Laboratory findings
After preliminary confirmation of the PEDV pathogen by the pig farm
laboratory, fecal samples, milk and serum of 20 sows, as well as the
feces and intestines of three infected piglets were sent to South China
Agriculture University for sequence analysis and serological testing. In
order to reveal the molecular characteristics of the virus and determine
the relationship between the current epidemic strains in China and other
subtype strains, the Bootstrap method of MEGA6.0 software was used to
analyze the evolutionary tree of domestic isolates and reference strains
from different sources at different times, which indicated that the
strain formed a new and unique branch in the PEDV genotype II (Figure
2), which had the closest genetic distance to the genotype I strain. The
S1 gene homology comparative analysis of this strain showed that it had
98.1%-100% homology with 59 epidemic strains uploaded by NCBI in the
past year, 91.8% homology with CV777 strain, 97.8 homology with the
AJ1102 vaccine strain, and 97.3%-98.1% homology with the 89 strains
prevalent in 2014-2015 measured by our laboratory (Table 1), indicating
that the PEDV of the current outbreak had mutated. Please see
Supplementary Figure 1 for specific similarities. The similarity with
the 2020 strain was also poorer by 1-2% compared to the 2014-2015
strain, indicating that PEDV had continuously mutated.
To evaluate the immunizing effect of the PED vaccine in this farm, 20
diseased sows in farrowing season were randomly selected before the
immunization with autogenous inactivated vaccines and numbered 1-20.
Colostrum, feces, and serum were collected respectively to test the
levels of PED antigen and IgA antibody in colostrum, as well as the
levels of PED antigen in feces and specific IgG antibody in serum. The
results showed that the pigs had a good immune response after
immunization with the PED vaccine, with high levels of IgA and IgG
antibodies. However, for the three sows numbered 8, 11, and 20, despite
the high levels of IgA in milk and serum IgG antibodies, there were
still detectable PED antigens (Table 2). PED antigens were detected in
feces of more sows, indicating that the antibody levels produced by the
existing commercial vaccines cannot completely neutralize the new
epidemic strain of PEDV.
To evaluate the fecal virus shedding of sows during the disease course,
we conducted a two-week monitoring of virus shedding on 7 diseased sows.
The results showed that the affected sows had intermittent virus
shedding within two weeks after delivery. Some of the sows shed a great
load of virus due to individual differences (Table 3).