Veterinary World, EISSN: 2231-0916 REVIEW ARTICLE Available at www.veterinaryworld.org/Vol.13/November-2020/37.pdf Open Access Megalocytiviruses in ornamental fish: A review Che Azarulzaman Che Johan and Sandra Catherine Zainathan Department of Fisheries and Aquaculture, Faculty of Fisheries and Food Science, University Malaysia Terengganu, Terengganu, Malaysia. Corresponding author: Sandra Catherine Zainathan, e-mail: [email protected] Co-author: CACJ: [email protected] Received: 09-07-2020, Accepted: 19-10-2020, Published online: 30-11-2020 doi: www.doi.org/10.14202/vetworld.2020.2565-2577 How to cite this article: Johan CAC, Zainathan SC (2020) Megalocytiviruses in ornamental fish: A review, Veterinary World, 13(11): 2565-2577. Abstract Iridoviruses, especially megalocytiviruses, are related to severe disease resulting in high economic losses in the aquaculture industry worldwide. The ornamental fish industry has been affected severely due to Megalocytivirus infections. Megalocytivirus is a DNA virus that has three genera; including red sea bream iridovirus, infectious spleen and kidney necrosis virus, and turbot reddish body iridovirus. Megalocytivirus causes non-specific clinical signs in ornamental fish. Cell culture, histology, immunofluorescence test, polymerase chain reaction (PCR) assay, and loop-mediated isothermal amplification assay have been used to diagnose megalocytiviruses. Risk factors such as temperature, transportation (export and import), and life stages of ornamental fish have been reported for the previous cases due to Megalocytivirus infections. In addition, other prevention and control methods also have been practiced in farms to prevent Megalocytivirus outbreaks. This is the first review of megalocytiviruses in ornamental fish since its first detection in 1989. This review discusses the occurrences of Megalocytivirus in ornamental fish, including the history, clinical signs, detection method, risk factors, and prevention measures. Keywords: clinical signs, detection, Megalocytivirus, ornamental fish, risk factors. Introduction One of these clusters (genotype I) is widely distributed Ornamental fish are known to be affected by among several Asian countries, including 13 isolates Megalocytivirus [1-5], which can cause systemic from Korea, nine isolates from Japan, one isolate from infections in a wide variety of freshwater and marine Thailand, one isolate from China, and one isolate from fish [6], including amphibians [7]. Megalocytivirus is the South China Sea [15]. In contrast, the other two an important genus of fish viruses in the Iridoviridae genotypes have smaller host ranges and are locally (the Iridoviruses) family [8]. The Iridoviridae family is distributed [15]. Genotype II megalocytiviruses infect divided into five genera, which include Chloriridovirus, freshwater fishes from Southeast Asia and Australia, Iridovirus, Lymphocystivirus, Megalocytivirus, and whereas genotype III megalocytiviruses primarily Ranavirus. Megalocytivirus is the most recently added infect flatfish in China and Korea [15]. genus [9,10]. Megalocytiviruses are large icosahedral The first outbreak of Megalocytivirus-induced DNA viruses measuring 120-200 nm in diameter [11] disease was recorded in the Japanese cultured red sea and have a large single linear dsDNA genome [12]. bream (Pagrus major) in 1990 and was designated Megalocytivirus isolates exhibit relatively few genetic as RSIV disease [8]. In 2001, the complete genome differences and have been divided into three major sequence of ISKNV was determined using the groups based on genetic sequencing data. These groups next-generation sequencing technique VIDISCA-454, are represented by infectious spleen and kidney necro- where sequences of an unknown virus were detected in sis virus (ISKNV), red sea bream iridovirus (RSIV), the serum of diseased fish. The near-complete genome and turbot reddish body iridovirus (TRBIV) [13]. The sequence of the virus was determined, which showed most frequently identified megalocytiviruses are RSIV a unique genome organization and low levels of iden- and ISKNV [14]. Song et al. [15] evaluated 48 Asian tity to known members of the Iridoviridae. Based on and Australian Megalocytivirus isolates with regard to the homology of a series of putatively encoded pro- their geographic locations and genetic variations in the teins, the virus was identified as a novel member of major capsid protein (MCP) gene. Based on the results, the Megalocytivirus genus in 2001 [8], and in the fol- they developed a phylogenetic tree that divided the 48 lowing year in Japan, Megalocytivirus was identified isolates into three distinct clusters based on genotype. in ornamental