Marine toxins are naturally occurring compounds that can contaminate some types of seafood. The seafood may not show any signs of contamination, but, if eaten, it can cause various human illnesses.
Marine toxins have probably existed for thousands of years. Biblical accounts of illnesses match the symptoms of paralytic shellfish poisoning, and the Red Sea may have been named “red” because of the frequent explosive growth of certain algae. Accounts of the consequences of marine toxins date back centuries. For example, a June 17, 1793, entry in a diary kept by the ship's surgeon during Captain George Vancouver's expedition off the West Coast of North America describes the death of a shipmate that is consistent with the effects of eating contaminated mussels.
In the coastal regions of the United States, the illnesses most frequently caused by marine toxins, from most common to least common, are scombrotoxic fish poisoning, ciguatera poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning, and amnesic shellfish poisoning.
Scombrotoxic fish poisoning is a bacterial illness caused by the degradation of fish (mainly tuna and bonito). The bacteria degrade fish proteins, and a by-product of the protein decomposition is a group of compounds called histamines. When the spoiled fish is eaten, the high histamine level causes poisoning. Symptoms may begin only a few minutes after eating the seafood or several hours later. The symptoms include the development of a rash, flushing of the skin, sweating, and headache. As the body tries to expel the poison, abdominal pain, vomiting, and diarrhea also can occur. Some people also experience a burning or metallic sensation in the mouth.
The symptoms of scombrotoxic fish poisoning are temporary, and tend to fade after a few hours. Usually no treatment is necessary, although some people do benefit from drugs called antihistamines that counteract the effects produced by the excess histamines, as well as from a drug called epinephrine. Symptoms can be more severe in those who are taking some medications that slow the breakdown of histamine.
The second most common type of illness caused by marine toxins in the United States is ciguatera poisoning. This type of poisoning is due to the contamination of tropical reef fish by tiny marine plants called dinoflagellates. The illness is an example of what is termed biological magnification or biomagnification. In this case, the dinoflagellates are present in fish species that are food for a larger species. That species in turn becomes food for a larger marine animal. This pattern continues, with the concentration of the poison increasing from creature to creature. The animal at the top of this food chain (with ciguatera poisoning, it is often the barracuda) may have a high concentration of the poison. The person who eats that animal ingests the accumulated load of toxin. In addition to the barracuda, other fishes may contain high levels of the dinoflagellate toxin, including grouper, sea bass, snapper, and mullet. These popular sport fishes are found in tropical waters off of Hawaii, the Virgin Islands, Puerto Rico, and islands in the South Pacific.
Symptoms of ciguatera poisoning usually appear within minutes of eating a contaminated fish. The symptoms include nausea with vomiting, abdominal cramps, diarrhea, sweating, headache, muscle aches, dizziness, itchy skin, and general weakness. More usual symptoms are possible, such as alterations in taste and temperature sensations, and nightmares and hallucinations may occur. The symptoms tend to fade in 1–4 weeks.
Paralytic shellfish poisoning is caused by another dinoflagellate that can explode in numbers during an event called a “red tide.” The name refers to the appearance of the water, which becomes discolored by the presence of vast numbers of the reddish brown dinoflagellates. The affected marine creatures are often filter-feeders—those that feed by straining sea water to remove tiny nutrients. The toxin-laden dinoflagellates accumulate in mussels, clams, oysters, crabs, and scallops. Lobsters also can become contaminated.
WORDS TO KNOW
BIOMAGNIFICATION: The increasing concentration of compounds at higher trophic level or the tendency of organisms to accumulate certain chemicals to a concentration larger than that occurring in their inorganic, non-living environment, such as soil or water, or in the case of animals, larger than in their food.
DEGRADATION: Degradation means breakdown and refers to the destruction of host cell components, such as DNA, by infective agents such as bacteria and viruses.
DIATOM: Algae are a diverse group of simple, nucleated, plant-like aquatic organisms that are primary producers. Primary producers are able to utilize photosynthesis to create organic molecules from sunlight, water, and carbon dioxide. Ecologically vital, algae account for roughly half of photosynthetic production of organic material on Earth in both freshwater and marine environments. Algae exist either as single cells or as multicellular organizations. Diatoms are microscopic, single-celled algae that have intricate glass-like outer cell walls partially composed of silicon. Different species of diatom can be identified based upon the structure of these walls. Many diatom species are planktonic, suspended in the water column moving at the mercy of water currents. Others remain attached to submerged surfaces. One bucketful of water may contain millions of diatoms. Their abundance makes them important food sources in aquatic ecosystems.
DINOFLAGELLATE: Dinoflagellates are microorganisms that are regarded as algae. Their wide array of exotic shapes and, sometimes, armored appearance is distinct from other algae. The closest microorganisms in appearance are the diatoms.
HISTAMINE: Histamine is a hormone that is chemically similar to the hormones serotonine, epinephrine, and norepinephrine. A hormone is generally defined as a chemical produced by a certain cell or tissue that causes a specific biological change or activity to occur in another cell or tissue located elsewhere in the body. Specifically, histamine plays a role in localized immune responses and in allergic reactions.
RED TIDE: Red tides are a marine phenomenon in which water is stained a red, brown, or yellowish color because of the temporary abundance of a particular species of pigmented dinoflagellate (these events are known as “blooms”). Also called phytoplankton, or planktonic algae, these single-celled organisms of the class Dinophyceae move using a tail-like structure called a flagellum. They also photosynthesize, and it is their photosynthetic pigments that can tint the water during blooms. Dinoflagellates are common and widespread. Under appropriate environmental conditions, various species can grow very rapidly, causing red tides. Red tides occur in all marine regions with a temperate or warmer climate.
