Distribution

Ciguatera can occur over a limited period of time and in a very restricted area in continental and insular reef regions between the latitudes of 35° north and 34° south, but it seems to expand now to the Eastern Atlantic (Canary Islands), Western Gulf of Mexico, Eastern Mediterranean (Israel) and elsewhere. The greatest concentration of epidemic-like outbreaks is in the Caribbean and Indo-Pacific. Ciguatera poisoning can also occur in all regions where reef fish from the above-mentioned zones are imported for human consumption.

Fig. 4.6 Body silhouettes of several typical reef fish that may temporarily become ciguatoxic.

a Acanthurus sp. (Surgeonfish)
b Balistoides sp. (Triggerfish)
c Scarus sp. (Parrotfish)
d Muraena sp. (Morays)
e Lutjanus sp. (Snapper)
f Scomberomorus sp. (Mackerel)
g Sphyrena sp. (Barracuda)
h Cephalophis sp. Grouper)
i Caranx sp. (Kingfish)

Biology

Besides scombroid poisoning, ciguatera is the most common cause of fish poisoning worldwide. The causative toxins are passed on via the food chain and can thus contaminate a large number of different animals within a reef population. Although the original description of this type of poisoning was traced to a Caribbean species of marine snail, named Cigua (Turbo pica), poisoning in humans occurs almost exclusively after the consumption of common edible marine fish. There are estimated to be up to 400 potentially ciguatoxic species (see also Table 4.1 in General information on poisonous animals.)

The organisms that actually produce the toxins, i.e. dinoflagellates of the species Gambierdiscus toxicus, and possibly also other species, are at the bottom of the food chain. They live on various macroalgae, which are eaten by herbivorous reef fish, such as Surgeonfishes (Acanthuridae) and Parrotfishes (Scaridae). Via these plant and debris feeders, toxins finally accumulate in predatory fish associated with the food chain of coastal regions. Towards the top of the food chain, the concentration of toxins increases, and thus larger and older predatory fish are the most dangerous for humans. Barracudas, Groupers, Snappers, Kingfishes and Mackerels are the most common causes of poisoning.

Ciguatera can occur suddenly and unexpectedly in a confined area. The reason for this is a drastic increase in dinoflagellates, together with so-called algal blooms (see General information on poisonous animals). There is increasing evidence that the factors that lead to a massive increase in dinoflagellates and thus toxin production are linked to destructive influences on coral reefs. Examples are hurricanes or heavy rainfall, but also human activities such as the construction of harbours, military campaigns, introduction of industrial sewage or water pollution. Attempts to explain these phenomena tend to argue that dead coral reefs offer a good substrate for fast-growing algae, which in turn attract epiphytic microorganisms such as Gambierdiscus toxicus. In addition, coral bleaching, which is being increasingly observed worldwide and is associated with global warming, would favour such a mass increase in unicellular organisms.

There is no such thing as classic ciguatera syndrome; on the contrary, it is a form of poisoning with vary variable symptoms. A number of toxins have been discovered to date that may cause ciguatera. These are heat-stable substances that are most strongly concentrated in the viscera (in particular the liver) of the fish and in the roe. Known toxins are the lipid-soluble ciguatoxin and scaritoxin, and the water-soluble maitotoxin and ciguaterin. Plant-eating fish contain more maitotoxin than ciguatoxin. Parrotfishes, which primarily feed on algae, smaller animals and coral polyps, accumulate scaritoxin in particular. Ciguatoxin is the primary toxin in omnivorous as well as carnivorous fish. In more recent times, palytoxin was determined to be the cause of several serious cases of poisoning due to Decapterus macrosoma (Scombridae), Ypsiscarus ovifrons (Scaridae), Melichthys vidua (Balistidae) and the crab Demania reynaudii. Palytoxin, which was first isolated from a soft coral (Palythoa sp.), can also enter the food chain, and is considerably more toxic than the above-mentioned toxins.

Risk

Ciguatoxic fish are not recognisable as such by means of external features. As the concentration of toxin is highest in the viscera, the consumption of whole, ungutted fish generally has the most severe consequences. Frying, boiling, deep-freezing or smoking the fish do not decontaminate it.

Feeding samples of fish to cats, chickens and mongeese is an old and questionable method of testing whether fish are ciguatoxic. In more recent times, various immunological laboratory tests have been developed, such as the "Hokama enzyme immunoassay stick test" (Dr. Y. Hokama, Department of Pathology, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI 96822, USA). For this test, a stick with a special coating for the adsorption of ciguatoxin is inserted into the fish. Toxin adhering to the stick is then made visible by means of a colour reaction via several steps with the use of ciguatoxin antibodies. A newer, simplified form of this test is the "Solid-phase immunobead assay", which can also be utilised on the spot by laypersons.

Ciguatera outbreaks are difficult to predict, and cases of poisoning often occur in the form of an epidemic. In more recent times, the annual number of cases has been estimated to be 50,000–100,000, with a mortality rate of under 1% (Russell and Egen 1991, Fleming et al. 2001). The last estimates on the annual global incidence of ciguatera poisoning range from 25,000 to 500,000 (Dickey and Plakas 2010).

Regular outbreaks are reported in the Caribbean, including Florida, the South Pacific Islands and Hawaii in the North Pacific, as well as the northeast coast of Australia (Queensland, Northern Territory). In the Pacific, the severity and incidence of poisoning increase from west to east.

Gambierdiscus species seem to expand into new areas. In the last years they have been detected in the Mediterranean (Crete island), at Brazil, Viet Nam and Hongkong. This expansion is thought to be the result of global warming and increasing nutrient overload.

Due to tourism and the increased export of tropical fish throughout the world, more and more doctors outside endemic regions are encountering cases of ciguatera poisoning.

Literature (biological)

Halstead 1967, 1988, 2001a, Dickey and Plakas 2010, Empey Campora et al. 2008, Fleming et al. 2001, Gillespie 1987, Hallegraeff 2006, Helfman et al. 2001, Hokama and Yoshikawa-Ebesu 2001, Ruff 1989, Russell and Egen 1991, Withers 1988, Bentur and Spanier 2007