Megalocytivirus infection in cultured Nile tilapia Oreochromis niloticus

Abstract

Megalocytiviruses, such as infectious spleen and kidney necrosis virus (ISKNV), induce lethal systemic diseases in both ornamental and food fish species. In this study, we investigated an epizootic affecting Nile tilapia Oreochromis niloticus cultured in the US Midwest. Diseased fish displayed lethargy, gill pallor, and distension of the coelomic cavity due to ascites. Histopathological examination revealed a severe systemic abundance of intravascular megalocytes that were especially prominent in the gills, kidney, spleen, liver, and intestinal submucosa. Transmission electron microscopic examination revealed abundant intracytoplasmic polygonal virions consistent with iridovirus infection. Comparison of the full-length major capsid protein nucleotide sequences from a recent outbreak with a remarkably similar case that occurred at the same facility many years earlier revealed that both epizootics were caused by ISKNV. A comparison of this case with previous reports suggests that ISKNV may represent a greater threat to tilapia aquaculture than previously realized. KEY WORDS: Iridoviridae · Megalocytivirus · Infectious spleen and kidney necrosis virus · Aquaculture · Ornamental fish Resale or republication not permitted without written consent of the publisherDis Aquat Org 119: [253][254][255][256][257][258] 2016 Although tilapia are considered relatively resistant to disease, they are susceptible to common aquatic pathogens (e.g. viruses, bacteria, fungi, water molds, parasites) when reared intensively. The primary microbial pathogens affecting cultivated tilapia include Gram-negative bacteria (Aeromonas hydrophila, Flavobacterium columnare, and Francisella noatunensis), Gram-positive bacteria (Streptococcus spp.), and water molds such as members of the family Saprolegniaceae (e.g. Achyla and Saprolgenia spp.; Plumb & Hanson 2010). Despite the global culture of tilapia, few viruses have been described from tilapia, and none has proven to be a serious threat to the industry (El-Sayed 2006).The first RNA virus discovered in tilapia was an aquabirnavirus isolated from apparently healthy Mozambique tilapia O. mossambicus cultured in Lukang, Taiwan (Hedrick et al. 1983). In 2007, a betanodavirus was detected by RT-PCR in Nile tilapia larvae following a mass mortality event at a western European fish farm (Bigarré et al. 2009), andEyngor et al. (2014) isolated a novel RNA virus (tilapia lake virus) from wild tilapia and confirmed its pathogenicity via experimental infection in Nile tilapia.The first DNA viruses discovered in tilapia were iridoviruses. Although the family Iridoviridae is composed of 5 genera, only members of the genera Megalocytivirus, Lymphocystivirus, and Ranavirus infect fish (Zhang & Gui 2015). Paperna (1973) detected lymphocystis virus in tilapine and haplochromine cichlids from the Rift Valley Lakes Kitangiri and Victoria. A severe (100% mortality) epizootic among Mozambique tilapia fry held in an Australian aquatic disease laboratory was tentatively attributed to a ranavirus, Bohle ir...

First description of a natural infection with spleen and kidney necrosis virus in zebrafish

“…With regard to the histopathological findings, the main lesion was the presence of infected hypertrophied cells displaying basophilic granular intracytoplasmic inclusions in various tissues, similar to descriptions in previous reports (Gibson‐Kueh et al., ; Lee, Do, Park, & Kim, ; Mahardika, Muzaki, & Miyazaki, ; Sriwanayos et al., ; Subramaniam et al., ; Weber et al., ; Yanong & Waltzek, ). Most of megalocytic cells in infected zebrafish did not correspond to IBCs, suggesting that increased size of infected cells is due to other factors other than the growth of a unique inclusion body in the IBCs (Chinchar et al., ), as suggested by Xu, Zhang et al.…”

Comparative evaluation of MCP gene in worldwide strains of Megalocytivirus (Iridoviridae family) for early diagnostic marker

“…The fish industry suffers an estimated loss of US $150 million annually (Shoemaker, Klesius, & Evans, ; Subramaniam et al., ). Therefore, there is an urgent need to develop rapid diagnosis methods for fish infections using universal primers, which may alarm the fish producers at very advanced stage.…”

The impact of pooling samples on surveillance sensitivity for the megalocytivirus Infectious spleen and kidney necrosis virus

“…Current evidence indicates that we must consider the three genotypes of ISKNV (i.e., RSIV, ISKNV and TRBIV) as potentially globally distributed pathogens, each able to infect fish in a variety of environments (Fusianto, Hick, & Becker, ). For example, ISKNV has been reported as the cause of significant mortality at marine farms holding barramundi ( Lates calcarifer ; Dong et al, ) whilst the geographic range is expanded with detections in ornamental fish in Brazil (de Lucca Maganha et al, ), Germany (Jung‐Schroers et al, ) and the USA (Subramaniam et al, ). TRBIV was recently reported in a marine food fish (starry flounder, Platichthys stellatus ) in Korea (Jin et al, ; Woong et al, ) and from archived clinical cases of freshwater ornamental fish from the late 1980s and early 1990s in the USA (Koda et al, ), Canada and Australia (Go et al, ).…”