Food Safety Focus (130^th Issue, May 2017 ) – Incident in Focus

Ciguatoxins and Coral Reef Fish

Reported by Mr. Johnny CHU, Scientific Officer,
Risk Assessment Section,
Centre for Food Safety

In late March/early April 2017, the Centre for Food Safety was notified of six suspected ciguatera fish poisoning (CFP) cases involving 11 persons. Investigations showed that they had consumed fish supplied by the same importer. This article describes the occurrence of CFP and provides advice to the public and the trade on the prevention of CFP.

Ciguatera Fish Poisoning

CFP is one of the most common types of marine food-borne poisoning worldwide; the Food and Agriculture Organization (FAO) estimated that 10 000 to 50 000 people worldwide annually suffer from this illness. CFP is caused by the consumption of coral reef fish that have become toxic from feeding directly or indirectly on toxic microalgae (Gambierdiscus toxicus) (Figure 1). The microalgae produce toxins known as ciguatoxins and the occurrence of these microalgae are mainly confined to specific patches of oceans in the tropics or subtropics.

Figure 1. Ciguatoxins produced by toxic single-celled algae and accumulated up the food chain.

There are some important features relating to CFP:

• The toxicity of coral reef fish depends on where the fish are caught. For CFP cases in Hong Kong, the most frequently identified at-risk contaminated fishing grounds are the Nansha Island, South Pacific and Hainan Island;
• Certain coral reef fish are more likely to cause CFP (e.g. Black fin red snapper, Tiger grouper, Lyretail, Leopard coral grouper, Areolated coral grouper and Moray eel);
• Some studies showed that the risk of CFP from farmed fish is extremely low;
• Depuration is not effective because contaminated fish tissue can remain toxic for years;
• Ciguatoxic fish cannot be identified by appearance, smell, or taste;
• Larger fish tend to accumulate more toxins. The fish involved in CFP is usually greater than 2 kg (3 catties);
• Ciguatoxins are up to 50 to 100 times more concentrated in the head, viscera, liver and gonads of affected fish;
• Ciguatoxins are heat stable and remain toxic after cooking.

Climate Change, Globalisation of Trade and Ciguatera

In recent years the toxic algae have been found spreading into new areas. In fact, evidence is accumulating that coral reef disturbance by climate change (such as increased water temperature and frequency of storms and hurricanes) impacts on the distribution and proliferation of the toxic algae and makes the occurrence of CFP less predictable. Besides, CFP is likely to increase with expanding international fish trade.

Public Health Significance

According to FAO, ciguatoxins, when present at concentrations of about 0.1 μg/kg or greater in fish will cause human poisoning. The symptoms of CFP include gastrointestinal, neurological and cardiovascular disturbances and often appear within 30 minutes and up to 24 hours after ingestion of the toxic fish. All humans are susceptible to ciguatera toxins. Sensitivity of an individual to CFP could be increased due to factors like previous CFP episode, and consumption of alcoholic beverages, nuts and seed products.

Regulatory Control

Codex is currently considering establishment of maximum limits for ciguatoxins and the possibility of development of risk management guidelines. There is no specific regulation on ciguatoxins in fish stipulated in the laws of Hong Kong. Nevertheless, all foods for sale in Hong Kong must be fit for human consumption.

Actions Taken

Upon notification of the food poisoning cases, the retailers/importer were instructed to stop selling coral reef fish of the same origin.

Preventive Measures

The trade should avoid importing fish from areas known to have a high chance of carrying ciguatoxins, or coral reef fish from unknown or suspicious sources. To allow prompt and effective control measures in case of the occurrence of CFP cases, the traders such as importers and retailers are required to keep proper and accurate transaction records.

Advice to the Trade

• Avoid sourcing fishes from areas known to be at risk of ciguatoxins.
• Adhere strictly to the Food Safety Ordinance in record keeping.

Advice to the Public

• Consume less coral reef fish and avoid eating the head, liver, skin, and roe of coral reef fish.
• Avoid alcoholic beverages, peanuts, nuts or beans when consuming coral reef fish.
• Seek medical treatment immediately when symptoms of CFP appear.