The consumption of animal foodstuffs can not only cause viral and parasitic infections as well as allergic reactions but can also lead to poisoning. Animals that possess toxins in their skin, muscles, organs or bodily fluids are grouped together as poisonous animals. In contrast to venomous animals they do not possess an apparatus for the injection of venom, such as venom fangs, venomous spines etc. Poisoning occurs after the animal has been consumed, when the toxins lead to functional or structural changes in the gastrointestinal tract and/or are absorbed from the gastrointestinal tract, are distributed throughout the body and become active systemically.

The great majority of poisonous animals live in the marine environment in tropical and subtropical coastal regions. From a taxonomic perspective, they form an extremely heterogeneous group, including the most varied invertebrates as well as various vertebrates. That said, the majority of medically significant cases of poisoning are caused by the consumption of shellfish and fish.

Worldwide, around 2,000 cases of poisoning due to the consumption of fish and shellfish are reported annually. Around 15% of these are believed to end fatally (Hallegraef 2006)! Estimates of the annual global incidence of ciguatera poisoning range from 25,000 to 500,000 (Dickey and Plakas 2010).

Origin of the toxins

In contrast to their great taxonomic diversity, the range of effects caused by poisonous animals is relatively small (see Clinical flowchart: Poisonous animals). It is often the case that many species carry chemically identical toxins or toxins with similar pathophysiological mechanisms. The reason for this is that only in a very few poisonous animals do the poisons originate from their own metabolism. Rather the toxins are often actually produced by bacteria or single-celled algae that can enter many different organisms within an ecosystem via the food chain. In the special case of scombroid poisoning, toxins (in particular histamine) are produced by bacterial decomposition of dead animals of the tuna family and other fish with histidine-rich flesh. The actual causes of shellfish poisoning and ciguatera (poisoning caused by tropical and subtropical reef fish), the epidemiologically most significant forms of poisoning due to poisonous animals, are dinoflagellates. These are single-celled, assimilating organisms that cannot be conclusively classified as either animals or plants. Various species of dinoflagellates are known to produce a number of highly toxic substances that lead to specific poisoning syndromes (Shellfish poisoning, Ciguatera). Dinoflagellates are planktonic organisms that float in the water and accumulate in filter-feeding shellfish or live epiphytically on macroalgae and thus find their way into plant-eating animals. Via the food chain, the toxins further accumulate in carnivorous fish that feed on toxic plant-eaters or filter-feeders and in turn are eaten by larger predatory fish. In this way, the toxins can find their way into entire ecological communities in offshore regions.

This process of accumulation is not so much a continuous course of events, but rather the result of sporadic and geographically confined phenomena, whereby, as a consequence of a sudden and unpredictable massive increase in the concentration of microalgae (so-called algal blooms, "red tides"), many otherwise edible marine animals become toxic. Functional classification of the poisonous animals must therefore be based on the specific types of poisoning rather than taxonomic criteria. Table 4.1 provides a summary of the different types of poisoning that can be caused by poisonous fish.

Table 4.1 Poisonous fish (arranged according to classes, orders and families) with regard to the possible types of poisoning they cause

1. Ciguatoxic
2. Scombrotoxic
3. Tetrodotoxic
4. Clupeotoxic
5. Gempylotoxic
6. Ichthyohemotoxic
7. Ichthyocrinotoxic
8. Ichthyohepatotoxic
   
9. Ichthyootoxic
10. Ichthyoallyeinotoxic

Prevention

Shellfish poisoning and ciguatoxic fish poisoning in particular usually occur as minor epidemics. It is important to remember that poisoning may not only occur in the places where these animals are caught, but also wherever they are imported. The increased supply of tropical fish to fish markets remote from the coast increases the risk of poisoning beyond the regions where it is endemic. Even tinned fish and shellfish can lead to poisoning. As a rule, frying, cooking or smoking the meat does not get rid of the toxins, and usually the toxic specimens are not noticeable as such on the basis of external features such as smell or taste. The pepper-like or metallic taste that sometimes occurs in poisonous tuna and mackerels or herrings and sardines is not always present as an indicator. When consuming fish and shellfish in warm to tropical coastal areas, it is important to pay attention to the advice of the local inhabitants.

Literature
Auerbach and Halstead 1989, Dickey and Plakas 2010, Estaugh and Shepherd 1989, Hallegraeff 2006, Halstead 1988, Hui et al. 2001, Mebs 1989b, Mills and Passmore 1988, Storch and Welsch 1997