Non-lethal sampling for Tilapia Lake Virus detection by RT-qPCR and cell culture

Tilapia Lake Virus (TiLV) is an emerging virus of tilapia fish. Recently, outbreaks of TiLV associated mortality have been reported in many countries including Israel, Ecuador, Colombia, Egypt and Thailand. However, little is known about the route of transmission and how the virus is spread in fish populations. In this study, TiLV was detected in liver and mucus samples from moribund tilapia using reverse transcriptase quantitative polymerase chain reaction and virus isolation in the cell culture. Comparison of virus detection in the liver and mucus of field samples revealed that mucus could be applied for TiLV diagnosis and the virus in mucus was still viable and could cause a cytopathic effect in E-11 cells. The cohabitation of TiLV-infected fish with healthy fish resulted in 55.71% cumulative mortality of cohabitating fish suggesting that direct contact of infected fish is sufficient for disease transmission. Notably, the TiLV genomic RNA was identified in the mucus of cohabitation challenge fish as early as 1 day post infection (dpi) and the virus was isolated from mucus samples collected at 5 dpi. The presence of TiLV persisted up to 12-14 dpi in the mucus, liver and intestines of cohabiting fish. Taken together, the detection of TiLV in the mucus of field samples and cohabitating fish suggested that horizontal transmission is one of the important routes for the spread of TiLV. Importantly, this study revealed that mucus could be used for non-lethal sampling in TiLV detection.