Food Safety Focus (7th Issue, February 2007) – Incident in Focus
Oilfish Consumption and Oily Diarrhoea
Reported by Ms. Joey KWOK, Scientific Officer,
Risk Assessment Section, Centre for Food Safety
On 23 January 2007, the Centre for Food Safety (CFS) announced that cases of oily diarrhoea had been reported among members of the public after consumption of fish products labelled as "cod fish". The fish products in question were suspected to be oilfish marketed as cod fish or using names similar to cod fish. The CFS alerted members of the public and the trade to the situation and the possible health effects associated with the consumption of oilfish and related fish species which contain high levels of indigestible wax esters. As of 31 January 2007, the CFS received about 700 complaints and enquiries about the incident. Because of the resulting market confusion, the trade agreed to stop importing and selling oilfish for the time being.
Occurrence of Wax Esters in Fish
Oilfish (Ruvettus pretiosus) and escolar (Lepidocybium flavobrunneum) belong to the family of Gempylidae; they are two of the more common fish species known to contain high levels of indigestible wax esters. According to literature, these fish species contain approximately 20% by weight of wax esters. As these fish species do not metabolise wax esters that occur naturally in their diet, these wax esters are accumulated in the fish body, including the skin and muscle meat. Geographical and seasonal variation may affect the level of wax esters in the fish.
Illustration: Oilfish and escolar which contain high levels of wax esters: oilfish (Ruvettus pretiosus) (top); escolar (Lepidocybium flavobrunneum) (bottom)
Source of Illustration: Regulatory Fish Encyclopedia: http://www.cfsan.fda.gov/~frf/rfe0.html
Office of Seafood and Office of Regulatory Affairs, U.S. Food and Drug Administration, 1993-2006
Oilfish and escolar are widely distributed in tropical and temperate waters of the world. Internationally food authorities from countries including the United States, Australia, the United Kingdom, Germany and Sweden have issued advisories to remind consumers and the trade of the possible health effects associated with consumption of oilfish and related species. Only two countries, namely Japan and Italy, find it necessary to ban the import and sale of oilfish and escolar.
Identification of fish species may sometimes be difficult; a single fish species is sometimes known with more than one common or market names. Consumers and traders, therefore, should be aware of the scientific names of fish species to avoid misidentification and mislabelling of their products. The CFS has set up a working group comprising representatives from the trade and academic institutes as well as government officials to prepare guidelines on identification and labeling of oilfish and ecolar.
Health Significance of Wax Esters
In humans, wax esters are not absorbed by the gut. These wax esters can have a laxative effect on some consumers, and cause symptoms which range from mild and rapid passage of oily yellow or orange droplets to severe diarrhoea with nausea, vomiting and headache. Not all individuals who eat these fish species are affected. It seems that there is a variation in sensitivity among individuals, and some people seem to tolerate the fish and show no ill effect after consumption. Symptom onset time ranges from 30 minutes to 36 hours after ingestion. Recovery is expected within 24 to 48 hours in affected individuals. Preliminary review of 70 local food complaint cases of some 100 people with symptoms showed that almost all of them presented with oily diarrhoea and about one third with abdominal pain. Reported median latent period was 13 hours.
Wax esters are not broken down by cooking or freezing. There is no well-proven ways to reduce wax esters in the fish to guarantee a no-effect level, although certain cooking methods that separate a large proportion of the oil from the fish (such as grilling) coupled with discarding the cooking liquid may reduce the risk to some extent.
Cases of oily diarrhoea in association with consumption of oilfish and related fish species have been reported in overseas countries such as Australia, New Zealand and the United States. As the nature of symptoms is often mild and short-lived, it is possible that there is a degree of under-reporting.
Further information about the incident can be obtained from the following webpages: