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By Fiskeriforskning - For a long time, vibriosis was the most serious bacterial disease in farmed cod, but now other diseases also result in significant mortality.

Preventing Disease in Farmed Cod - By Fiskeriforskning - For a long time, vibriosis was the most serious bacterial disease in farmed cod, but now other diseases also result in significant mortality.

Extensive research is being done to prevent outbreaks of disease. Development of effective vaccines requires long-term research and is crucial for us to succeed with cod as a farmed species.

Effective vaccines are the most important safeguard for preventing disease and the spreading of infection and maintaining the use of antibiotics at a minimum level.

Improving vaccine against vibriosis

Classical vibriosishas been a problem ever since the first days of cod farming and is caused by several variants of the vibriosis bacteria Vibrio anguillarum. Today, the majority of Norwegian farmed cod are vaccinated with vibriosis vaccines developed for cod, which has considerably reduced the number of outbreaks.

In collaboration with the the Norwegian College of Fishery Science and vaccine manufacturer PHARMAQ, Fiskeriforskning has studied how early the cod juveniles can be vaccinated to achieve the best possible protection, and how long the protection lasts. As a result of this work, the manufacturer can now recommend an improved vaccine strategy.

Nevertheless, several outbreaks of classical vibriosis in vaccinated cod were reported in 2004/05. There can be several reasons for this, e.g. other isolates of the vibriosis bacteria than those that are included in the vaccines have been demonstrated in diseased, vaccinated fish.

"In collaboration with PHARMAQ, we are therefore studying whether the vibriosis vaccine can be improved by also including the new isolates. We are also assessing whether new formulations can provide long-term protection against vibriosis in the sea", says Scientist Vera Lund at Fiskeriforskning.

Three types of furunculosis bacteria

Atypical furunculosis was proved for the first time a few years ago in farmed cod in Norway. Since then, the disease has been reported in an increasing number of fish farms along the coast. Up to now, three different variants of the furunculosis bacteria atypical Aeromonas salmonicida have been registered in diseased cod, and one of the variants seems to be dominating in fish farms in Norway and Iceland.

Experimental tests have shown that the furunculosis vaccine that has been developed for salmon does not provide satisfactory protection for cod against the most common variant. "Knowledge about similarities and differences in the variants of the bacteria are therefore important with an eye to development of a vaccine against atypical furunculosis in cod", says Lund.

Viral diseases not yet a problem

According to our knowledge, viral diseases such as IPN (infectious pancreatic necrosis) and VNN (viral nerve necrosis) have not yet been reported in cod farming in Norway. The IPN virus has only been proved in adult wild cod without signs of the disease, but VNN outbreaks have already been reported in cod in Scotland, Canada and the USA, and are an increasing problem. We have therefore conducted a number of challenge experiments to study how great a risk viruses, which have been found in other marine species in Norway, can constitute for farmed cod in different stages of life", says Senior Scientist Ann-Inger Sommer at Fiskeriforskning.

Generally, the smallest cod larvae are most susceptible to outbreaks of disease caused by viruses. Challenge tests with the IPN virus from wild cod and farmed halibut in Norway resulted in disease and mortality in small cod larvae (0.3 grams), while slightly larger cod had a virus infection without mortality. However, the same IPN virus that in experimental challenge results in high mortality in post-smolt salmon, did not result in any registerable mortality in the smallest cod larvae, and seems therefore to be less dangerous for cod.

We have also shown that a nodavirus, which has caused VNN outbreaks in small halibut in Norway, has resulted in mortality in cod larvae when they were experimentally infected.

Challenge models

By studying the cod's susceptibility to viruses and bacteria, we have been able to develop suitable challenge models for some of the current diseases in cod. These are necessary tools to be able to study the effects of experimental vaccines, amongst other things.

The results have principally appeared in two joint venture projects between the Norwegian College of Fishery Science c/o Associate Professor Merete B. Schrder and Fiskeriforskning.

The projects are financed by Innovasjon Norge, Norges forskningsrd and PHARMAQ AS

Contact persons at Fiskeriforskning: Scientist Vera Lund and Senior Scientist Ann-Inger Sommer.

Source: Fiskeriforskning - March 2006

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