Updates on the nervous necrosis virus and the koi herpesvirus in Southeast Asia

Abstract

In Southeast Asia, the Viral Nervous Necrosis (VNN) or Viral Encephalopathy and Retinopathy (VER) and the Koi herpesvirus (KHV) infection are currently economically-important diseases of fishes. The VNN affects groupers (Epinephelus akaara, E. coioides, E. tauvina, E. fuscogutatus, E. septemfasciatus, E. malabaricus, E. moara and Cromileptes altivelis), Asian sea bass (Lates calcarifer), mangrove red snapper (Lutjanus argentimaculatus) and milkfish (Chanos chanos) in Thailand, Taiwan, Indonesia, Brunei, Malaysia, Singapore, Philippines and Vietnam. The Piscine nodavirus of the genus Betanodavirus, genotype red-spotted grouper nervous necrosis virus (RGNNV) is predominantly involved. Research on fish species pathogenicity, optimum temperature, cell susceptibility, organ predeliction, pathology, virus reservoirs, experimental infection, vaccination and diagnosis have been reported. The Koi herpesvirus (KHV) infection causes significant mortalities in common carp (Cyprinus carpio carpio), koi carp (Cyprinus carpio koi) and ghost carp (common x koi cross, Cyprinus carpio goi). Outbreaks have been reported among koi in Hongkong in 2001; common carp in Indonesia, in 2002; koi in Taiwan in 2002; and common carp in Japan, in 2003. A dramatic spread of the disease was subsequently observed among most prefectures in Japan, with outbreaks that eventually involved koi carp. In Thailand, KHV was first diagnosed in March 2005 while in Singapore, in Sept 2005. By Feb 2006, two batches of Thai koi exported to Singapore, tested KHV PCR positive from which the virus was successfully isolated on KF-1 cells. In Malaysia, no KHV outbreak was reported but the presence of the virus was detected among koi carp exported to UK in 2000 and in 2001. In 2004, koi carp in Malaysia tested positive for KHV by nested PCR. To date, annual active surveillance of the virus in Cambodia, Lao PDR, Myanmar, Philippines and Vietnam from 2004 to 2008 showed these countries are free of KHV. Recent developments on research of KHV focused on pathogenicity, cell line susceptibility, fish size susceptibility, predilection to fish organs, persistence in fish, vaccine development and application, surveillance and gene sequence analyses of KHV strains. The extensive international trading of live ornamental koi fish has largely contributed to the global spread of KHV. Hence, KHV was recently added to the list of notifiable diseases to the World Organisation of Animal Health or the Office International des Epizooties (OIE).