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Delousing Medicine Found to Have Only Minor Effects on Cod

17 February 2014

NORWAY - A new study by NIFES has found that the delousing agent diflubenzuron is not damaging to Cod.

“We are aware that the use of diflubenzuron has increased, and that this substance can be distributed in the environment, so we wanted to study its effects on fish species that we know feed near sea-cages; in this case, cod,” said NIFES scientist Bjørn Tore Lunestad.

The study, which was based on cod kept in tanks, showed that diflubenzuron will be absorbed by cod that consume feed containing this agent. The trials also showed that it is gradually excreted, and that after three weeks the concentration of diflubenzuron in the Cod was no longer detected.

The scientists gave the cod a standard treatment consisting of a daily dose of 3 mg active agent per kilo body weight for 14 days, which is similar to the recommended treatment of salmon with diflubenzuron in aquaculture.

Diflubenzuron inhibits the formation of the hard protective shell of the salmon louse, which is the most challenging parasite of farmed salmon. Diflubenzuron is often regarded as an environmental problem, because it can also have similar effects on the shells of other crustaceans that occupy the water around fish farms.

In the present cod study, the research team also looked at how diflubenzuron affected the expression of genes that are involved in liver cell detoxification, but found only minor effects.

“This suggests that diflubenzuron, in the doses that we have studied, should have only minor effects on cod,” said Mr Lunestad.

Large variations in the concentration of diflubenzuron could be seen in the analysed samples, and parallel trials conducted at NIFES further showed that the concentration in the filet was lower than what was found in samples of liver from the same fish. Typical maximum concentrations in cod liver were approximately 0.2 milligram/kg, whereas the corresponding concentration in fillet was around 0.04 milligram/kg sample.

According to EU regulations, the acceptable daily intake of diflubenzuron via food has been set to 0.0124 milligram/kg body weight. To exceed this limit a person of 60 kg, would have to eat 18.5 kg cod fillet, or 3.7 kg cod liver having the typical maximum concentrations found in this study. This would be provided that the cod were the only source for diflubenzuron. This agent is also used for agricultural purposes as an insecticide, and it is likely that vegetable products would also be a source for diflubenzuron exposure in humans.

After diflubenzuron has been used on farmed salmon, the fish must not be slaughtered until 105 degree-days (water temperature multiplied by days) have passed since the last day of treatment. Diflubenzuron has never been detected in the random samples of Norwegian farmed salmon collected every year by NIFES for the European Union and the Norwegian Food Safety Authority.

This research activity has been conducted in cooperation with The Institute of marine research, Bergen (IMR), and financed by NIFES and IMR.

The study is published in Aquatic Organisms and Diseases.

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