4 DISCUSSION
ARV, MDRV, and NDRV are classified into the genus Orthoreovirus , members of the family Reoviridae(Benavente and Martínez-Costas, 2007, Chen et al., 2012b). ARV was first isolated from the respiratory tract of chickens suffering from chronic respiratory diseases in 1954 (Fahey and Crawley, 1954). MDRV was isolated from Muscovy ducklings for the first time in 1972 (Gaudry et al., 1972). In 2005, a novel duck reovirus disease was discovered in duck flocks in China, characterized by irregular hemorrhage and liver necrosis. The pathogen of this disease was isolated in 2011 and named NDRV distinguished from the classical MDRV (Chen et al., 2012a). Since then, NDRV has become the predominant strain of reovirus that emerged in duck flocks in China (Ma et al., 2012, Li et al., 2016, Zhu et al., 2015, Wang et al., 2019, Luo et al., 2021, Wang et al., 2020c). Previous studies have demonstrated that NDRV has a broader host spectrum than MDRV (Cao et al., 2019, Zhang et al., 2019, Luo et al., 2021), and different pathogenicity (Wang et al., 2020a, Zheng et al., 2016, Luo et al., 2021, Wang et al., 2020c, Farkas et al., 2018). In recent years, the diseases caused by NDRVs broke out in waterfowl flocks worldwide, have led to significant economic losses to the waterfowl industry.
As the cell attachment protein of ARVs, σC proteins play an essential role in viral fusion, invasion, and pathogenicity (Du et al., 2020, Ma et al., 2012, Shih et al., 2004, Benavente and Martínez-Costas, 2007). Additionally, the σC protein possesses the highest sequence variability among ARVs (Benavente and Martínez-Costas, 2007). The 18-aa deletion in the σC protein significantly enhanced the virulence of reovirus (Zheng et al., 2016). In this study, two NDRV isolates were identified, and the σC genes were amplified and sequenced. Phylogenetic analysis based on the σC genes deduced amino acid sequences demonstrated that Chinese NDRVs had formed two distinct clades, with late 2017 as the turning point, suggesting that Chinese NDRVs have been evolving in different directions. More importantly, NDRV strains within clade 2 have become predominant in duck flocks in China, and emerged in the major duck production regions in China, such as Shandong, Henan, Jiangsu, Hubei, and Guangxi. Remarkably, four amino acid mutation sites located in the head domain, and one amino acid mutation site located in the shaft domain were discovered in both σC proteins of the newly identified NDRV strains, which might have changed the virulence and led to differences in the epidemic.
Previous studies have demonstrated that the genetic diversity of NDRV circulating in China is complex (Yun et al., 2013, Zhu et al., 2015, Cao et al., 2019, Zhang et al., 2019, Wang et al., 2020c). Since using the attenuated MDRV vaccine in 2013, MDRV infection in waterfowl has significantly reduced in China in recent years. However, NDRV and N-MDRV emerged and spread widely in waterfowl in China (Luo et al., 2021, Ji et al., 2020, Cao et al., 2019, Wang et al., 2019, Zhang et al., 2019), which caused significant economic losses in China. The complexity of epidemic strains undoubtedly facilitated virus mutation and recombination(Wang et al., 2020c, Luo et al., 2021), and complicated the genetic diversity of DNRV. To date, no effective NDRV vaccine is available, the complex genetic diversity of NDRV hampers the development of an effective vaccine. Though diverse vaccines against NDRV infection are under development, none is commercially available. The complex genetic diversity of NDRV poses more challenges to vaccine development, including poor cross-protection and the lack of markers for sero-surveillance.
With African swine fever outbreak in China in 2018, China’s swine industry has suffered devastating destruction (Wang et al., 2018). After that, the waterfowl industry developed considerably in China. China has the largest waterfowl population worldwide. However, small-scale farms with poor biosecurity produce more than 70% of waterfowl in China, resulting in a higher risk of spreading NDRV. Furthermore, NDRV can spread vertically, as well as horizontally (Wang et al., 2020b). Therefore, NDRV infection is much more challenging to prevent and control in duck production.
In conclusion, two NDRV strains were isolated from duck farms during NDRV outbreaks in central China. Phylogenetic analysis revealed that Chinese NDRVs had formed two distinct clades. The both newly identified NDRV strains, which belong to clade 2, are current predominant strains. This study highlights the importance of continuous surveillance and evaluation of the epidemiology of NDRV in ducks. It improves the understanding of the genetic heterogeneity of NDRV in China, providing a foundation for developing effective prevention and control strategies for this persistent disease.