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.

Many species of ornamental freshwater fishes are imported into Japan from all over the world. We found African lampeye Aplocheilichthys normani and dwarf gourami Colisa lalia suffering from an iridovirus infection just after being imported by tropical fish wholesalers from Singapore. African lampeye were cultured on the Indonesian Island of Sumatra and dwarf gourami were cultured in Malaysia before export. Diseased fishes displayed distinct histopathological signs of iridovirus infection: systemic appearance of inclusion body-bearing cells, and necrosis of splenocytes and hematopoietic cells. Electron microscopy revealed viral particles (African lampeye:180 to 200 nm in edge to edge diameter; dwarf gourami: 140 to 150 nm in diameter) in an inclusion body within the cytoplasm of inclusion body-bearing cells as well as in the cytoplasm of necrotized cells. Experimental infection with an iridovirus isolate from African lampeye (ALIV) revealed pathogenicity of ALIV to African lampeye and pearl gourami Trichogaster leeri. Polymerase chain reaction (PCR) products from ALIV and an iridovirus isolate from dwarf gourami (DGIV) using iridovirus-specific primers were indistinguishable. The nucleotide sequence of PCR products derived from ALIV (696 base pairs) and DGIV (701 base pairs) had 95.3% identity. These results indicate that ALIV and DGIV have a single origin. KEY WORDS: Tropical iridovirus · African lampeye · Dwarf gourami · Inclusion body-bearing cells · Polymerase chain reaction assay · DNA sequence Resale or republication not permitted without written consent of the publisherDis Aquat Org 48: [163][164][165][166][167][168][169][170][171][172][173] 2002 brown-spotted groupers Epinephelus tauvina (Chua et al. 1994) and E. malabaricus (Danayadol et al. 1996), farmed in Singapore and Thailand, respectively; sea bass, Lateolabrax sp., captured in the South China Sea ; mandarin fish Siniperca chuatsi, reared in offshore pens in the South China Sea in China (He et al. 2000); red sea bream Chrisophylus major (= Pagrus major, Inouye et al. 1992), yellowtail Seriola quinqueradiata, and striped jack Caranx delicatissimus, etc., farmed in Japan (Miyazaki unpubl.); and striped beakperch Oplegnathus fasciatus, cultured in Korea (Jung & Oh 2000).Here we present the results of histopathological and electron microscopic (EM) examinations of the diseased African lampeye and dwarf gourami. We report the results of artificial infections with an isolate from African lampeye to African lampeye and pearl gourami. We also describe a polymerase chain reaction (PCR) assay and DNA sequence analysis of the PCR products for identification of iridovirus isolates from African lampeye (ALIV) and dwarf gourami (DGIV). MATERIALS AND METHODSHistopathological and EM examinations, virus culture in natural outbreaks. Diseased African lampeye and dwarf gourami were found just after being imported to Japan from Singapore by tropical fish wholesalers. The African lampeye had been cultured in freshwater ponds on Sumatra Island, Indonesia, a...

Tropical iridovirus infection causes severe epizootic resulting in mass mortalities and large economic losses in freshwater ornamental fishes cultured in Southeast Asian countries, in wild fish seedlings captured in South China Sea, and in marine fishes farmed in Japan, Singapore, and Thailand. All of tropical iridovirus-infected fishes histopathologically showed the systemic formation of inclusion body-bearing cells and necrosis of virus-infected splenocytes and hematopoietic cells. We designed primer sets for the ATPase gene and the major capsid protein (MCP) gene and sequenced the PCR products derived from 5 iridovirus isolates from sea bass in South China Sea, red sea bream in Japan, brown-spotted grouper with a grouper sleepy disease in Thailand, dwarf gourami from Malaysia and African lampeye from Sumatra Island, Indonesia. The ATPase gene and the MCP gene of these 5 viral isolates were highly homologous (> 95.8%, > 94.9% identity, respectively) and the deduced amino acid sequences of the ATPase and the MCP were also highly identical (> 98.1%, > 97.2% identity, respectively). Based on the high homology, these 5 isolates of tropical iridovirus from various fishes in geographically different regions were determined to have a single origin and to be native to Southeast Asian regions. However, these sequences were far different from those of members of the genera Ranavirus, Lymphocystivirus and Iridovirus in the Family Iridoviridae. We propose a new genus "Tropivirus" for tropical iridovirus in the Family Iridoviridae.

ABSTRACT:The annual mortality of cultured Japanese pearl oysters Pinctada fucata martensii in all western regions of Japan was over 400 million in both 1996 and 1997. The main pathological signs of the diseased oysters were atrophy in the adductor muscle, the mantle lobe and the body accompanied by a yellowish to brown coloration. Histological studies revealed necrosis and degeneration of muscle fibers of the adductor, pallial and foot musculatures as well as the cardiac muscle. Electron microscopy revealed the presence of small round virions approximately 30 nm in diameter within intrasarcoplasmic inclusion bodies in necrotized muscle fibers of the adductor and pallial musculatures, and the heart. The causative virus was isolated and cultured in EK-1 (eel kidney) and EPC (epithelioma papilosum cyprini) fish cell lines. Marked mortalities occurred in pearl oysters that had been experimentally inoculated with the cultured virus; these oyster displayed the same pathological signs of the disease as oysters in natural infections. These results inbcate that a previously undescribed virus caused the mass mortalities in cultured pearl oysters. KEY WORDS:New virus disease -Japanese pearl oyster. Mass mortality . Virus in musc1.e fiber INTRODUCTIONresults of histopathological and electron microscopic studies, and report the isolation and culture of the Mass mortalities of the cultured Japanese pearl oyscausative virus. The results of infectivity experiments ter Pinctada fucata martensii (hereafter akoya oyster)

Iridovirus infection causes serious economic damage in marine cultured fish in Japan, Hong Kong and Singapore, and the incidence of this disease has been increasing. Iridovirus of sea bass from offshore Hong Kong was isolated to determine the genetic similarities of the causative agents. The genomic DNA of iridovirus was purified and cloned. Four DNA clones were randomly chosen and sequenced to generate primers for the polymerase chain reaction (PCR). Corresponding DNA fragments of iridoviruses from sea bass from offshore Hong Kong, red sea bream in Japan and grouper in Thailand were detected. The analogous PCR products from geographically diverse iridoviruses may indicate a widespread distribution of an iridovirus of a single origin.