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.