fish, African lampeye Aplocheilichthys normani, and dwarf gourami Colisa lalia imported Copyright: Johan and Zainathan. Open Access. This article is from Singapore [16]. Next, in 2005, the identification distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/ of genotypes within the species of Megalocytivirus we by/4.0/), which permits unrestricted use, distribution, and made [17], and in 2006, genetic vaccines for RSIV reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the were generated from DNA vaccines against RSIV Creative Commons license, and indicate if changes were made. infection in fish using the MCP gene and an open The Creative Commons Public Domain Dedication waiver (http:// creativecommons.org/publicdomain/zero/1.0/) applies to the data reading frame (ORF) containing a transmembrane made available in this article, unless otherwise stated. domain [18]. Veterinary World, EISSN: 2231-0916 2565 Available at www.veterinaryworld.org/Vol.13/November-2020/37.pdf In 2006, it was understood that global trade presence of numerous virions in large groups in the of ornamental fish may facilitate the spread of cell cytoplasm. The structure and size of the virions Megalocytivirus and enable the emergence of dis- suggested that they were Iridoviruses, yet that was ease in new host species in distant biogeographic not confirmed until much later. In 2001, there was regions [19]. In the same year, an outbreak of irido- an outbreak caused by Iridoviruses in tropical orna- virus was reported among various fish species in mental fish farms in Israel. A systemic viral infection Taiwan. These megalocytiviruses showed high in Trichopodus trichopterus, Trichopodus leerii, and homology (97% identity) to RSIV with the presence swordtail Xiphophorus hellerii has occurred in endo- of unidentical nucleotide sequences [20]. In 2008, a thelial cells that become hypertrophic, and an outbreak phylogenetic tree revealed three clusters: Genotype I, of lymphocystis in freshwater angelfish P. scalare and including nine Japanese isolates, 13 Korean isolates, gourami was also reported [26]. one Chinese isolates, one Thai isolate, and one South ISKNV-like viruses, African lampeye iridovirus, China Sea isolate; genotype II, including five fresh- and dwarf gourami iridovirus (DGIV) were detected water fish isolates in Southeast Asian countries and in freshwater African lampeye (A. normani) and dwarf Australia; and genotype III, consisting mainly of flat- gourami (C. lalia), which were imported into Japan fish isolates in Korea and China. This suggests that in 2002 [16]. A. normani was cultured in freshwater viruses belonging to genotype I were widely distrib- ponds in Indonesia Island of Sumatra and C. lalia uted among various fish species in many Asian coun- was cultured in freshwater ponds in Malaysia before tries [15]. Furthermore, in 2009, a detailed study of tropical fish wholesalers from Singapore exported it ISKNV infection in an imported ornamental fish was to Japan. The histology result for A. normani showed conducted. Phylogenetic analyses using sequences many inclusion body-bearing cells (IBCs) and necrotic from a portion of the DNA polymerase, MCP, and cells in the pulp from spleen. Meanwhile, the spleen ATPase genes of Banggai cardinalfish iridovirus in of C. lalia displayed many early-stage IBCs, matured Pterapogon kauderni with the presence and systemic IBCs, and necrotic cells in the pulps, and the kidney distribution of enlarged virus-infected cells, demon- showed many matured IBCs in the hematopoietic tis- strated a close relationship with ISKNV [21]. sue. Gibson-Kueh et al. [27] examined four species of This review is intended to provide an update tropical freshwater ornamental fish, including dwarf on the prevalence of megalocytiviruses, including gourami (C. lalia), pearl gourami (T. leeri), Siamese the history, clinical signs, detection method, risk fac- fighting fish (Betta splendens), and angelfish (P. sca- tors, and prevention measures, which will of interest lare). The samples were derived from newly dead fish. to aquaculturists that are involved in ornamental fish The histological results showed that all of the affected farming and trading around the world. fish demonstrated basophilic and hypertrophied cells in various organs. Petty and Fraser [28] found irido- History of Diagnosis and Cases of virus in freshwater angelfish that showed an enlarged Megalocytivirus Infection in Ornamental Fish abdomen and abnormal body in a high mortality Many cases have reported on the infection of occurrence at the farm. Megalocytivirus in ornamental fish. In 1989, the pres- In 2005, the International Committee on ence of numerous virions as large aggregates