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Diarrhoeic Shellfish Poisoning (DSP)
Hazard Identification
What is diarrhoeic shellfish poisoning? Diarrhoeic shellfish poisoning (DSP) is a foodborne intoxication associated with the consumption of contaminated shellfish harvested from waters affected by growth of certain types of toxic algae. DSP has been known for around 30 years and is most common in Europe and Japan, but DSP toxins are being increasingly reported in shellfish from previously unaffected areas DSP is caused by the ingestion of toxins that accumulate in certain types of shellfish that have been feeding on the algae that produce the toxins. DSP is a non-lethal form of food poisoning with symptoms typical of gastroenteritis, especially diarrhoea. There are a number of chemically different toxins associated with DSP. They are lipophilic and polyether compounds and can be divided into three main groups:
The DTXs are the most important group in causing DSP symptoms and it is possible that the other two groups are not involved in illness, but are often found in association with DTXs. Occurrence in foods Most cases of DSP are related to bivalve molluscs, especially mussels, but also scallops, oysters and clams. These species are filter feeders and accumulate toxins when the water contains sufficient levels of toxin-producing algae. Toxicity is seasonal and tends to be highest during the summer months in Europe and Japan, although DSP cases in Scandinavia have been reported in February and in October. Predatory fish and other marine animals that prey on toxic shellfish may also accumulate DSP toxins, especially in liver tissue, but the significance of this for human health is uncertain. DSP toxins are fat soluble and so tend to accumulate in the fatty tissue of affected shellfish. The highest levels are normally found in the viscera and shellfish can accumulate enough toxin to cause illness within hours when large populations of toxic algae are present in the water. DTXs may also be metabolised in the digestive gland (hepatopancreas) of contaminated shellfish, producing related toxic by-products. Toxin levels as high as 10 mg OA/g hepatopancreas have been reported in mussels grown in Japanese waters. DSP toxin levels in shellfish do reduce naturally after they stop feeding on toxic algae, but there is little definite information on how this process occurs or on toxin retention times in different species. It is likely that some toxin is excreted in faeces before it can be assimilated. Hazard Characterisation Effects on health DSP toxins are powerful phosphatase inhibitors and this property is associated with inflammation of the gut in humans. This leads to fluid loss from intestinal cells resulting in diarrhoea. A minimum dose of OA for toxic effects to occur is estimated to be 48 µg, whereas for its derivative DTX1 the minimum it is 38.4 µg. Levels of DSP toxins are commonly expressed as toxic equivalents of OA (mg OA eq/kg) or as Mouse Units (MU/kg) relating to a standard mouse bioassay method. The onset of symptoms of DSP may occur between 30 minutes and 12 hours after ingestion of toxic shellfish. The main symptoms of DSP include diarrhoea, nausea and vomiting and abdominal pain. The severity of symptoms depends on the amount of DSP toxins ingested, but complete recovery typically occurs within three days. No fatalities caused by DSP have been reported to date and hospital treatment is not usually needed. DSP toxins have also been shown to have other effects in animals and in cell cultures. For example OA and DTX1 are probable carcinogens, but the significance of this for human health is unknown. PTXs and YTX are lethal in mice and are certainly toxic, but the effects of oral doses in humans are not known. Incidence and outbreaks DSP mainly affects Western Europe and Japan, but DSP toxin-contaminated shellfish and toxin-producing algae have been found in more widespread locations, including Canada, Mexico, South America, India, Thailand, China and Australia, and incidences of DSP toxin seem to be increasing. There have been a number of major outbreaks of DSP in Europe. Mussels imported from Denmark caused 415 cases of illness in France in 1990. In 1984, 10,000 people in France were affected by DSP symptoms caused by domestically produced mussels and a further 2,000 became ill the following year in a similar outbreak. 