Marine food toxins

Saxitoxin (STX) is a toxin which is found in marine microorganisms. It is most likely synthesized by bacteria which live in symbiosis with dinoflagellates, a component of phytoplankton. Through the marine food chain, it can lead to poisoning of humans. The mechanism of toxicity of saxitoxin is vety similar to that of tetrodotoxin. Saxitoxin binds from the outside of the membrane to various forms of voltage-sensitive Na+channels and blocks the channel in an activation state-independent manner. 

The use of high performance liquid chromatography (HPLC) for the study of paralytic shellfish poisoning (PSP) has facilitated a greater understanding of the biochemistry and chemistry of the toxins involved. HPLC enables the determination of the type and quantity of the PSP toxins present in biological samples. An overview of the HPLC method is presented that outlines the conditions for both separation and detection of the PSP toxins. Examples of the use of the HPLC method in toxin research are reviewed, including its use in the determination of the enzymatic conversion of the toxins and studies on the movement of the toxins up the marine food chain. 

In general, the clinical presentation of the human diseases associated with the ingestion of marine seafood toxins is similar to that of any other food poisoning disease. However, a number of clinical issues make these diseases particularly difficult to diagnose and treat. For example, the neurotoxic syndromes associated with CFP, PSP, and NSP represent points along a continuum of disease severity rather than clinically exclusive diseases. Even if fish or other seafood is the suspected source of a disease outbreak, diarrhea associated with the outbreak could be misdiagnosed as originating from bacterial rather than from phycotoxin contamination. 

Diseases associated with marine seafood toxins appear to have high attack rates. An attack rate is the proportion of a well-defined population that develops a disease over a specific period of time (where the numerator is the number of new cases during that period and the denominator is the size of the population at risk, e.g., the number of people who ate a contaminated food at the start of the time period of interest) (Goodman and Peavy, 1996). Physicians therefore need to ask about disease cases among people sharing the same seafood meal. 

Baden, D.G. 1983. Marine food-borne dinoflagellate toxins. IntRev Cytol 82, 99-150. 

Baden DG. Marine food-home dinofiagellate toxins. IntRev Cytol 82 99-150, 1983. 

Cruz-Rivera, E. and Villareal, T.A. Macroalgal palatabUity and the flux of ciguatera toxins through marine food webs, Harmful Algae, 5,497-525, 2006. 

A number of human illnesses are caused by ingesting seafood contaminated with toxins produced by marine phytoplankton [9-11], The phytoplankton is the base of the marine food web, and the toxins it produces can accumulate and concentrate in higher organisms that can become lethal if ingested. Although mainly neurotoxins, these toxins can cause a wide range of acute and chronic health effects in humans and other species. They are tasteless, odorless, and heat and acid stable. Therefore, conventional food methods are unable to detect and destroy them in contaminated seafood. 

Arakawa, O., Mahmud, Y., Tanu, M.B., Tsuruda, K., Okada, K., Kawatsu, K., Hamano, Y., Takatani, T., and Noguchi, T. 2003. Micro-distribution of tetrodotoxin in puffers and newts. In Proceedings of International Scientific Symposium on Marine Toxins and Marine Food Safety (D.F. Hwang and T. Noguchi, eds), pp. 57 5. National Taiwan Ocean University, Keelung. 

Large, generalist marine grazers such as fishes and urchins attempt to choose foods that maximize nutritional input (e.g., protein, lipids, and carbohydrate) (Mattson 1980 Choat and Clements 1998) and minimize intake of secondary metabolites (Hay 1991). The untested assumption underlying these optimal foraging decisions is that detoxification and excretion rates are a constraint on toxin intake and thus drive feeding choice (Freeland and Janzen 1974). However, we have virtually no information on such constraints in marine herbivores, because it requires an understanding of the metabolic fate of secondary metabolites. 

Saxitoxins are water-soluble compounds that prevent proper nerve functioning. They are produced in nature by plant-like marine protozoa called dinoflagellates. Humans typically acquire such toxins by eating bivalve mollusks fed on dinoflagellates. A terrorist would likely deliver a saxitoxin as an aerosol or use it as a poison to contaminate food or water. 

People eating fish cannot detect the marine phyeotoxins associated with seafood diseases, and preparation procedures do not remove the toxins (Baden et al., 1995 Baden and Mende, 1982 Baden and Trainer, 1993 Sakamoto et al., 1987). Severe heating processes, such as retorting, may reduce the levels of some toxins (U.S. Food and Drug Administration, 2001) but this is not a practical method for protecting public health. 

Laboratory methods for detecting marine toxins (Table 7.2) can be characterized into two types of analyses indirect assays and direct measurement analyses. Indirect assays, or bioassays, measure the biological effect of a toxin on a system and can implicate, but not verify, the presence of a particular toxin. By contrast, direct measurements can both confirm and quantify the amount of a specific toxin in a food or biological specimen. 

Foodborne poisoning may result from the ingestion of food contaminated with marine toxins (Butterton and Calderwood, 2001 CDC, c). 

In addition to serving as the major food source to heterotrophic bacteria, DOM plays an important ecological role in enabling marine organisms to control various aspects of their environment including trophic interrelationships. This is accomplished by the secretion or exudation of specific molecules, called secondary metabolites. These are generally LMW compounds that tend to be species specific in their source and targets. Some act as toxins that repel or kill competitors or predators. As noted earlier, some diatoms... 

The investigation of the presence of marine biotoxins in water, phytoplankton, and food has been achieved by several in vitro assays. However, alternatives to the animal bioassay for marine toxins have not been sufficiently evaluated in interlaboratory studies needed to demonstrate their scientific validity. In addition, these methods continue to be time consuming and expensive for intensive monitoring programs, and present some difficulties for their automation. 

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