Turbot Reddish Body Iridovirus in Asian Seabass-An overview

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Hello everyone!


Hope you’re all doing great. Let’s start the week with something exciting with a new post about Turbot Reddish Body Iridovirus (TRBIV) and its pathogenicity in Asian seabass.

Ever since I arrived in Thailand, I’ve become a huge fan of seabass dishes they’re absolutely delicious!😋 But today, we’ll be talking about a not-so-tasty side of seabass the viral infections that affect them. 

We previously covered two important viral disease in Asian Sebass. If you didn't read yet, Kindly find here 1.SDDV-Asian seabass 2.ISKNV-Asian seabass

Let's jump into the topic...

Know little about the virus:

Viruses belonging to the family Iridoviridae are known to cause infectious diseases in Asian seabass (Lates calcarifer). The family Iridoviridae is classified into two subfamilies: Alphairidovirinae and Betairidovirinae. The Turbot Reddish Body Iridovirus (TRBIV) belongs to the genus Megalocytivirus within the Alphairidovirinae subfamily. Members of the Iridoviridae family infect a wide range of both vertebrate and invertebrate hosts, including amphibians, insects and fish.


                               

                                               Image : Asian seabass

Viruses in the genus Megalocytivirus are large, icosahedral, double-stranded DNA viruses with a genome size of approximately 110,104 base pairs and a GC content of 55%. In the case of TRBIV, 115 open reading frames (ORFs) have been identified, encoding polypeptides ranging from 40 to 1,168 amino acids. 

Close identity with other virus of same genus:

Phylogenetic analysis of TRBIV indicates a close relationship with other Megalocytivirus species, including

  •  Red Seabream Iridovirus (RSIV)
  •  Infectious Spleen and Kidney Necrosis Virus (ISKNV)
  •  Rock Bream Iridovirus (RBIV)
  • Taiwan Grouper Iridovirus (TGIV)
  •  Dwarf Gourami Iridovirus (DGIV)
  •  Large Yellow Croaker Iridovirus (LYCIV)
Host range:

The known host range of TRBIV includes Asian seabass (Lates calcarifer), striped beakperch (Oplegnathus fasciatus), starry flounder (Platichthys stellatus), turbot (Scophthalmus maximus), and olive flounder (Paralichthys olivaceus).

Significant oubreak and signs&symptoms of infection:

 A significant outbreak was reported when a large number of Asian seabass fingerlings were transported from Thailand to taiwan, during which most of the fish were infected with the Turbot Reddish Body Iridovirus genotype II. Clinical signs observed in affected fish included lethargy,

  1. Loss of appetite
  2. Pale gills with visible spots
  3. Abdominal swelling
  4. Severe anemia

The spleen and kidney are the primary target organs for infection, while the liver serves as a secondary target organ.

Mortality range : Higher mortality of 80-90% observed in the giant sea perch farm in taiwan. 

WOAH recognition:

Due to its relatively narrow host range, TRBIV is not listed as a notifiable pathogen by the World Organisation for Animal Health (WOAH, formerly OIE). 

However, infections caused by Megalocytivirus species exhibit similar clinical signs, making differentiation based solely on external or internal symptoms challenging. Among Megalocytivirus species, TRBIV shows a closer phylogenetic relationship to RSIV than to ISKNV.

Polymerase Chain Reaction (PCR) is considered the most effective diagnostic method for the early detection of TRBIV in Asian seabass. Given the similarity of clinical symptoms among Megalocytivirus species, molecular diagnosis is essential for accurate species identification.

Using specific primer set for this TRBIV genotype is really important in the disgnosis. 

Better management practices are highly recommended to protect your seabass.


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Be in touch,

Dhivagar P

E-mail : dhivagarfcri2000@gmail.com


References:

1. Tsai, J.-M., Huang, S.-L., & Yang, C.-D. (2020). PCR Detection and Phylogenetic Analysis of Megalocytivirus Isolates in Farmed Giant Sea Perch Lates calcarifer in Southern Taiwan. Viruses, 12(6), 681. https://doi.org/10.3390/v12060681

2. Eaton, H. E., Ring, B. A., & Brunetti, C. R. (2010). The Genomic Diversity and Phylogenetic Relationship in the Family Iridoviridae. Viruses, 2(7), 1458-1475. https://doi.org/10.3390/v2071458

3. Shi, CY., Jia, KT., Yang, B. et al. Complete genome sequence of a Megalocytivirus (family Iridoviridae) associated with turbot mortality in China. Virol J 7, 159 (2010). https://doi.org/10.1186/1743-422X-7-159

4. Kurita, J., & Nakajima, K. (2012). Megalocytiviruses. Viruses, 4(4), 521-538. https://doi.org/10.3390/v4040521

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