Symptoms of paralytic shellfish poisoning begin within several minutes to several hours of eating contaminated seafood. Initially, the symptoms are mild and include numbness of the face, arms, and legs; more severe symptoms follow, including dizziness, nausea, and loss of coordination. Some people become paralyzed and can die when they become unable to breathe.
Neurotoxic shellfish poisoning is another illness that is caused by a dinoflagellate. Shellfish are involved; they concentrate the toxin during filter-feeding. As with the other illnesses caused by marine toxins, symptoms tend to occur soon after consuming the contaminated seafood. Symptoms include dizziness, numbness, a tingling sensation in the mouth, arms, and legs, and loss of coordination. Recovery occurs within several days.
Finally, amnesic shellfish poisoning is a rare event that is caused by a microscopic plant (diatom) called Nitzchia pungens. Concentration of the diatom in shellfish, such as mussels, also concentrates a component of the diatom known as domoic acid. When contaminated mussels are eaten, the domoic acid causes an intestinal upset, dizziness, headache, and loss of orientation. In severe cases, there can be brain damage when domoic acid attaches to chemical receptors in brain cells, disrupting cell function. Permanent loss of memory, paralysis, and death can result.
Marine toxins are found in coastal regions in almost all parts of the globe, except at the higher latitudes of the Arctic and Antarctic. Different illnesses often have different distributions. For example, neurotoxic shellfish poisoning tends to occur in the Gulf of Mexico and along the southern Atlantic Coast of the United States.
Illnesses caused by marine toxins occur in greater numbers in more equatorial regions, since the warmer waters encourage the growth of the microorganisms that produce the toxins. However, outbreaks occur during the warmer months in other regions.
There is no evidence that gender or race influences a person's susceptibility to marine toxins. The elderly and those with a less efficient immune system may be more at risk.
Treatment typically involves making the patient as comfortable as possible and waiting for the illness to pass. Scombrotoxic fish poisoning can be treated with drugs aimed at neutralizing the effects of the excess histamine.
There are no vaccines that provide protection against poisoning by marine toxins. The best prevention strategy is to use caution when eating seafood. For example, eating raw shellfish is risky and should be avoided. Warnings about algal blooms and reports of seafood-related illnesses should be taken seriously. Seafood from the affected region should be avoided until public health officials have determined that the danger is over.
In the United States, about 30 people are poisoned by the toxins in seafood each year. The consequences of this poisoning can range from a short-term and inconvenient illness to permanent damage, memory loss, and death.
Because coastal areas often attract tourists and tourists often want to sample the local seafood, an outbreak of poisoning by a marine toxin can affect the local economy. For example, in 1987 there was an outbreak of amnesic shellfish poisoning on Prince Edward Island, Canada, which sickened more than 100 people and caused several deaths. In the years following the outbreak, fear over consumption of seafood and a lingering perception that the coast of the province was dangerous caused a marked drop in visitors. This adversely affected the island’ economy, which heavily relies on summer tourism.
Periodic outbreaks involving larger numbers of people also occur. In fact, studies by the Woods Hole Oceanographic Institution and the U.S. National Oceanographic and Atmospheric Administration indicate that the frequency of algal blooms has been increasing along the coasts of the United States and other countries since the 1970s. While the cause of the increased number of blooms is not absolutely certain, a general consensus among scientists is that the documented warming of the coastal oceans has made conditions more favorable for algal growth. If so, a consequence of global warming could be more algal blooms and more cases of marine toxin-related illness.
IN CONTEXT: MARINE MICROORGANISMS
Marine microorganisms often inhabit a harsh environment. Ocean temperatures are generally very cold—approximately 37.4° F (about 3° C) on average—and this temperature tends to remain this cold except in shallow areas. About 75% of the oceans of the world are below 3,300 ft (1,000 m) in depth. The pressure on objects like bacteria at increasing depths is enormous.
Some marine bacteria have adapted to the pressure of the ocean depths and require the presence of the extreme pressure in order to function. Such bacteria are barophilic if their requirement for pressure is absolute or barotrophic if they can tolerate both extreme and near-atmospheric pressures. Similarly, many marine bacteria have adapted to the cold growth temperatures. Those which tolerate the temperatures are described as psychrotrophic, while those bacteria that require the cold temperatures are psychrophilic (“cold loving”).
Marine microbiology has become the subject of much commercial interest. Compounds with commercial potential as nutritional additives and antimicrobials are being discovered from marine bacteria, actinomycetes and fungi. For example the burgeoning marine nutraceuticals market represents millions of dollars annually, and the industry is still in its infancy. As relatively little is still known of the marine microbial world, as compared to terrestrial microbiology, many more commercial and medically-relevant compounds undoubtedly remain undiscovered.
In coastal regions of the United States, Canada, and other maritime countries, government agencies monitor ocean catches and aquaculture facilities for the presence of the various toxic species. Detection of these toxic species can lead to the closure of a region to fishing and the sale of commercially raised seafood until the problem is resolved.
The health risk posed by marine toxins has been balanced somewhat by the discovery that some marine toxins can act as anti-cancer drugs. A 2006 University of Wisconsin study reported that marine toxins can bind to a cell component called actin and that this interaction can disable rapidly growing cells, such as cancer cells.
See AlsoWater-borne Disease.
Sindermann, Carl J. Coastal Pollution: Effects on Living Resources and Humans. Boca Raton: CRC, 2005.
Allingham, J. S., et al. “Structures of Microfilament Destabilizing Toxins Bound to Actin Provide Insight into Toxin Design and Activity.” Proceedings of the National Academy of Science 102 (2005): 14527–14532.