Distribution

1. Worldwide in coastal areas from cold to subtropical regions
2. Outbreaks known to date from the Gulf of Mexico and the east coast of Florida
3. Outbreaks known to date only from the east coast of Canada
4. Occurrence to date in coastal areas in Europe, Japan and Chile

Fig. 4.1 Bivalve molluscs commonly associated with shellfish poisoning. a Mytilus sp. (mussels); b Saxidomus sp. (clams).

Biology

Bivalve molluscs are filter-feeding animals, i.e. they take in tiny food particles and plankton organisms from the water they need to breathe. In this way, they also take in toxic substances, which accumulate primarily in the hepatopancreas ("liver"). Apart from heavy metals or chemicals, these organic toxins are also derived from single-cell phytoplankton algae, namely from certain dinoflagellates (Dinophyceae) and to a lesser extent also from diatoms (Diatomeae).

Due to various factors that are still not understood, localised algal blooms of limited duration occur in the form of an explosive increase in phytoplankton. This phenomenon is also known as "red tides", as the massive increase in algae may actually turn the water red. Of the 5,000 species of phytoplankton, 300 can proliferate to such an extent that they discolour the surface of the sea, producing these "red tides". About 80 species produce toxins that are potent enough to cause poisoning in humans. Following toxic algal blooms, there is a massive increase in algal toxins in the bivalve molluscs in the area, although the molluscs themselves are not affected. However, these toxins are very toxic for fish and can cause mass mortality. Algal blooms can also cause large losses in fish farms. Mass mortality of fish and invertebrates may also occur after non-toxic algal blooms if the algae are so dense that they consume all the oxygen in an area and thus cause the local aquatic fauna to asphyxiate.

In humans, toxins dissolved in sea spray can lead to irritation of the mucous membranes and airways, but by far the greatest danger is the consumption of poisonous shellfish. Popular edible molluscs such as blue mussels (Mytilus edulis) or clams (Saxidomus sp.) are well-known causes of poisoning but not the only causes. Significant outbreaks of PSP in the absence of red tides have also been described.

Algae that are associated with the various types of shellfish poisoning are the dinoflagellates Protogonyaulax tamarensis (= Gonyaulax tamarensis), Protogonyaulax catenella and Pyrodinium bahamense, among others (at least 12 different species), for PSP; Ptychodiscus brevis (= Gymnodinium breve) for NSP; Dinophysis acuminata and Dynophysis forti for DSP, and diatoms (Diatomeae) of the genus Nitzschia for ASP. The responsible toxins are the strongly toxic saxitoxin and its derivatives (gonyautoxins) in PSP, brevetoxins in NSP, okadaic acid, pectenotoxin and yessotoxin in DSP and domoic acid in ASP. Saxitoxin, gonyautoxins and brevetoxins are neurotoxins that specifically bind to sodium channels of excitable membranes.

Risk

In moderate and colder regions, the occurrence of shellfish poisoning is largely limited to the warm summer months -- hence the rule of thumb that shellfish should not be eaten during the months in which there is no "r" (May, June, July, August in the Northern hemisphere). In tropical and subtropical zones, there is no seasonal dependency. Shellfish poisoning usually occurs in the form of epidemics in these regions (see Distribution), but outbreaks in areas remote from the coast, where poisonous shellfish have been imported, are also possible.

In recent decades there has been a clear trend of an increase in the occurrence of algal blooms that cannot only be the result of more intensive monitoring. The frequency, intensity and duration of spontaneous mass increases in microalgae appear to be on the increase, and toxic algae are advancing into new regions. This is chiefly blamed on increasing pollution and eutrophication of the water. The spread of algal blooms can be ascribed to the dispersal of resting forms of algae (so-called resting cysts) in the ballast water of ships and infected mussel stocks in aquacultures. The increase in shellfish farming worldwide is leading to more reports of paralytic, diarrhoetic, neurotoxic and amnesic shellfish poisoning. Until 1970, cases of PSP had only been reported in the Northern hemisphere, but by 1990, PSP had spread to southern Africa, Australia, New Zealand, India, Thailand, Brunei, Sabah, the Philippines and Papua New Guinea. Since 1990, PSP has continued to spread. There was a clear correlation between the number of red tide outbreaks per year in Tolo Harbour between 1976 and 1986 and population growth in Hong Kong. The number of outbreaks increased steadily from 0 to 18 per year. In the same time the human population increased from <1 million to nearly 5 million (Hallegraeff 2006).

Clinically documented PSP outbreaks with several dozen to around 200 patients have been described in western Europe (McCollum 1968, Zwahlen et al. 1977), Taiwan (Cheng et al. 1991) and Guatemala (Rodrigue et al. 1990). Cases have also been described in Massachusetts and Alaska (MMWR 1991, Gessner and Schloss 1996).

An NSP outbreak with 48 documented cases in North Carolina was reported in 1987 (Morris et al. 1991).

Literature

Sakamoto et al. 1987, Auerbach and Halstead 1989, Fleming et al. 2001, Godan 1999, Hallegraeff 1998, 2006, Jaime 2003, Llewellyn 2001, Mebs 1991b, 1992