The role of live fish movements in spreading koi herpesvirus throughout England and Wales

Abstract

Carp (Cyprinus carpio L.,) positive for koi herpesvirus (KHV) antibodies are widely distributed across England and Wales in fisheries (Taylor, Dixon, Jeffery, Peeler, Denham & Way 2010). The majority of carp farms, however, remain negative. Although being antibody positive for KHV does not mean that waters still harbour the virus, it is a good indicator of prior exposure, and based on the current understanding of KHV and other herpesviruses, it seems likely that antibody-positive fish can become latent carriers of the virus (Gray, Williams, Jordan & Griffin 1999; St-Hilaire, Beevers, Way, Le Deuff, Martin & Joiner 2005 &; Thompson, Khoo, Wise & Hanson 2005 Field, Biswas & Mohammad 2006; Yuasa, Kawana, Ito, Sano & Iida 2007; St-Hilaire, Beevers, Joiner, Hedrick & Way 2009; Uchii, Matsui, Iida & Kawabata 2009). Because of the low proportion of infected farms in England and Wales, there may be prospects to control the disease, but to do so an understanding of the way in which the disease has spread is required (Taylor et al. 2010). Of the sites experiencing clinical outbreaks of KHV in England and Wales between 2003 and 2006, 66% had received a consented movement of carp from another site in the 2 years prior to the outbreak (Taylor et al. 2010). These introductions were thought to be the most likely route of introduction of the virus, although the disease status of the supplying sites remains unknown. The remaining cases were attributed to unrecorded movements or the stocking of imported ornamental carp. Taylor et al. (2010) also identified 30 antibody-positive sites (excluding sites known to hold fish vaccinated against KHV); however, the route of introduction to these sites remains unknown, and it is not known whether these sites have further spread the virus to other sites. This study therefore aimed at assessing the role of consented live fish movements in the spread of KHV antibody-positive fish. Using the live fish movements database (LFMD) held by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and Environment Agency (EA), a contact matrix was developed that included the 30 antibody-positive sites identified by Taylor et al. (2010) and any sites that they had supplied carp to, or received carp from, in the 5 years prior to testing. This was used to determine the proportion of positive sites for which a recorded fish movement was a possible source and to determine what proportion of sites supplied by a positive site tested positive. Permission to take blood samples to be analysed for KHV antibody by ELISA was requested for each contact site. Where permission was granted, fish were captured by EA netting or electric fishing teams in 2008. Blood samples were taken by a Cefas fish health inspector and returned to the Weymouth laboratory to be analysed by the ELISA method described in Taylor et al. (2010). As for Taylor et al. (2010), a target sample size of 30 fish was sought; however, the mean sample size obtained was 19 (SD = 11), which provides an 86% chance of detection if the Journal of Fish Diseases 2010, 33, 1005–1007 doi:10.1111/j.1365-2761.2010.01198.x