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Replication Cycle

Viral replication and transcription of associated virus orthomyxovirus.

Attachment and Entry

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There is limited information of the replication cycle specific to TiLV, but it is known to be in the family Orthomyxoviridae due to its single-stranded negative-sense RNA genome. Typical Orthomyxoviridae carry several surface glycoproteins that recognize and bind to sialic acid receptors on the target cell membrane.[1] The target cell transports the virus into the cell by receptor-mediated endocytosis, initiating endosome acidification.[2] This acidification results in a conformational change of the viral glycoprotein, initiating membrane fusion of the viral envelope and endosomal membrane. Once fusion is complete, the viral genome, accessory proteins, and RNA dependent RNA polymerase are released into the host cell cytoplasm.[2]

Viral Replication and Transcription

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Through in situ hybridization, it is found that transcription of the TiLV viral genome occurs in the nucleus, this is common among all Orthomyxoviridae.[3] The nucleocapsid of an orthomyxovirus is transported to the nucleus where it is transcribed by viral enzymes resulting in the production of viral mRNA.[4] Cap sequences are taken from the host cell mRNA during transcription and bound to viral mRNA, this allows the viral mRNA to exit the nucleus and return to the cytoplasm of the host cell where it will by recognized and translated into proteins by host cell ribosomes.[4] The 5' and 3' noncoding termini of TiLV include 13 similar nucleotides, this enables base pairing and replication, transcription, and packaging of viral RNA as a result of the formation of secondary structures.[5] In addition, all of the 5′ ends of TiLV genomic RNA segments contain a brief uridine stretch (3 to 5 bases long). This short uninterrupted sequence resembles that of the 5 to 7 uridine nucleotides found in many other orthomyxoviridae, this occurs when the viral polymerase "stutters" while assembling poly(A) tails.[3]

Assembly and Release

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Orthomyxovirus viral glycoproteins then travel to the cell membrane where they form a spherical bud to transport negative-stranded vRNA out of the host cell. After the new viral material leaves the host cell, the host cell is terminated.[6]

Interactions with Host

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In cell culture, the affected cells exhibit significant cytopathic effect (CPE), structural changes of the host cell due to viral infection.[7] Clear and rapid CPE development occurs primarily at the E-11 cell line, cell lines of the brain and liver have been shown to be highly permissive at propagating TiLV.[8][9] Cases of infection note syncytium formation, the fusion of infected neighboring cells to produce multi-nucleated cells. Syncytial cells of this species are characterized by swollen mitochondria.[10] Hepatocytes of infected tilapia are swollen and dissociated, [10] with significant cytoplasmic accumulation of yellow to brown pigment (MMC) in both the spleen and liver of naturally and experimentally infected fish.[9] 'in addition, experimental infection shows histologic lesions on the brain such as edema, focal hemorrhages in the leptomeninges, and capillary congestion in both the white and gray matter.[7]

Studies have shown that upon experimental infection of TiLV, histopathological lesions have been found similar to those seen in natural outbreaks.[9] These natural outbreaks have been characterized with lethargy, discoloration, ocular alterations, skin patches, and ulcerations of the digestive tract[7]. The main organs where pathology is observed are the brain, eyes, and liver. Gross lesions are commonly visible in infected species such as ocular opacity of the cataract, and skin erosions such as,[7] loss of scales or discoloration, skin hemorrhages, abdominal swelling, scale protrusion, and exophthalmia.[11]

Associated Diseases

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There is little known of the relationship of TiLV to other viral aquaculture diseases, but viruses such as infectious salmon anemia orthomyxoviruses, infectious hematopoietic necrosis virus, and viral hemorrhagic septicemia virus are common causes of illness and death in cultured fish.[12] Salmon anemia orthomyxoviruses, influenza, and Thogoto have also been said to be have similar replication to that of TiLV due to organization of nucleotide sequences in transcription enabling base pairing.[3] Specific to tilapia, Tilapia iridovirus is the only significant viral pathogen known to cause severe disease and die-offs.[13] Other pathogens known to cause viral diseases in this species are betanodavirus and herpes-like virus.[12]

Viral Transmission and Control

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The virus is found to be transmitted through direct horizontal transmission by cohabitation or transfer of live aquatic animals, although these viral pathogens have been found in fresh and preserved tilapia.[14][15] There is limited information on the biophysical properties and risks of TiLV associated with animal product but research suggests that the eye, brain and liver are likely to contain highest concentrations of TiLV and thus solid and liquid waste are likely to be contaminated.[14] There is currently no evidence of vertical transmission of TiLV.

