Shedding of Viral Haemorrhagic Septicaemia Virus (VHSV) from Rainbow Trout, Oncorhynchus mykiss, and Quantification in Waste from Processing Preclinical Fish

Viral haemorrhagic septicaemia virus (VHSV) is a fish disease notifiable to the World Organisation for Animal Health. The United Kingdom is currently free from VHSV, and the introduction and onward spread of this disease could cause major economic losses in aquaculture facilities. Legislation in Great Britain requires that imports of live fish for aquaculture purposes into declared disease-free areas are of equivalent disease-free status. However, conditions on fish products are less stringent, whereby eviscerated fish or fillets can be transported from areas with disease to areas declared disease-free. Market-size rainbow trout were experimentally infected with VHSV to investigate two important factors relevant for pathogen introduction and transmission: (1) VHSV shedding, quantified by daily assessment of viral titres in tank water samples, and (2) VHSV concentrations in liquid and solid processing waste. Evisceration and filleting preclinical fish, maceration, and wastewater separation processes within a facility were mimicked, and VHSV was quantified in each fraction of the wastewater. Shedding was detected 25 hr post-challenge. Levels increased daily to peak on day 5 post-challenge, with a calculated average titre of 1.35 x 10(3) TCID50 mL(-1) kg(-1) fish, 1 day before clinical signs of disease. Preclinical fish contained virus levels in their kidney, skin, and muscle >10(7) TCID50 g(-1). The fish had significantly higher levels in the kidney, and evisceration led to higher VHSV concentrations in the waste compared to filleting. However, there was no significant difference in levels in wastewater released from the two processes after the removal of solids, even when macerated; average titres were >10(4) TCID50 mL(-1). The quantities of VHSV from shedding and processing can be utilised when modelling transmission and undertaking more accurate risk assessments for imports and processing of commodities, with the ultimate aim of reducing the global risk of disease from international trade and processing.