Eradication of VHS in Denmark

​It was realised that more stringent methods requiring investments beyond the capacity of the industry had to be implemented to obtain total eradication of VHS. Therefore, the DVFA decided in collaboration with the Danish aquaculture industry to apply the European Fisheries Fund (EFF) for co-financing a project, which would hopefully finally eradicate the disease in Denmark.

 

The EU approved the Danish programme for the eradication of VHS and granted co-financing with reference to Article 16 of Commission Decision 2008/897/EC.

 

Former eradication strategy
The original eradication strategy for VHS was a stepwise eradication starting with the fish farm along the river closest to the source and subsequently the other farms downstream. The fish farms involved in the eradication programme were fallowed for a period of at least six weeks in April and May when the risk of spread of the infection from the feral fish stock was the highest. Prior to the fallow period, the farms were emptied of fish, dried out and disinfected. After the fallow period, the farms were restocked with fish from farms registered as free from VHS. After a few years, eradication of VHS  was the normal time frame for fish farms situated along small river systems, whereas it could take several years to successfully eradicate the disease in large river systems, due to the presence of large populations of potential infected feral fish.

All costs associated with the eradication programme were covered by the individual fish farmer, and the farmers, therefore, tried to minimise costs by feeding as much of the fish stock as possible to marketable size and selling the remaining part to farms having specialised in the summer production of VHS-infected fish. This production was situated in brackish water near the mouth of one of the biggest Danish river systems – Skjern River – which is intensively used for fish farming.

The postponement of the fallowing to reduce costs posed a high risk for feral fish; especially ascending salmonids might be infected and become carriers of the infection. Furthermore, the summer production based on VHS-infected fish also contributed to maintaining the infection in the environment.

New eradication strategy
Complete eradication of VHS would need another approach implying that all production based on VHS-infected fish should come to an end, and that the affected fish stock should be culled immediately after the detection of VHS. These two measures should effectively reduce the risk of maintaining chronically infected feral fish in the river system.

Such a change in strategy would, however, require a shift from voluntary to compulsory disease eradication and imply a need for compensating the affected fish farmers. The funding from the EFF made this change possible.

The final eradication of VHS has been fully endorsed by the fish farmers and their organisation. In order to demonstrate their great interest in the project and to reduce costs, the fish farmers have agreed that compensation will only cover the value of the fish and costs for removal of the fish. The farmers bear all costs for cleaning and disinfecting the fish farms and the loss of income during the fallowing period.

Risk-mitigating measures
The brackish water fish farms producing fish on the basis of discarded undersize fish from fresh water farms fallowed due to VHS eradication have been closed since 1 April 2009 for a period of two years. This decision was based on the risk following from the location of these farms that any VHS outbreak in these farms would endanger the whole project as ascending salmonids would have to pass virus-infected waters before entering the river system where the VHS eradication programme was being carried out. Based on a risk assessment, approximately 20 fish farms regarded as high-risk farms for contracting VHS infection in the spring period would be fallowed for at least six weeks following 1 April 2009. As an additional risk mitigating procedure, the river system feeding the high-risk fish farms was electro-fished to remove any escaped rainbow trouts, which are the main source of the VHS infection.

Work conducted in 2009 and 2010
Electro-fishing was conducted in February and March 2009 prior to the fallowing of the fish farms. In January 2010, electro-fishing was also conducted.

The brackish water farms were fallowed, and the fallowing was approved by DVFA inspectors. The fallow period of these brackish water farms will last two years. The risk assessment for 2009 concluded that 18 freshwater fish farms should be fallowed for a period of at least six weeks. The risk assessment for 2010 concluded that 13 farms should be fallowed for a period of at least six weeks, and the fallowing was approved by DVFA inspectors.

During the autumn period, an intensified inspection scheme was carried out on the farms in the eradication area to ensure that there were no signs of VHS and that the appropriate biosecurity measures (removal of dead fish and disinfection procedures) were maintained. It was important to carry out the inspections during the period when the water temperature decreased​ as VHS outbreaks are mainly observed at water temperatures below 12°C. At the inspections, the official veterinarians collected samples for laboratory testing. The inspection and monitoring activities were not funded by the project but under the governmental co-financing scheme.