Restriction of the movement of live tilapines between farms or fisheries is thought to limit the spread of the viral disease to new species, as well as maintaining clean practices and sanitizing equipment in these areas. There is still no evidence that there are practices to limit viral spread in an infected farm.[14]

References

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  1. ^ Wagner, Ralf; Matrosovich, Mikhail; Klenk, Hans-Dieter (2002). "Functional balance between haemagglutinin and neuraminidase in influenza virus infections". Reviews in Medical Virology. 12 (3): 159–166. doi:10.1002/rmv.352. ISSN 1099-1654.
  2. ^ a b "ScienceDirect". www.sciencedirect.com. Retrieved 2019-03-12.
  3. ^ a b c Bacharach, Eran; Mishra, Nischay; Briese, Thomas; Zody, Michael C.; Kembou Tsofack, Japhette Esther; Zamostiano, Rachel; Berkowitz, Asaf; Ng, James; Nitido, Adam (2016-05-04). "Characterization of a Novel Orthomyxo-like Virus Causing Mass Die-Offs of Tilapia". mBio. 7 (2). doi:10.1128/mBio.00431-16. ISSN 2150-7511. PMC 4959514. PMID 27048802.{{cite journal}}: CS1 maint: PMC format (link)
  4. ^ a b Couch, Robert B. (1996), Baron, Samuel (ed.), "Orthomyxoviruses", Medical Microbiology (4th ed.), University of Texas Medical Branch at Galveston, ISBN 9780963117212, PMID 21413353, retrieved 2019-03-12
  5. ^ Kembou Tsofack, Japhette Esther; Zamostiano, Rachel; Watted, Salsabeel; Berkowitz, Asaf; Rosenbluth, Ezra; Mishra, Nischay; Briese, Thomas; Lipkin, W. Ian; Kabuusu, Richard M. (2017-3). Fenwick, Brad (ed.). "Detection of Tilapia Lake Virus in Clinical Samples by Culturing and Nested Reverse Transcription-PCR". Journal of Clinical Microbiology. 55 (3): 759–767. doi:10.1128/JCM.01808-16. ISSN 0095-1137. PMC 5328443. PMID 27974544. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  6. ^ Nayak, Debi P.; Hui, Eric Ka-Wai; Barman, Subrata (2004-12). "Assembly and budding of influenza virus". Virus Research. 106 (2): 147–165. doi:10.1016/j.virusres.2004.08.012. {{cite journal}}: Check date values in: |date= (help)
  7. ^ a b c d Eyngor, M.; Zamostiano, R.; Kembou Tsofack, J. E.; Berkowitz, A.; Bercovier, H.; Tinman, S.; Lev, M.; Hurvitz, A.; Galeotti, M. (2014-12-01). "Identification of a Novel RNA Virus Lethal to Tilapia". Journal of Clinical Microbiology. 52 (12): 4137–4146. doi:10.1128/JCM.00827-14. ISSN 0095-1137. PMC 4313277. PMID 25232154.{{cite journal}}: CS1 maint: PMC format (link)
  8. ^ Jansen, Mona Dverdal; Dong, Ha Thanh; Mohan, Chadag Vishnumurthy. "Tilapia lake virus: a threat to the global tilapia industry?". Reviews in Aquaculture. 0 (0). doi:10.1111/raq.12254. ISSN 1753-5131.
  9. ^ a b c Eyngor, M.; Zamostiano, R.; Kembou Tsofack, J. E.; Berkowitz, A.; Bercovier, H.; Tinman, S.; Lev, M.; Hurvitz, A.; Galeotti, M. (2014-12-01). "Identification of a Novel RNA Virus Lethal to Tilapia". Journal of Clinical Microbiology. 52 (12): 4137–4146. doi:10.1128/JCM.00827-14. ISSN 0095-1137. PMC 4313277. PMID 25232154.{{cite journal}}: CS1 maint: PMC format (link)
  10. ^ a b Dong, H.T.; Siriroob, S.; Meemetta, W.; Santimanawong, W.; Gangnonngiw, W.; Pirarat, N.; Khunrae, P.; Rattanarojpong, T.; Vanichviriyakit, R. (2017-7). "Emergence of tilapia lake virus in Thailand and an alternative semi-nested RT-PCR for detection". Aquaculture. 476: 111–118. doi:10.1016/j.aquaculture.2017.04.019. {{cite journal}}: Check date values in: |date= (help)
  11. ^ "ScienceDirect". www.sciencedirect.com. doi:10.1016/j.aquaculture.2017.11.025. Retrieved 2019-03-12.
  12. ^ a b Al, W. Surachetpong et. "Outbreaks of Tilapia Lake Virus Infection, Thailand, 2015–2016 - Volume 23, Number 6—June 2017 - Emerging Infectious Diseases journal - CDC". doi:10.3201/eid2306.161278. {{cite journal}}: Cite journal requires |journal= (help)
  13. ^ "Tilapia Diseases". AmeriCulture, Inc. Retrieved 2019-03-12.
  14. ^ a b c "TILAPIA LAKE VIRUS (TiLV)—A NOVEL ORTHOMYXO-LIKE VIRUS" (PDF). World Organisation for Animal Heath. February 2018.
  15. ^ Kembou Tsofack, Japhette Esther; Zamostiano, Rachel; Watted, Salsabeel; Berkowitz, Asaf; Rosenbluth, Ezra; Mishra, Nischay; Briese, Thomas; Lipkin, W. Ian; Kabuusu, Richard M. (2017-3). Fenwick, Brad (ed.). "Detection of Tilapia Lake Virus in Clinical Samples by Culturing and Nested Reverse Transcription-PCR". Journal of Clinical Microbiology. 55 (3): 759–767. doi:10.1128/JCM.01808-16. ISSN 0095-1137. PMC 5328443. PMID 27974544. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)