First report of Infectious Spleen and Kidney Necrosis Virus in Nile tilapia in Brazil.

In June 2020, an atypical fatal outbreak in the Nile tilapia farm was investigated. Twenty-three animals were collected and different tissues were used for bacterial isolation, histopathological and electron microscopic examination, and viral detection using molecular methods. A large number of megalocytes were observed in the histopathological analysis of several tissues. Icosahedral virions, with a diameter of approximately 160 nm, were visualized inside the megalocytes through transmission electron microscopy of the spleen tissue. The virions were confirmed to be infectious spleen and kidney necrosis virus (ISKNV) through PCR and sequencing analyses of the fish samples. This viral pathogen is fatal in the early stages of Nile tilapia farms in the USA, Thailand, and Ghana; however, until now, there have been no reports from Brazil. In 14 of the sampled fish, Streptococcus agalactiae, Edwardsiella tarda, or Aeromonas hydrophila infections were also detected. This is the first report of fatal ISKNV infections in the Brazilian Nile tilapia fish farms.