A vast array of marine-derived nutrients has exhibited significant health benefits ranging from cardioprotective and anti-inflammatory effects to improvements in cognitive function. As sources of the omega-3 fatty acids docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), oils from fish and algae are well known for their substantiated heart and mental health benefits. Numerous marine compounds have also been identified for their bioactive antioxidant qualities, most notably the carotenoids and phenolic compounds found in crustaceans, seaweed, and a variety of compounds isolated from microalgae and fish protein hydrolysates. In addition, various polysaccharides, including fucoidans derived from algae and chitin derived from crustacean shells, have demonstrated both antiviral and antitumor activity in mammals. Consumption of health promoting marine-derived foods has been recommended and encouraged for all persons, but they are particularly beneficial for pregnant women and young children due to their positive effects on prenatal and postnatal neural development.

Marine toxins are chemicals and bacteria that can contaminate certain types of seafood. Eating the seafood may result in food poisoning. The seafood may look, smell, and taste normal. There are five common types of marine toxins, and they all cause different symptoms. Food poisoning through marine toxins is rare. Marine toxin poisoning occurs most often in the summer.

Increased consumption of fish and shellfish has, however, recently prompted concerns by the FDA for communication of the safety of these foodstuffs, particularly for consumption during pregnancy and childhood, and the agency is currently considering updates to its nearly 20-year old recommendations for manufacturers and consumers. Though shark, tilefish, swordfish, king mackerel, and canned tuna are fishes traditionally associated with the threat of high levels of mercury, other species of fresh and salt water fish and seafood may also contain levels far in excess of the recommended limit for safe mercury ingestion. This limit, originally calculated by the EPA in 1999, was updated by the World Health Organization in 2003 to 1.6 µg/kg body weight/week, but as fish consumption becomes more prevalent in an increasingly populated world, this figure is currently being reconsidered. Ironically, since mercury is neuro-, nephro-, and immunotoxic, any potential health benefits of marine-derived foods could be negated by exposure to this toxic substance, as reported most markedly in children.

In addition to the potential risks of mercury exposure, adverse outcomes associated with fish and seafood ingestion include food-induced anaphylactic allergic reactions, and less commonly, seafood toxin-induced neurological poisoning. Confounding the classification of negative health effects arising from seafood is the fact that seafood toxin poisoning in its milder form may frequently mimic allergic reaction. In exoskeleton-containing shellfish, cross-reactivity of the tropomyosin protein has been identified as the major allergen associated with IgE-mediated hypersensitivity. Marine allergy prevalence in the general population is estimated at 0.5%–2.5%, with shrimp, crab, lobster, clam, oyster, and mussel as causative agents in order of decreasing frequency. Shellfish allergies are most prevalent in Asia, most likely due to comparatively high reliance on seafood as a major component of the diet in this area. Overall, the condition is less common in children in spite of the observation that affected children have higher IgE levels than affected adults, suggesting a decreased sensitivity with age.

Seafood toxins may be produced by the fish itself, the plankton or algae consumed by the fish, or by the action of bacteria on muscle histidines to produce histamine by the fish. Generally, the greatest toxicological impact arises the five types of shellfish poisoning as well as from fish-borne scombroid poisoning, as these are most prevalent.

Scombroid poisoning is the most widespread fish poisoning and has been linked to the action of bacteria on muscle histidines. Its causative agents are improperly preserved abalone, tuna, mackerel, and other sea fish, and it typically presents with symptoms including diarrhea, nausea, and gastro intestinal distress, with respiratory failure occurring in its most severe forms.

Of the five classified adverse shellfish reactions, diarrhetic shellfish poisoning, occurring with the consumption of mussels, scallops, clams, and oysters, is the most innocuous. The causative toxins, which include okadaic acid, pectenotoxins, and dinophysistoxin-1, are generated by unicellular marine dinoflagellates that proliferate during summer blooms. Presenting symptoms include flu-like gastrointestinal cramping, vomiting, and nausea, accompanied by fever and chills. Shellfish poisoning also includes the more severe amnesic, neurotoxic, paralytic, and azaspiracid disease entities. Paralytic shellfish poisoning is the best characterized and most widespread. Though most prevalent on the east and west coasts of North America, these incidents occur worldwide, causing various degrees of neurologic dysfunction, from numbness to neuromuscular and respiratory paralysis. The toxins from the amnesic, neurotoxic, and paralytic varieties are heat-/acid-stable tetrahydropurines, of which at least 12 are known. The most common and well characterized include saxitoxins (paralytic), brevetoxin (neurotoxic), and domoic acid (amnesic) from dinoflagellates (paralytic, neurotoxic) and phytoplankton (amnesic). Less is known about azaspiracid intoxication, which principally affects areas of western Europe and Africa.

Marine neurotoxins more common to Asia include ciguatoxins, produced by the microalgae; Gambierdiscus toxicus, found in sea bass, snapper, grouper, and others; and tetrodotoxin, found in puffer fish. All of these toxins accumulate in fish liver and act to adversely affect sodium channel permeability. Less common are poisonings from fish roe, moray eels, sea urchins, and sea turtles, any of which may occur as a result of various toxins specific to the organism, or seasonally, as a result of algal bloom–derived bioaccumulation.

The presence of bacteria, bacterial toxins, and viruses in fish and seafood may also result in adverse reactions upon consumption of contaminated raw or undercooked food. Foodborne pathogens such as Norwalk virus, bacteria including Vibrio vulnificus, and bacterial toxins such as botulism toxin produced by Clostridium botulinum and Staphylococcal enterotoxin produced by Staphylococcal aureus bacteria are all common sources of coastal contamination and possible causes of severe illness. The intestinal symptoms of infectious contamination may mimic toxin-induced disease; however, unlike the presence of pathogenic microorganisms, toxins do not alter the taste and appearance of food and are not inactivated by standard cooking or food processing techniques.

How is marine toxin poisoning diagnosed and treated?

Your doctor will do a medical history and a physical exam and ask you questions about your symptoms and any fish you have recently eaten. Laboratory testing is typically not needed.

There are no specific treatments for marine toxin poisoning. Treatment generally consists of managing complications and being supportive until the illness passes. Dehydration caused by diarrhea and vomiting is the most common complication.

To prevent dehydration, take frequent sips of a rehydration drink (such as Pedialyte). Try to drink a cup of water or rehydration drink for each large, loose stool you have. Soda and fruit juices have too much sugar and not enough of the important electrolytes that are lost during diarrhea, and they should not be used to rehydrate.

Try to stay with your normal diet as much as possible. Eating your usual diet will help you to get enough nutrition. Doctors believe that eating a normal diet will also help you feel better faster. But try to avoid foods that are high in fat and sugar. Also avoid spicy foods, alcohol, and coffee for 2 days after all symptoms have disappeared.