Abstract
Infectious spleen and kidney necrosis virus (ISKNV) is a fish-pathogenic virus belonging to the genus Megalocytivirus of the family Iridoviridae. In 2018, disease occurrences (40-50% cumulative mortality) associated with ISKNV infection were reported in grown-out Asian sea bass (Lates calcarifer) cultured in an inland freshwater system in Thailand. Clinical samples were collected from seven distinct farms located in the eastern and central regions of Thailand. The moribund fish showed various abnormal signs, including lethargy, pale gills, darkened body, and skin hemorrhage, while hypertrophied basophilic cells were observed microscopically in gill, liver, and kidney tissue. ISKNV infection was confirmed on six out of seven farms using virus-specific semi-nested PCR. The MCP and ATPase genes showed 100% sequence identity among the virus isolates, and the virus was found to belong to the ISKNV genotype I clade. Koch's postulates were later confirmed by challenge assay, and the mortality of the experimentally infected fish at 21 days post-challenge was 50-90%, depending on the challenge dose. The complete genome of two ISKNV isolates, namely KU1 and KU2, was recovered directly from the infected specimens using a shotgun metagenomics approach. The genome length of ISKNV KU1 and KU2 was 111,487 and 111,610 bp, respectively. In comparison to closely related ISKNV strains, KU1 and KU2 contained nine unique genes, including a caspase-recruitment-domain-containing protein that is potentially involved in inhibition of apoptosis. Collectively, this study indicated that inland cultured Asian sea bass are infected by homologous ISKNV strains. This indicates that ISKNV genotype I should be prioritized for future vaccine research.
Highlights
Asian sea bass (Lates calcarifer Bloch, 1790), known as barramundi, is a species native to Thailand and widely distributed in the Indo-West Pacific region from the Arabian Gulf to China, Taiwan, and northern Australia [14]
Infectious spleen and kidney necrosis virus (ISKNV) was identified from diseased Asian sea bass cultured in the inland freshwater system
The most common histopathological lesions from ISKNV-infected Asian sea bass collected in this study, i.e., severe necrosis and the appearance of basophilic inclusion bodies in gills, liver, and kidney, was consistent with the pathognomonic lesions of megalocytivirus infection reported previously [32, 45]
Summary
Asian sea bass (Lates calcarifer Bloch, 1790), known as barramundi, is a species native to Thailand and widely distributed in the Indo-West Pacific region from the Arabian Gulf to China, Taiwan, and northern Australia [14]. The total production of Asian sea bass in Thailand was approximately 39,500 tons in 2018, increased from the year 2017 (20,500 tons) by over 90% [9]. Asian sea bass has been cultivated mainly in the coastal area along the Gulf of Thailand and the Andaman Sea, where the fish can grow in open sea cages or brackish/marine water earthen ponds. The main practice applied for Asian sea bass grow-out has been shifted to the inland freshwater pond system, owning to the catadromous nature of the fish and the advancement of pelleted feed quality, to align with the continual increase of consumer demand [14]. Asian sea bass farming in inland freshwater in Thailand is usually applied in an intensive system. Maximum yield can be expected with high stocking density, this aquaculture system is undeniably posing a high risk for the emergence of infectious diseases due to the stressful conditions caused by the accumulation of metabolic waste, organic matters, and fluctuation of water qualities
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