|
Introduction
Red tide is a
seasonal natural phenomenon caused by explosive population growth of
planktonic algae, imparting a red tinge to the surface of the water. The
name is a misnomer because it does not associated with tides. The
scientific community prefers the term "Harmful Algal Bloom "
(HAB). Ever since the first reported cases of HAB detected in the shore of
Sabah in 1976, this situation is constantly monitored by the Department of
Fisheries of Sabah.
The problem with Algae
Algae are basically a
single-celled plants that live in the sea and they serve as the energy
producers at the base of the food chain. Of the three hundreds species
that are known to cause bloom, only one quarter are capable of producing
toxins. As they are consumed by other higher marine life forms, these
toxins may accumulate in their organs, particularly among the
bivalves (e.g., clams, oysters, mussels, scallops). As a result they
become contaminated and the toxin is concentrated as it moves up the food
chain. Fish and shellfish can tolerate high levels of toxins, thereby
appearing healthy while posing a significant danger to man. The toxin
unfortunately cannot be detected by sight, smell, or taste and is not
destroyed by cooking or freezing. Thus man becomes an unsuspecting victim.
Health problem related to
HAB
To date there are 5
distinct clinical syndromes known caused by contaminated seafood. They are
mainly named according to
the main clinical toxic effects produced by the different toxins
in the human body. However, sources and symptoms may be similar among
these poisonings making
them difficult to diagnose. These intoxications can vary in
severity from mild to fatal depending on the type and amount of toxin
ingested.
 |
Amnesic
shellfish poisoning |
|
Ciguatera
fish poisoning |
|
Diarrhoetic
shellfish poisoning |
|
Neurotoxic
shellfish poisoning |
|
Paralytic
shellfish poisoning |
Paralytic shellfish
poisoning (PSP)
This has so far been
the most important type of HAB in Sabah and her surrounding regions.
Several dinoflagellates species are responsible
for PSP, notably Pyrodinium bahamense var compressum. About 20
toxins, all derivatives of saxitoxins, can cause PSP. Saxitoxin
blocks neuromuscular
transmission in the motor
axon and muscle membrane while leaving the end-plate unaffected; it
also inhibits the respiratory
centre. PSP is a potentially life-threatening condition. Symptoms
of the disease develop fairly
rapidly, within 0.5 to 2 hours after ingestion
of the shellfish, depending on the amount of toxin consumed. The effect
is purely neurological and includes tingling, numbness and burning of
the perioral region, giddiness, ataxia (motion imbalance), drowsiness,
fever and rash. In severe
cases respiratory paralysis is common, and death may occur if respiratory
support is not provided. There is no known antidote for the toxin.
Supportive measures including ventilatory support is needed. If treated
early, recovery is usually complete, with no permanent side effects.
In unusual cases, because of the weak hypotensive action of the toxin,
death may occur from cardiovascular collapse despite respiratory support.
Amnesic shellfish poisoning
(ASP)
ASP is also a potentially
lethal condition caused by the
toxic domoic acid, which are synthesised by pseudo-nitzscia species.
This condition is commonly
reported in Canada, but the organism has been found
in many parts of the world. It is characterized
by 2 phases: gastrointestinal disorders (nausea, vomiting, diarrhoea,
abdominal pain) which present within 24 hours; and later, within
48 hours, neurological problems (confusion, memory loss, disorientation,
seizure, coma). The poisoning is particularly serious
in elderly patients, and includes symptoms resembling Alzheimer's disease.
All fatalities to date have involved elderly patients.
Ciguatera fish poisoning
(CFP)
This poisoning,
transmitted by several tropical reef fish, is potentially lethal,
although the symptoms are less
debilitating compared to PCP and ASP. It is caused
by Gambierdiscus toxicus,
Ostreopsis sp and Prorocentrum sp.
Ciguatera produces gastrointestinal,
neurological and cardiovascular disturbances. It usually starts
with vomiting, abdominal pain and diarrhoea. Neurological symptoms follows
with altered temperature sensation, muscles aches, anxiety, dizziness,
sweating and numbness
of mouth and digit. Treatment is generally supportive as there is no
antidote. Recovery often takes
months or even years.
Diarrhoetic shellfish
poisoning (DSP)
This is a relatively
benign non-fatal poisoning characterised by gastrointestinal
disturbances with diarrhea,
vomiting, and abdominal cramps within 0.5 to 3 hours
following ingestion of the contaminated seafood. The patients usually
recover within a few days
with or without medical treatment. It
is usually mistaken as ordinary stomach ailment so many cases will go
unreported. The
causative dinoflagellate are dinoflagellate Dinophysis acuta,
Dinophysis acuminata,
Dinophysis fortii, Dinophysis mitra and Prorocentrum lima. DSP
is presumably caused by a group of high molecular weight polyethers,
including okadaic acid, the
dinophysis toxins, the pectenotoxins and yessotoxin.
Chronic exposure to DSP is
suspected to promote tumour formation
in the digestive system.
Neurotoxic shellfish
poisoning (NSP)
NSP produces a
clinical picture almost similar to CFP with gastrointestinal and
neurological involvement. Unlike CFP, no fatality
related to NSP have been reported so far. The toxicity is due to
brevetoxin, a group of
polyethers produced by dinoflagellate Gymnodinium breve, Gimnodinium cf.
Breve (New Zealand). It present within
a few minutes to a few hours after ingestion of contaminated source and
the duration of illness is fairly short, ranging from few hours to several
days. Recovery is complete without any significant sequalea.
Asthma-like symptoms caused
by toxic aerosols formed by wave action have also
been reported.
Diagnosis
Diagnosis of
shellfish poisoning is based entirely on observed symptomatology and
recent dietary history. If you
suspect that you have shellfish poisoning go
to the nearest healthcare providers and volunteer the dietary information.
Further information
Red
tide monitoring program by the Department of Fisheries of Sabah
First
aids for paralytic shellfish poisoning
The Harmful Algae Page
The
IOC harmful algal bloom programme
US
FDA's "bad bug book"
|
king
for
