Abstract

In spring 2019, diseased, 4-month-old Nile tilapia (Oreochromis niloticus) from an aquaculture farm in Southern California were collected and submitted for diagnostic evaluation. The fish were experiencing lethargy, anorexia, abnormal swimming behavior, and low-rate mortalities. Water parameters were within normal limits according to the farmer and temperature, at the time of collection, ranged from 24–26°C. Diseased fish presented with non-specific gross lesions including fin erosion, pale gills, and ascites. Gross necropsy revealed hepatomegaly, splenomegaly, and yellowish-white nodules on spleens (3/5) and head kidneys (1/5). Swabs of spleen and posterior kidney were inoculated onto Modified Thayer Martin Agar (MTA) and incubated at 25°C for 72 hours. After 72 hours, greyish, translucent colonies appeared. Samples of gill, brain, liver, spleen, and kidney were collected, homogenized, inoculated onto Mozambique tilapia (O. mossambicus) brain cells (OmB), and incubated at 25°C. Cytopathic effects developed after 10 days in the OmB cells characterized by rounding of cells and detaching from the monolayer. Bacterial colonies were identified as Francisella orientalis by quantitative PCR. Total nucleic acids extracted from the infected cell culture supernatant was used to generate a DNA library for sequencing on an Illumina MiSeq sequencer. De novo assembly of the resulting data set recovered the complete genome of a novel member of the genus megalocytivirus (MCV; Family Iridoviridae; subfamily Alphairidovirinae). Phylogenetic analysis based on the alignment of the iridovirus core genes supported the novel MCV as the closest relative to the scale drop disease virus, previously reported in cultured Asian seabass (Lates calcarifer) in southeast Asia. Nile tilapia fingerlings were challenged by immersion (F. orientalis) and intra-coelomic injection (MCV), individually and in combination. Mortality rates were highest in fish challenged by the two pathogens in combination.

*Presenting author
⁺Student presenter