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If you're travelling to tropical waters of the Pacific and Indian Oceans or the Caribbean Sea you need to be aware that certain types of fish and shellfish may contain biotoxins that are dangerous to your health.

The Center for Disease Control (CDC) says poisoning from fish toxins "is an under recognised hazard for travellers". As the recent tragic deaths of Queenslanders Noelene Bischoff and her daughter Yvana in Bali seem to indicate, the ramifications can be extreme.

There are 3 main types of fish toxin poisoning:

  • Ciguatera poisoning
  • Scombroid poisoning
  • Shellfish posioning

CIGUATERA POISONING

Around 50,000 cases of ciguatera poisoning are reported to the CDC each year. It is particularly common in reef fish but occurs in more than 400 species of fish, including:

  • gropers
  • barracudas
  • moray eel
  • sturgeon
  • sea bass
  • red snapper
  • amberjack
  • mackerel
  • parrot fish
  • surgeonfish
  • triggerfish

How ciguatera poisoning occurs

Smaller herbivorous fish eat algae containing the toxin from coral. The smaller herbivores are eaten by the larger carnivorous fish and the toxins accumulate in them (especially in the liver, intestines, roe and head of the fish).

The toxin does not alter the texture, taste or smell of the fish and is not destroyed by cooking, freezing, pickling or canning.

Symptoms of ciguatera poisoning

These typical symptoms can appear within a few minutes and up to six hours after consumption:

  • Nausea
  • Vomiting
  • diarrhoea
  • Headache
  • Numbness and tingling about the mouth and extremities

In severe cases; muscle pains, dizziness, irregular heartbeat and low blood pressure may occur. Some victims may feel a bizarre sensation where hot things seem cold and cold things seem hot. 

Treatment for ciguatera poisoning

There's no "antidote" and the only treatment is to relive the symptoms which typically last several days, but may persist for up to four weeks. Ciguatera poisoning may resemble other medical conditions. Always consult your doctor for a diagnosis.

Preventing ciguatera poisoning

  • The CDC recommends never eating moray eel or barracuda.
  • Avoid or limit consumption of the reef fish.
  • Avoid the parts of the fish that concentrate ciguatera toxin: liver, intestines, roe, and head.

SCOMBROID POISONING

This occurs after eating fish that has been subject to improper food handling, especially lack of refrigeration. 

How scombroid poisoning occurs

Certain species of fish have  high levels of a naturally occurring substance called histidine. When subjected to temperatures above 16ºC the histidine converts to histamine - which is why scombroid poisoning is often mistaken for an allergic reaction.

Fish which contain histidine include:

  • mahi mahi (dolphin fish)
  • albacore tuna
  • bluefin and yellowfin tuna
  • bluefish
  • mackerel
  • sardines
  • anchovy
  • herring
  • marlin
  • amberjack
  • abalone

This form of fish poisoning occurs worldwide in temperate and tropical waters. Cooking, smoking, canning, or freezing will not destroy histamine in contaminated fish.

Symptoms of scombroid poisoning

Symptoms generally appear within minutes to an hour after eating affected fish. They typically last three hours, but can last several days.

  • Tingling or burning sensations in the mouth
  • Rash on the face and upper body
  • Drop in blood pressure
  • Throbbing headache
  • Hives and itchiness
  • Nausea
  • Vomiting
  • Diarrhoea

The symptoms may resemble other medical conditions. Always consult your doctor for a diagnosis.

Treatment for scombroid poisoning

Administering antihistamines such as diphenhydramine (Benadryl) will relieve symptoms. However, treatment is usually unnecessary because symptoms rarely last longer than 12 hours.

Preventing scombroid poisoning

  • Avoid fish with a peppery, sharp, salty, or “bubbly” feel but be aware, contaminated fish may also look, smell, and taste normal. 
  • Make sure that the fish has been properly iced, refrigerated, or frozen after it is caught. 

SHELLFISH POISONING

Several forms of shellfish poisoning may occur after ingesting filter-feeding bivalve mollusks such as mussels, oysters, clams, scallops, and cockles. 

How shellfish poisoning occurs

The toxins originate in small marine organisms that are ingested and concentrated by shellfish.

Symptoms of shellfish poisoning

Symptoms usually appear 30–60 minutes after consumption including: numbness and tingling of the face, lips, tongue, arms, and legs. In some forms it presents as gastroenteritis with headache, nausea, vomiting, and diarrhoea.

Marine shellfish toxins cannot be destroyed by cooking or freezing.

Prevention of shellfish poisoning

Avoid eating shellfish from areas shortly after an algal bloom (red tide).

Travellers to developing countries should avoid eating all shellfish, because of the high risk of viral and bacterial infection.

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