The emergence of White Spot Syndrome Virus: Identifying ancestral hosts to prevent, mitigate, and combat emerging diseases in aquaculture.

Abstract

Aquaculture is a rapidly growing industry but is negatively affected by viral diseases that lead to a major economic loss. This thesis examines natural viruses of aquatic species. In Chapter 2 a comparative genomics approach was applied to study the genetic variations of 30 WSSV isolates from different geographical locations. About 50 % of the WSSV genome was shared by all 30 isolates. A total of 1109 SNPs were detected across the coding regions; of these, 66 % were non-synonymous missense mutations and the majority were in structural proteins. A total of 86 core genes were identified, 9 of which have a known protein function so far. Consistent deletions of whole genes were detected among isolates which could be due to adaptation to new hosts in this emerging disease. In Chapter 3 a survey was conducted to examine the prevalence of the novel WSSV-like virus -referred to here as Carcinus maenas nimavirus (CmNiV) - among crab species populations in the UK. The virus was found only in European shore crabs with a mean prevalence of 60%. Males as well as individuals infected with the Dinoflagellate parasites were found to be less likely to be infected with CmNiV. The virus showed no clear pathology in tissues from analysed animals. In Chapter 4 European shore crabs were stressed by exposing them to a prolonged exposure of increased temperatures to determine if this may enhance the replication of CmNiV. We found no effect of temperature on the viral load between treatments over time. However, there was a significant reduction in the viral load in females than males over time irrespective of treatment. Moreover, we observed no definitive viral histopathology. Further work is needed to investigate the phylogenetic relationships between the WSSV isolates and other Nima-like viruses. Moreover, further investigations are needed to study the tolerant behaviour of European shore crabs to CmNiV and its pathogenesis.