by contamination from the environment, the handlers or the water. Protection from the environment, personal hygiene, education of fish handlers and water treatment (e.g. chlorination) are therefore essential in the control of fish-borne diseases.
|Types of fish-borne illnesses |
|Type of illness ||Causative agent |
|Infections ||infections ||Listeria monocytogenes, Salmonella sp., Shigella sp. |
|toxi-infections ||Vibrio paraheamolyticus, V. cholerae, E. coli, Salmonella sp. |
|Intoxications ||chemical ||Heavy metals: Hg, Cd, Pb. Dioxines and PCBs. Additives: nitrites, sulfites |
|biotoxins ||Ciguatera, Paralytic shellfish poisoning (PSP), Diarrheic (DSP), Amnesic (ASP), Neurotoxic (NSP), Histamine |
|microbial ||bacterial ||Staphylococcus aureus, Clostridium botulinum, histamine |
|viral ||Hepatite type A, Norwalk, Calicivirus, Non-A and Non-B |
|parasitic ||Nematodes (round worms), Cestodes (tape worms), Trematodes (flukes) |
Biotoxins are marine toxins that accumulate in fish (ciguatoxin, tetrodotoxin) or shellfish (PSP, DSP, ASP, NSP). Illnesses caused by biotoxins are rare. Tetrodotoxin in mainly found in the liver, ovaries and intestines of various species of pufferfish, the most toxic being the members of the family tetraodontidae.
Marketing of these fish for human consumption is forbidden or strictly regulated. Ciguatera and the other biotoxins accumulate in fish (ciguatera) or in shellfish (mussels, oysters, clams, scallops, cockles) that feed on microscopic marine planktonic algae called dinoflagellates. Being filter-feeders, shellfish can accumulate high levels of these dinoflagellates when they occur in the sea. Prevention of biotoxin accumulation in fish and shellfish requires the implementation of a surveillance program to monitor the harvesting areas and the products.
Finally, physical contaminants such as glass, metal and wood pieces, nails, bones in fish fillets have also been at the origin of consumer health distress and need to be considered when designing a safety assurance programme.