Isolation, identification and characterization of a novel megalocytivirus from cultured tilapia (Oreochromis spp.) from Southern California, USA

Khalid Shahin, Kuttichantran Subramaniam, Alvin C. Camus, Zeinab Yazdi, Susan Yun, Samantha A. Koda, Thomas B. Waltzek, Felipe Pierezan, Ruixue Hu, Esteban Soto

Research output: Contribution to journalArticlepeer-review


In spring 2019, diseased four-month-old tilapia (Oreochromis spp.) from an aquaculture farm in Southern California, USA were received for diagnostic evaluation with signs of lethargy, anorexia, abnormal swimming, and low-level mortalities. At necropsy, non-specific external lesions were noted including fin erosion, cutaneous melanosis, gill pallor, and coelomic distension. Internal changes included ascites, hepatomegaly, renomegaly, splenomegaly, and multifocal yellow-white nodules in the spleen and kidney. Cultures of spleen and kidney produced bacterial colonies iden-tified as Francisella orientalis. Homogenized samples of gill, brain, liver, spleen, and kidney inocu-lated onto Mozambique tilapia brain cells (OmB) developed cytopathic effects, characterized by rounding of cells and detaching from the monolayer 6–10 days post-inoculation at 25 °C. Transmission electron microscopy revealed 115.4 ± 5.8 nm icosahedral virions with dense central cores in the cytoplasm of OmB cells. A consensus PCR, targeting the DNA polymerase gene of large double-stranded DNA viruses, performed on cell culture supernatant yielded a sequence consistent with an iridovirus. Phylogenetic analyses based on the concatenated full length major capsid protein and DNA polymerase gene sequences supported the tilapia virus as a novel species within the genus Megalocytivirus, most closely related to scale drop disease virus and European chub iridovirus. An intracoelomic injection challenge in Nile tilapia (O. niloticus) fingerlings resulted in 39% mortality after 16 days. Histopathology revealed necrosis of head kidney and splenic hematopoietic tissues.

Original languageEnglish (US)
Article number3524
Issue number12
StatePublished - Dec 2021


  • Co-infection
  • Francisella
  • Megalocytivirus
  • Tilapia

ASJC Scopus subject areas

  • Animal Science and Zoology
  • veterinary(all)


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