Research seminar 8.12.2009: Importance of wild fish, blue mussels and farmed whitefish in the epidemiology of the VHS virus


<p>Viral haemorrhagic septicaemia, (hereafter VHS) is a fish disease caused by a virus that is a rhabdovirus. The VHS virus is found as four distinct genotypes (VHS GI-GIV). In Europe the VHS GI type is the most important as the fish species most sensitive to it is the rainbow trout, which is farmed extensively in Europe.</p>

Researcher Pia Vennerström, Fish and Wildlife Health Research Unit
Evira/Helsinki Viikki, Auditorium C111 Kalevi
Tue 8.12.2009, 3:00 – 4:00 pm

Viral haemorrhagic septicaemia 
VHS has been found to be the most significant viral disease on European trout farms and is classified in the EU as a list II disease, for which an official control programme has been set up (concerns all genotypes). Several other fish species have been found to be sensitive to the VHS GI virus types, e.g. pike, whitefish and Arctic char (or charr). The different genotypes of the VHS virus have been isolated from at least fifty different fish species worldwide. The disease has a direct effect on the trade of live fish within the EU area and even more extensively.

The background for the research project
In Finland, VHS disease was detected for the first time in 2000 on a fish farm on Åland and it is still causing significant financial losses for the infected fish farms there. VHS has also been detected in Pyhämaa and Pyhtää. In Pyhtää the disease has been eradicated. In Pyhämaa, the disease was not found for five years after the first case, but in the last control sample before the area was released, a VHS positive sample was detected. The VHS virus that has been detected in trout in Finland belongs to genotype I and is further identified as belonging to subtype Id. Finland is adhering to the disease eradication programme approved by the EU in the restriction zones set due to VHS, but the industry has not managed to eradicate the disease on Åland and in Pyhämaa. The reason for this failure has been unclear, and as a consequence Evira started a three-year research project in the spring of 2006, with the main goal being to clarify which factors may possibly affect this. The significance of wild fish, blue mussels and farmed whitefish as potential carriers of VHS was one of the factors that were to be clarified in the research project.

In the fish cage environment there is a significant wild fish population consisting of different species, most of which are local species which do not migrate long distances. These fish stay in the area, even if the cages are emptied and all of the fixtures are removed for the fallow time. It is quite likely that these wild fish will return to the farm when the cages with their new VHS free fish are put back after sanitation.

There is no researched information available on the importance of blue mussels as carriers of the VHS virus. The chains and ropes that anchor the frames of the fish cages are covered with a thick layer of blue mussels. It is generally impossible to remove these anchoring structures when the farming equipment is taken for sanitation. Blue mussels are also found in large numbers on the sea bed in the vicinity of the farm.

Whitefish is farmed widely on Åland along with rainbow trout. According to practical experience, whitefish appear to be resistant to the VHS virus. In the research project we wanted to clarify whether the virus in question is transmitted to whitefish and whether it susceptible to VHS disease.

How the research was carried out
The significance of wild fish, blue mussels and farmed whitefish as vectors of the disease was clarified by catching fish from the immediate vicinity of the fish farms that had been infected with VHS and by collecting mussels from the structures of the fish farms. The fish were caught with nets used in exploratory fisheries, which have four meshes of different sizes (10, 20, 30, 40 mm). 14 species of wild fish were caught, as well as blue mussels and rainbow trout that had escaped from fish farms. In addition, wild perch, roach and farmed whitefish were brought from outside the restriction zone for carrying out the exposure test in the field, which was carried out at a VHS positive fish farm. The mussels were exposed to the VHS virus in aquariums kept in a laboratory.

Results of the research
VHS was not detected in wild fish which had been caught in the immediate vicinity of VHS infected fish farms. Traces of the viral genotype were found in rainbow trout which had escaped from the farms, but it was not possible to isolate the virus.

The whitefish that had been used for the exposure test fell ill with clinical VHS disease about a week after the exposure had started, but after that the virus was not found in them.

In the exposure tests on blue mussels small numbers of the RNA of the VHS virus were found in live samples still 27 days after exposure, but the live virus was only found in the samples 6 hours after exposure and the RNA of the virus was no longer found in the water samples of the aquarium after two days.

Discussion of the results
The results of the study of wild fish do not completely preclude the possibility that wild fish would be carriers of the VHS virus. The results indicate, however, that the 14 species studied are not sensitive to the VHSV genotype Id. It is quite unlikely that they would have a significant role as a maintaining factor of the disease.

The rainbow trout caught were at the moment of catching clinically healthy and in good condition. The results indicate that the escaped rainbow trout may be significant vectors of the disease. Escaped schools of VHS positive rainbow trout has to be considered to be one of the most significant vectors of VHS and a factor that maintains it in the zone.

Whitefish farmed as a food fish in Finland may become infected by VHS and become clinically ill. Whitefish are however not particularly sensitive to the disease. Whitefish still has to be considered a significant carrier and vector of VHS when it is in contact with a VHS positive rainbow trout population. Farming of rainbow trout and whitefish should therefore be kept separate from one another.

The importance of blue mussels as a maintaining factor of VHS has to be considered to be insignificant.

The lecture is in Finnish.

For more information, please contact:
Researcher Pia Vennerström, Fish and Wildlife Health Research Unit
Tel. 050 354 4603, pia.vennerstrom


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