1 Introduction
Avian orthoreovirus (ARV) is non-enveloped double-srtanded RNA (dsRNA)
virus, which is classified into the genus Orthoreovirus , familyReoviridae (Jones, 2000). The ARV genome is about 23 kilobase and
contains ten dsRNA segments, including three L-class (L1, L2 L3), three
M-class (M1, M2, M3) and four S-class (S1, S2, S3, S4), which encode
eight structural proteins (λA, λB, λC, μA, μB, σA, σB, σC,) and four
non-structural proteins (μNS, p10, p17, σNS) (Nick et al., 1975; Joklik,
1982; Su et al., 2006). ARV can infect a variety of poultry including
chicken (Tang and Lu, 2015a), duck (Malkinson et al., 1981), goose
(Vilmos et al., 2003; Yun et al., 2012), pigeon (Vindevogel et al.,
1982) and other domestic poultry, and has been associated with a variety
of clinical symptoms (Jones and Guneratne, 1984).
In recent years, there have been reports of new genomic variations in
these novel ARV strains isolated from vaccinated broiler flocks (Su et
al., 2006; Tang and Lu, 2015b; Tang et al., 2015; Hao Chen et al.,
2019). Since the first report of Muscovy duck infection with ARVs in
China in 1997, waterfowl-derived ARVs have already spread in most parts
of China. And in the process of epidemic, mutants with different
pathogenic characteristics appeared. The representative strains mainly
including a new type of Muscovy orthoreovirus (N-MDRV) that causes
necrotic and bleeding spots in the liver of Muscovy ducks, also known as
liver white spot disease, DRV that causes hemorrhagic symptoms of
multiple organs (Zheng et al., 2016), DRV causes hemorrhagic necrotic
hepatitis which characterized by liver necrosis and bleeding in infected
ducks (Z. Chen et al., 2012), and new type duck orthoreovirus (N-DRV)
that causes necrosis in the spleen of Pekin ducks (Zheng et al., 2016).
Since the above strains are viruses that have appeared in Chinese duck
flocks in recent years, the current research on DRV is far less
comprehensive than the chicken-derived ARV. DRV needs to continue
in-depth research in terms of virus transmission mode, gene-function
relationship, antigen characteristics and so on.
Since 2017, a new epidemic disease characterized by spleen necrosis and
swelling has emerged in China’s main meat duck breeding provinces, which
mainly causes large groups of ducks with poor mental state, appetite
loss and even death (Wang et al., 2019). Since the outbreak of the
disease, the incidence of this new disease has been rising rapidly and
expanding geographically. The mortality rate of infected ducks can reach
50-60%. Due to the necrosis of spleen, the weakened immunity of the
body is easy to cause secondary infection, which brings great economic
losses (Montgomery et al., 1986). This disease has become one of the
major diseases that seriously harm the development of China’s duck
breeding industry.
Regarding the study of ARV transmission methods, researchers have
confirmed that ARV can be transmitted vertically through eggs as early
as 1970s (Menendez and Cowen, 1975), and carried out research on the
protective effect of maternal antibodies (Heide, 2000). But at the same
time, some researchers believe that the rate of transmission through egg
is extremely low, less than 2%. Regarding the transmission method of
N-DRV, we had determined that healthy ducks can cause infection through
ingestion of fecal contaminated water and feed, or direct contact with
ducks infected with N-DRV and cause respiratory infections, which shows
N-DRV is spread horizontally (Wang et al., 2020). The epidemiological
survey results of wild waterfowl show that N-DRV can be detected in the
wild waterfowl feces such as wild ducks and swans, indicating that the
migration of wild waterfowl may play an important role in the spread of
N-DRV. However, there is no definite report on whether N-DRV can spread
through vertical transmission.
In order to investigate whether N-DRV can vertically spread in breeding
ducks, through experimental infection of healthy breeding ducks during
the laying period, a N-DRV artificial infection animal model was
successfully established (Chen et al., 2016). Through the hatching of
eggs, the detection of pathogens in hatching eggs, embryos, and
ducklings, we found that the fertilization and hatching rates of the
ducks in the infected group were lower than those in the control group,
and the samples of hatching eggs, embryos and ducklings in the infected
group were all tested positive for N-DRV. This discovery provides
detailed evidence for the vertical transmission of N-DRV and provides an
important reference for the prevention and control of the disease.