Abstract

From 1992 onwards, outbreaks of a previously unknown illness have been reported in Asian seabass (Lates calcarifer) kept in maricultures in Southeast Asia. The most striking symptom of this emerging disease is the loss of scales. It was referred to as scale drop syndrome, but the etiology remained enigmatic. By using a next-generation virus discovery technique, VIDISCA-454, sequences of an unknown virus were detected in serum of diseased fish. The near complete genome sequence of the virus was determined, which shows a unique genome organization, and low levels of identity to known members of the Iridoviridae. Based on homology of a series of putatively encoded proteins, the virus is a novel member of the Megalocytivirus genus of the Iridoviridae family. The virus was isolated and propagated in cell culture, where it caused a cytopathogenic effect in infected Asian seabass kidney and brain cells. Electron microscopy revealed icosahedral virions of about 140 nm, characteristic for the Iridoviridae. In vitro cultured virus induced scale drop syndrome in Asian seabass in vivo and the virus could be reisolated from these infected fish. These findings show that the virus is the causative agent for the scale drop syndrome, as each of Koch’s postulates is fulfilled. We have named the virus Scale Drop Disease Virus. Vaccines prepared from BEI- and formalin inactivated virus, as well as from E. coli produced major capsid protein provide efficacious protection against scale drop disease.

Highlights

  • Scale drop syndrome in Lates calcarifer, Asian seabass, was first reported in 1992 in Penang, Malaysia, and since outbreaks have been seen in Indonesia and in the Strait of Malacca

  • As Asian seabass is a large, valuable fish kept in maricultures, scale drop syndrome currently results in significant economic losses for affected farms

  • This study describes the characterization of a virus which infects Lates calcarifer and causes scale drop syndrome

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Summary

Introduction

Scale drop syndrome in Lates calcarifer, Asian seabass, was first reported in 1992 in Penang, Malaysia, and since outbreaks have been seen in Indonesia and in the Strait of Malacca. The phenotypic symptoms and pathology of the syndrome were described in detail by GibsonKueh et al [1]. The syndrome is so far described to affect Asian seabass only, both juvenile and adult fish, and seems to follow a seasonal pattern: the south-west monsoon/inter-monsoon season starting around September may be a trigger. It is an illness with unknown etiology of which the incidence is on the rise in commercial fish farms. As Asian seabass is a large, valuable fish kept in maricultures, scale drop syndrome currently results in significant economic losses for affected farms. The identification of the cause of the disease, and if possible, the development of a vaccine, are highly desired

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