1984 also saw a major outbreak in Norway affecting at least 300 people. Over 5,000 cases of DSP-related gastroenteritis were reported in Spain in 1981, and DSP toxins have repeatedly been found at high levels in shellfish from the Galician region, resulting in prolonged disruption to local fisheries from 1993 onward. DSP cases were not reported in the UK until 1997, when 49 people were made ill after eating mussels in a London restaurant. Since then, the frequency of DSP events in UK waters has increased and shellfish harvesting has been restricted in several areas on a regular basis. In Japan, cases of DSP were first reported in 1976 and 1977 when more than 150 people were affected by vomiting and diarrhoea. A total of at least 1,300 cases were reported between 1976 and 1981. Elsewhere, outbreaks have been reported in Australia, Canada, Chile and the USA. Although the Northeast USA, especially New York and New Jersey, experienced large outbreaks of DSP-like illness between 1980 and 1985, outbreaks of human illness have not been reported since then, although DSP toxins have been found occasionally in US waters. Sources DSP toxins are produced by dinoflagellates of the genus Dinophysis. Seven species have been shown to produce the toxins. These are D. acuminata (Europe) D. acuta, D. fortii (Japan), D. mitra, D. norvegica (Scandinavia), D. rotundata and D. tripos. Three other species are also suspected of being able to produce toxins. Certain Prorocentrum species (P. concavum, P. lima and P. redfieldi) also produce DSP toxins. If conditions are favourable exponential growth of these species may occur resulting in an algal bloom. However, it is not necessary for visible blooms to occur for DSP toxins to be present at harmful levels. The production of toxins by different dinoflagellate species is highly variable and the same species may produce widely varying quantities of toxin in different locations. Some Dinophysis spp. can produce sufficient toxin in shellfish to cause illness in consumers at populations as low as 200 cells per litre. On other occasions much greater densities (>20,000 cells per litre) may be involved. Stability in foods DSP toxins are all relatively heat stable and are not destroyed by practical cooking processes. Natural detoxification in shellfish does occur, but the rate of this process varies greatly with the species, the season (low water temperature slows toxin loss) and with the site of toxin accumulation. It has been reported that the retention time of DSP toxins in mussels can vary from one week to six months. Control Options The stability of DSP toxins and the variability of natural detoxification mean that neither depuration in clean water nor cooking processes are effective or economically viable methods of reducing the toxicity of affected shellfish to safe levels. The only effective controls available currently are the monitoring of the marine environment and the testing of shellfish flesh for DSP toxins. Regular inspection of the waters where shellfish are harvested, or produced by aquaculture, for the presence of toxic algae can be a useful source of data and indicate when a risk of toxicity is present. The routine testing of shellfish, especially mussels, for DSP toxins by chemical, immunological, or bioassay methods is the key prevention measure. However, the variability of toxin production by the algae and other factors must be taken into account when designing a suitable sampling plan When toxic conditions are detected, bans on harvesting shellfish have to be imposed until toxicity can be shown to have returned to safe levels and contaminated shellfish should not be allowed to enter the human food chain. Legislation There are regulations relating specifically to PSP toxins in shellfish in a number of countries. In the EU the European Commission has set a maximum limit for combined OA, DTXs and PTXs in molluscs, echinoderms, tunicates and marine gastropods of 160 mg STX eq/kg of edible tissues. The maximum level of YTXs in edible tissues is set at 1 mg YTX eq/kg. The mouse bioassay method is the official reference method of analysis, if required in association with a chemical detection method. Monitoring programmes for toxic dinoflagellates are in place in most European countries where shellfish are harvested. Japan actively monitors both phytoplankton and shellfish and applies a tolerance level for DSP toxins of 5 MU/100g whole meat, when detected by the mouse bioassay method. This equates to approximately 0.2 µg/g. In the USA, there is no current monitoring programme or limit for DSP toxins in shellfish, although monitoring is carried out in Canada. Sources of further information Published Sobel J., & Painter J. Illnesses caused by marine biotoxins Clinical Infectious Diseases, 2005, 41(9), 1290-6 Australia New Zealand Food Authority Shellfish toxins in food: A toxicological review and risk assessment Technical Report Series, No. 14, 2001 On the web US Centers for Disease Control & Prevention (CDC) - Marine toxins factsheet http://www.cdc.gov/ncidod/dbmd/diseaseinfo/marinetoxins_g.htm FAO Food and Nutrition Paper 80 - Marine Biotoxins (2004) http://www.fao.org/docrep/007/y5486e/y5486e00.htm
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