temperature, light and nutrients are appropriate. Gonyaulax catenella
andG. tamarensis(now both referred to the genusAlexandrium) are the
best known of a number of dinoflagellates responsible for paralytic
shellfish poisoning.
This can be a serious illness with a high mortality rate. The toxic
metabolites of these algae, which include saxitoxin and gonyautoxin
(Figure 8.4) block nerve transmission causing symptoms such as tingling
and numbness of the fingertips and lips, giddiness and staggering,
incoherent speech and respiratory paralysis. Saxitoxin, so named because
it was isolated directly from the Alaskan butter clam, Saxidomas
giganteus, before it was recognized that it is actually a dinoflagellate
metabolite, is a very toxic compound having a lethal dose for the mouse
of as little as 0.2mg. Recent studies have even thrown doubt on whether
saxitoxin is, in fact, a dinoflagellate metabolite for it has been shown to
be produced by a species of bacteria of the genusMoraxellaisolated
from the dinoflagellates.
To control paralytic shellfish poisoning, the collection and sale of
bivalves in areas affected by algal blooms is banned.
Neurotoxic shellfish poisoning is less severe and less common than
paralytic shellfish poisoning and is associated with the brevitoxins, which
are complex cyclic polyethers produced by the dinoflagellatePtychodis-
cus brevis(¼Gymnodinium breve) one of the species responsible for the
phenomenon known as a ‘red tide’.
A third quite different form of poisoning associated with eating
shellfish which have accumulated dinoflagellates is diarrhoeal shellfish
poisoning caused by lipophilic toxins such as dinophysistoxin produced
byDinophysis fortii(Figure 8.5). The major symptoms, which usually
occur within an hour or two of consuming toxic shellfish, include
diarrhoea, abdominal pain, nausea and vomiting and may persist for
several days.
Figure 8.4 Toxic metabolites of algae (a) saxitoxin and (b) gonyautoxin 2
278 Non-bacterial Agents of Foodborne Illness