A dissertation about viral haemorrhagic septicaemia virus surveillance in Finnish brackish water fish farms has been published in the series Finnish Food Authority Research Reports

November 19/2020

Head of Production and Companion Animal Pathology Section Pia Vennerström (DVM, Specialist in Veterinary Medicine, Infectious Animal Diseases) from the Finnish Food Authority will defend her doctoral dissertation entitled “Viral haemorrhagic septicaemia in Finnish brackish water fish farms – Studies on disease surveillance and epidemiology of viral haemorrhagic septicaemia virus” in the Faculty of Veterinary Medicine, University of Helsinki, on 19 November 2020 at 12:00. The public examination will take place at the following address: Biocenter 3, room 2402 (Viikinkaari 1, Helsinki). Professor Manfred Weidmann, University of Stirling, will serve as the opponent, and Professor Olli Vapalahti as the custos.

The dissertation has been published in the series Finnish Food Authority Research Reports 3/2020 (pdf).

Viral haemorrhagic septicaemia (VHS) was isolated for the first time in Finland in 2000 from a Finnish brackish water fish farm farming rainbow trout in net pens in the Province of Åland, Baltic Sea. The efforts to eradicate the disease from the Åland islands were not successful. Epidemical factors, needed for VHS management in viral haemorrhagic septicaemia virus (VHSV) positive brackish water fish farms, were studied in a 3-year project, the results of which are presented in this thesis. The study compared the ability of four different surveillance procedures and three diagnostic tests to reveal whether a fish population was infected with VHSV. The programme that was conducted as syndromic surveillance, where the farmers sent in samples for diagnostics if any signs of possible fish disease were noticed, clearly outperformed the other three programmes, which were based on active surveillance.

During the epidemics it was a common suspicion wild fish being the most likely source of the reinfections of VHSV. Wild fish of 17 different species from VHS-positive fish farms were screened for VHSV during 2005-2008. In addition, uninfected wild perch, roach and farmed whitefish were introduced to a fish farm with rainbow trout experiencing a clinical outbreak of VHS. The wild fish did not test positive on any occasion, but whitefish were infected and started to replicate VHSV for a short time.

The presence of VHSV in the environment on fish farms or processing plants farming or handling VHSV-positive fish was studied by testing samples for VHSV from wild blue mussels (Mytilus edulis) living in infected fish farms. Sea water and sediment from infected fish farms were also tested for VHSV. Wastewater from a processing plant was tested before and after disinfection treatment. Blue mussels were not found to be carriers of VHSV on any occasion. Sea water tested positive for VHSV RNA more often during the wintertime. Most wastewater samples collected before the disinfection treatment were positive for VHSV, but samples collected after disinfection were all negative regarding VHSV RNA. Contacts between the processing plants and the fish farms in the restriction area of VHS were very common during this study. Processing plants are usually the place where fish food and farming equipment are stored, including boats that are used for the daily servicing of the farming localities. According to the results of this study, this contact was considered a major risk for disease spread, especially during the cold part of the year when daylight is also short.

The study was funded by the Ministry of Agricultural and Forestry and performed in the National Veterinary and Food Research Institute (EELA), later the Finnish Food Safety Authority Evira, in the Research Department at the Pathology Research Unit and finalised in The Finnish Food authority, in the department of Laboratory and Research, Veterinary Bacteriology and Pathology Unit.

Further information

Pia Vennerström, Head of Production and Companion Animal Pathology Section, tel. +358 295 205 150 or +358 50 354 4603, pia.vennerstrom@foodauthority.fi