The conditions best suited for toxic dinoflagellate growth are not well
understood. There have been a number of conflicting observations concerning
optimal growth conditions, some of which are as follows:
G. toxicus is negatively influenced by land runoff and high light
intensity (Yasumoto et al., 1980 as cited in Carlson and Tindall, 1985).
Dinoflagellates replicate rapidly when disturbed, such as, after major
storms, and areas of construction or dredging (Craig, 1980).
Highly toxic sites seem to be toward the leeward side of islands.
Populations of dinoflagellates are affected by rainfall in varying
degrees. Moderate rainfall may promote dinoflagellate growth by increasing
dissolved nutrients in the water, through increased terrestrial runoff.
However, heavy rainfall may inhibit growth by increasing dilution and/or
turbidity (Carlson and Tindall, 1985).
G. toxicus may specifically associate with macroalgae where high
concentrations of nutrients are available for growth (Carlson and Tindall,
1985).
There are at least four known toxins which appear to be
concentrated in the viscera, head or central nervous system of affected fish
(Tosteson et al., 1988): ciguatoxin, scaritoxin, maitotoxin and ciguaterin.
Ciguatoxin, the principal toxin, is lipid soluble (Kantha, 1987). Studies
indicate that oral intake of as little as 0.1ug (11MU) of ciguatoxin could cause
illness in an adult human (Yasumoto, 1985). Maitotoxin is water soluble and
approximately three times more toxic than ciguatoxin (Yasumoto, 1985).
Contaminated Species
Tropical and subtropical coral reef fish can become ciguatoxic. The incidence
of poisonous fish, however, is sporadic. All fish of the same variety and caught
in the same area may not necessarily be toxic (Hokama et al., 1983). A study
done in Hawaii indicated that if fish in one location are toxic, other fish in
the vicinity are approximately 60% likely to be toxic. Both herbivorous and
carnivorous fish can become toxic. Herbivorous fish become toxic by eating the
toxic algae itself. Carnivorous fish become toxic by consuming toxic herbivorous
fish. Generally, large fish are more poisonous than small fish because they
consume greater amounts of the toxins (Craig, 1980). The fish most often
implicated in cases of ciguatera include: barracuda (Olson et al., 1984 as cited
in Tosteson et al., 1988); grouper (Craig, 1980; Lawrence et al., 1980); snapper
(Craig, 1980; Lawrence et al., 1980); surgeon fish (Miyahara et al., 1987);
jack (Craig, 1980; Hokama et al., 1983; Miyahara et al., 1987); and parrot fish
(Bryan, 1986; Bryan, 1988).
Geographic Area
Ciguatera is found world-wide in fish between 35N and 34S latitude (Craig,
1980). It is a problem in the South Pacific, Japan Islands, U.S. and Bahamas.
The only areas of the U.S. affected by ciguatera are: Florida, Hawaii, Puerto
Rico and the U.S. Virgin Islands. There is evidence that ciguatoxic
dinoflagellate populations experience seasonal fluctuations. In Hawaii ( ) and
the Virgin Islands (Carlson and Tindall, 1985), the algae exhibits a bimodal
pattern of abundance with population maxima occurring in conjunction with the
peak periods of rainfall; April to May, and August to October. Studies in Puerto
Rico indicate that populations of Ostreopsislenticularis and
Gambierdiscus toxicus experience a seasonal trend, although densities are
highly variable (Ballantine et al., 1988). Peak populations tend to occur during
the late summer and fall, and do not appear to be correlated to rainfall.
Although additional data are needed, there appears to be a seasonal fluctuation
in the toxicity of Ostreopsis as well. In a three year period in Puerto
Rico, toxicity of Ostreopsis ranged from nontoxic to 182 MU/1,000,000
cells. In two of the three years, peak toxicities occurred in October.
Symptoms & Treatment
Ciguatera exhibits both gastrointestinal and neurological symptoms (Lawrence
et al., 1980). The time of onset is usually less than 24 hours. Gastrointestinal
symptoms, which usually persist for 12 hours (range < 1 hour - 7 days),
include: diarrhea, abdominal pain, nausea and vomiting. The most common
neurological symptoms include: paresthesia (abnormal or impaired skin
sensations), vertigo, ataxia (lack of muscle coordination), cold-to-hot sensory
reversal, myalgia (muscular pain), itching (especially during any activity that
increases skin temperature and blood flow). Neurological symptoms may recur
intermittently with gradually diminishing severity for a long as six months. No
deaths have been reported from ciguatera in the U.S. (Morris, 1980), although
world-wide the mortality rate of ciguatera is 7-20% (Craig, 1980).
