RESULTS AND DISCUSSION
The clinical symptoms and pathological changes in the dead rabbits were similar to those of rabbit hemorrhagic disease. The mortality rate was more than 70% (approximately 1300 rabbits died), although weaning rabbits had been immunized with a commercial inactivated RHD vaccine. Importantly, most of the unweaned rabbits died of the disease, indicating that RHDV2 might be the causal pathogen.
Compared with the positive control, the new isolate failed to hemagglutinate at 4°C, 25°C, and 37°C (hemagglutination titers ≤ 1:20). Given that the hemagglutination test remains the routine diagnostic method for RHDV in China, this non-hemagglutinating characteristic warrants further attention in the detection of clinical samples.
The new isolate exhibits the highest nucleotide sequence identity with the NL2016 strain from the Netherlands (98.3%; GenBank accession number: MN061492), which corresponds to RHDV2. Phylogenetic analysis was employed to determine the evolution of the new isolate. As shown in Figure 1, the new isolate is in the same branch of the other RHDV2 strains. These results support the conclusion that the isolate collected from the Sichuan province of China in 2020 belongs to the RHDV2 (GI.2) genogroup, which was designated strain SC2020/04 (GenBank accession number: MT383749). This represents the first outbreak of RHDV2-induced RHD in rabbit farms in China. We previously classified all RHDV isolates in China collected before 2017 in GI.1 (Hu et al., 2017); therefore, the present finding indicates the potential for co-circulation of RHDV and RHDV2 in China. Indeed, RHDV2 (GI.2) was reported to replace RHDV (GI.1) in some countries, including Sweden and Australia (Mahar et al., 2018; Neimanis et al., 2018). In addition, recombinant events between GI.2 and other genotypes have been reported in Europe (Almeida et al., 2015; Lopes et al., 2015). Considering the distinct serotype from RHDV (GI.1) and the high risk of RHDV2 (GI.2) to the Chinese rabbit industry, ongoing surveillance is important to better understand the epidemic of RHDV in China.
Acknowledgment: This work was supported by the funds earmarked for the China Agriculture Research System (No. CARS-43-C-1) and National Key R&D Program of China (No. 2018YFD0502203). We would like to thank Editage for English language editing.
Conflict of interests: The authors declare no conflict of interest regarding the publication of this manuscript. All authors have read and approved the final manuscript.
Ethical approval: The authors confirm that the ethical policies of the journal have been adhered to. The collection of the liver samples was performed in strict accordance with the guidelines of Jiangsu Province Animal Regulations (Government Decree No. 45).
Data availability statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.