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.