4. Discussion
Since 2017, the prevalence of duck spleen necrosis and swelling has occurred in the main meat duck breeding provinces in China, causing infected ducks growth retardation, the feed return rate drops (Wang et al., 2019), and causing immune suppression to be susceptible to other disease (Montgomery et al., 1986). In previous studies, the epidemiological investigation of serum antibody of ducklings found that there was a potential N-DRV infection in the breeding ducks. In order to confirm that N-DRV can be transmitted vertically by breeding eggs, we carried out the artificial infection experiment of breeding ducks.
In the artificial infection test, it can be found that N-DRV infection can cause obvious ovarian atrophy and bleeding, fallopian tube bleeding, and typical spleen swelling and necrosis of the breeding duck, and the fertilization rate and hatching rate of the eggs are significantly reduced. The results of viral RNA detection of Ovary, fallopian tube, egg yolk membrane, duck embryo, duck spleen samples revealed that the N-DRV can spread vertically through the eggs. Although there are still some duck embryos that can be hatched normally, and there are no obvious clinical symptoms in the ducklings, but the spleen of the ducklings shows necrotic lesions of different degrees in the spleen, and the ducklings are not as good as healthy ducks in terms of growth rate and feed return rate (Landman et al., 1994). The damage of immune organs also poses a greater risk for the prevention and control of other epidemics in the later stage (Kibenge et al., 1987). In the comparison of infection routes, the orally admins group has a more significant pathogenic effect than the intramuscular injection group, which is consistent with the main routes of ARVs infection ingesting virus-contaminated feed and drinking water mentioned in previous studies (Landman et al., 1994).
DRV continues to be popular among Chinese ducks, causing various diseases including liver necrosis, splenic necrosis, and hemorrhagic lesions, causing serious economic losses to the Chinese duck farming industry (Z. Chen et al., 2012; Zheng et al., 2016; Wang et al., 2019). In the previous epidemiological survey, we found that the DRV infection rate of duck flocks in some parts of China (Shandong, Hebei, Jiangxi, Inner Mongolia, Guangxi, Jiangsu, and other provinces) had reached 22.8%, and the DRV infection rate of breeding ducks in some areas exceeds that of meat ducks (Zhang et al., 2013). In another study five years later, it was found that the detection rate of DRV in spontaneous disease ducks reached 38.54%, and the detection rate of DRV in clinically healthy ducks also reached 15.5%, and the co-infection of DRV and other pathogens was widespread (Yang et al., 2018). In terms of seasonal popularity, the incidence of DRV in summer was significantly higher than in other seasons.
In most cases, after the ducks are infected with DRV, these ducks do not show typical symptoms and lesions, and present subclinical infections (Li et al., 2016). Only through serological testing can the pathogenic infections be detected. This study found that there were high antibody positive rates in breeding ducks and meat ducks in some areas of China. Since the sampled ducks and meat ducks had not been immunized with N-DRV related vaccine products, it means that N-DRV antibody positive was a natural infection resulting in. This also means that N-DRV infection was already widespread in breeding ducks and meat ducks in some regions, and the infection situation was already very serious. The positive rate of infection of breeding ducks was higher than that of meat ducks, which may be related to factors such as feeding cycle. Infection with ARV reduces the body’s immune function, leading to increased susceptibility to other infectious diseases (Kibenge et al., 1987). When vaccinating ducks with other epidemic vaccines, it is easy to cause the immunization effect to fail to reach the standard. Practitioners in the aquaculture industry need to correctly understand and scientifically prevent and control N-DRV infection.
N-DRV is widespread in nature and has strong tolerance to the environment (Heide, 2000). In addition, the virus can spread through horizontal and vertical transmission, making it difficult to prevent and control the disease. Immunization of the breeding ducks is an effective means of controlling the disease, which not only enables the breeding duck population to gain resistance to the virus, but also protects the ducklings through the maternal antibody route and limits the spread of the virus through the vertical transmission route (Menendez et al., 1975). Based on this, the development of vaccines has a very important application value. In addition, the continuous viral shedding in cloaca indicates that feces were non-negligible source of N-DRV infection, prompting us that comprehensive and effective biosecurity measures have a positive effect on the prevention and control of the disease. Carrying out population purification will help reduce or even eliminate the harm of the disease to the duck farming industry.