Experimental infection reveals transmission of tilapia lake virus (TiLV) from tilapia broodstock to their reproductive organs and fertilized eggs

Early developmental stages of tilapia, including fertilized eggs were tested positive for TiLV in our previous study (Dong et al., 2017a). We, therefore, hypothesized that infected broodstock is able to pass the virus to their reproductive organs and then to the fertilized eggs. In order to prove this hypothesis, Nile tilapia (Oreochromis niloticus) broodstock were experimentally infected with TiLV by intramuscular injection and non-infected broodstock were used as control group. At day 6 post infection, eggs and semen from each breeding pair were aseptically collected for in vitro fertilization. Fertilized eggs at 3, 12 and 64 h post-fertilization were subjected to detection of TiLV by PCR, ISH, and cell culture. In parallel, blood, serum, liver and reproductive organs from each broodstock were subjected to TiLV analysis. The results revealed that all collected tissues (liver, blood, ovary and testis) from infected broodstock tested positive for TiLV by PCR, ISH, and cell culture. ISH revealed strong positive signals in hepatocytes surrounding blood vessels in the liver, connective tissue and membrane surrounding the oocytes in the ovary and the connective tissue close to blood vessels in the testis. These findings suggested that TiLV causes systemic infection in tilapia broodstock with the virus being able to spread into the reproductive organs, most likely through the blood circulatory system. Subsequently, the fertilized eggs produced by infected broodstock tested positive for TiLV by PCR and ISH revealed location of the virus inside the fertilized eggs. The results of this study suggested that TiLV can be transmitted vertically. We thus recommend for hatchery and multiplication center to use TiLV-tested negative broodstock for the production of TiLV-free tilapia seeds.