Transmission routes of grass carp reovirus type Ⅱ in rare minnow Gobiocypris rarus

Grass carp reovirus (GCRV) causes severe grass carp hemorrhagic disease, which is one of the main diseases that threaten grass carp culture in our country. In order to explore the transmission of grass carp reovirus type Ⅱ (GCRV-Ⅱ), we established the horizontal and vertical transmission models of GCRV-JX02 infection in rare minnow Gobiocypris rarus using molecular biology detection and real-time fluorescent quantitative (RT-qPCR) methods. We compared the transmission efficiency of GCRV-JX02 in different models, and found that immersion infection and co-culture infection could turn the rare minnow into asymptomatic carriers of GCRV-JX02 in horizontal transmission. The positive detection rates were 50% and 80%, while the mortality rates of rare minnow in co-culture infection were 8%. When the asymptomatic rare minnow in the immersion and co-culture groups were treated with heat shock, the mortality rates were 57.14% and 100%, and the overall positive detection rates were 80.95% and 100%. Copy numbers of GCRV-JX02 in the testis and ovary of asymptomatic rare minnow after being injected intraperitoneally of GCRV-JX02 were 3.64×10⁶ and 6.84×10⁶ copies/0.01g. In the vertical transmission experiment, average copy numbers of GCRV-JX02 in ova, unfertilized eggs, fertilized eggs and juvenile fish were 1.98×10³, 1.15×10⁴, 4.75×10³ and 6.74×10⁴ copies. The copy number of GCRV-JX02 in single juvenile fish was significantly higher than that of ovum and fertilized egg. The results indicated that GCRV-JX02 could not only transmit vertically in rare minnow, but also persist continuously with the development of juvenile fish. This study elucidates the transmission efficiency of GCRV-Ⅱ in multiple transmission routes, which is helpful in evaluating the epidemic risk of grass carp hemorrhagic disease and provide an effective animal model for screening drugs to block the transmission of GCRV.