Ciguatera from consumption of herbivorous fish has reportedly been associated
with more severe gastrointestinal complaints, whereas neurological and
cardiovascular effects often predominate in poisoning by carnivores (Bagnis,
1968 as cited in Miyahara et al., 1987). This observation was supported in a
study which demonstrated that different species of ciguatoxic fish accumulate
different toxins (Miyahara et al., 1987).
Statistics
World-wide, there may be as many as 50,000 cases of ciguatera per year
(Ragelis, 1984). In the U.S., between 1970 and 1980, 94 outbreaks (418 cases) of
ciguatera were reported to the CDC, making it the most frequently reported
food-borne illness associated with consumption of seafood (Morris, 1980).
Detection & Prevention
MOUSE BIOASSAY (Kimura et al., 1982) - For lack of a better technique,
the mouse bioassay is currently the laboratory method used to detect ciguatera.
Concentrated lipid extracts of fish tissue are injected intraperitoneally into a
20 g mouse and the mouse is observed for toxic symptoms for 24 - 48 hours.
Listed below are a number of general disadvantages of the mouse bioassay to
detect marine toxins:
Need to maintain a mouse colony and have 19 - 22 g mice always available,
limit of sensitivity dependent on mouse strain,
the onset of toxic symptoms is subjective,
high incidence of false positives due to other contaminants,
assay is not linear,
labor intensive and expensive,
cannot be used in the field,
the use of mammals in experiments is becoming controversial with the
public,
poor sensitivity,
A number of other laboratory methods have been
suggested as a replacement for the mouse bioassay and are described below: MOSQUITO BIOASSAY (Chungue et al., 1984) - Toxins are extracted from
fish and injected intrathoracically into mosquitoes. The mosquitoes are observed
for one hour for signs of death. This technique requires only a small amount of
fish tissue and results can be obtained within 2 hours. IN VITRO GUINEA PIG ATRIUM ASSAY (Miyahara et al., 1979 as cited in
Kimura et al., 1982) - Crude lipid extracts of fish are added to an isolated
guinea pig atrium. The effects are expressed as the ratio of the amplitude of
contraction occurring after the addition of extract as compared to the initial
amplitude of untreated atrium. "STICK" TEST (Hokoma et al., 1985; Hokoma et al., 1987a; Hokoma et
al., 1987b; Hokoma et al., 1989) - Bamboo sticks, pretreated to help adsorb the
toxin, are stuck into the fish flesh for 1 second. The sticks are then fixed
with methyl alcohol and immersed in a solution of blue latex beads and
ciguatoxin antibody. A positive result will change the bamboo stick to a
dark-blue or purple color within 10 minutes. This procedure does not require
extraction of tissue (although it is capable of testing extracted tissue), gives
rapid results, and is inexpensive. RADIOIMMUNOASSAY (Hokama et al., 1977; Kimura et al., 1982) - Sheep
anti-ciguatoxin serum coupled with iodine-125 is added to a sample of fish
tissue extract. Excess antibody is removed and the samples are analyzed with a
scintillation counter. If ciguatoxin is present in the fish flesh, the DPM will
be high. If the fish is free of toxin the DPM will be low. This procedure is
sensitive and relatively specific, however, it is economically unfeasible for
testing fish weighing <9 kg (Hokama et al., 1983). ENZYME-IMMUNOASSAY (Hokama et al., 1983) - Sheep anti-ciguatoxin serum
coupled to horseradish peroxidase is added to a sample of fish tissue extract
and incubated at room temperature for 1 hour. The amount of toxin in the tissue
is determined by measuring the absorbance at 405 nm.
Ciguatera toxins impart no unusual tastes, odor or color to the fish (Craig,
1980) and ciguateric fish cannot be made safe to eat by cooking, freezing,
drying or smoking (Tosteson et al., 1988). Listed below are methods which have
been suggested to avoid ciguatera:
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