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.