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.