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Vet. Res. (2010) 41:51 P.J. Walker, J.R. Winton from Hong Kong, rock bream (Oplegnathus fasciatus) iridovirus (RBIV) from Korea and the orange-spotted grouper (Epinephelus coiodes) iridovirus (OSGIV) from China [27, 45, 83, 94]. Sequence analysis showed these and several additional, but related, viruses formed a novel group and have been assigned to the genus Megalocytivirus in the family Iridoviridae, with ISKNV as the type species [27, 46, 69, 83]. In addition to causing outbreaks associated with severe necrosis and high mortality in a wide range of cultured marine fish, these viruses have emerged to affect freshwater species such as the African lampeye (Aplocheilichthys normani) and dwarf gourami (Colisa lalia) in which they have caused additional losses [132]. Molecular epidemiological studies have shown that megalocytiviruses form at least three genetic lineages. There is evidence that some of the initial outbreaks in marine species were due to spill-over from viruses endemic among free-ranging fishes; in other cases, there are clear links to the international movement of both marine and ornamental fish [132, 150, 152]. 2.2.8. Viral nervous necrosis and other nodavirus diseases Viral nervous necrosis (VNN) has emerged to become a major problem in the culture of larval and juvenile marine fish worldwide [101]. Initially described as a cause of substantial mortality among cultured barramundi (Lates calcarifer) in Australia where the disease was termed vacuolating encephalopathy and retinopathy, the condition was shown to be caused by a small, icosahedral virus that resisted cultivation in available cell lines, but appeared similar to picornaviruses [38, 100]. Around the same time, efforts to expand marine fish aquaculture in other regions of the world revealed disease conditions associated with similar viruses in larvae or juveniles from a range of species including turbot (Scophthalmus maximus) in Norway [8], sea bass (Dicentrarchus labrax) in Martinique and the French Mediterranean [12], and parrotfish (Oplegnathus fasciatus) and redspotted grouper (Epinephelus akaara) in Japan [97, 163]. In Japan, the disease was termed nervous necrosis and the virus infecting larval striped jack (Pseudocaranx dentex), now assigned as the species Striped jack nervous necrosis virus (SJNNV), was shown to be a putative member of the family Nodaviridae [98]. Following isolation in cell culture [34], sequence analysis of the coat protein gene supported the creation of a novel genus, Betanodavirus, within the family Nodaviridae to include isolates of fish nodaviruses from various hosts and geographic locations [104, 105]. These and other phylogenetic analyses [23, 122, 139] revealed that genetic lineages of the betanodaviruses show low host specificity and generally correspond to geographic location, indicating they emerged due to spill-over from reservoirs that include a broad range of wild marine fish, although some isolates revealed links to commercial movement. 2.3. Emerging viral diseases of shrimp Shrimp is the largest single seafood commodity by value, accounting for 17% of all internationally traded fishery products. Approximately 75% of production is from aquaculture which is now almost entirely dominated by two species – the black tiger shrimp (Penaeus monodon) and the white Pacific shrimp (Penaeus vannamei) that represent the two most important invertebrate food animals [32]. Disease has had a major impact on the shrimp farming industry. Since 1981, a succession of new viral pathogens has emerged in Asia and the Americas, causing mass mortalities and threatening the economic sustainability of the industry [148]. Shrimp are arthropods and most shrimp viruses are either related to those previously known to infect insects (e.g., densoviruses, dicistroviruses, baculoviruses, nodaviruses, luteoviruses) or are completely new to science and have been assigned to new taxa (Tab. II). Several important characteristics are common to shrimp diseases and distinguish them from most viruses of terrestrial or aquatic vertebrates. Firstly, as invertebrates, shrimp lack the key components of adaptive and innate immune response mechanisms (i.e., antibodies, lymphocytes, cytokines, interferon) and, although Toll-like receptors have been identified, there Page 8 of 24 (page number not for citation purpose)
(page number not for citation purpose) Page 9 of 24 Table II. Emerging viral pathogens of marine and freshwater shrimp. Virus Abbreviation Genome Taxonomic Year emerged Known geographic classification 1 distribution OIE listed disease 2 DNA viruses Monodon baculovirus MBV dsDNA Baculoviridae 1977 Asia-Pacific, Americas, Africa No Baculoviral midgut gland necrosis virus BMNV dsDNA Baculoviridae 1971 Asia, Australia No White spot syndrome virus WSSV dsDNA Nimaviridae, 1992 Asia, Middle-East, Yes Whispovirus Mediterranean, Americas Infectious hypodermal and haematopoietic necrosis virus IHHNV ssDNA Parvoviridae, Densovirus 1981 Asia-Pacific, Africa, Madagascar, Middle-East, Americas Yes Hepatopancreatic parvovirus HPV ssDNA Parvoviridae, Densovirus 1983 Asia-Pacific, Africa, Madagascar, Middle-East, Americas RNA viruses Yellow head virus YHV (+) ssRNA Nidovirales, Roniviridae, 1990 East and Southeast Asia, Mexico Yes Okavirus Taura syndrome virus TSV (+) ssRNA Picornavirales, 1992 Americas, East and Southeast Yes Dicistroviridae Asia Infectious myonecrosis virus IMNV (+) ssRNA Totivirus (unclassified) 2002 Brazil, Indonesia, Thailand, Yes China Macrobrachium rosenbergii nodavirus MrNV (+) ssRNA Nodavirus (unclassified) 1995 India, China, Taiwan, Thailand, Yes Australia, Caribbean Laem-Singh virus LSNV (+) dsRNA Luteovirus-like 2003 South and Southeast Asia No (unclassified) Mourilyan virus MoV ( ) ssRNA Bunyavirus-like (unclassified) 1996 Australia, Asia No 1 ICTV, 2009. 2 OIE, 2009. No Emerging viruses of fish and shrimp Vet. Res. (2010) 41:51
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