Discussion and conclusions
In recent years, PEDV has repeatedly broken out in China and has become
one of the deadliest pathogens infecting pig herds. Existing measures
such as immunization, biosafety, and feeding with tissues from diseased
animals cannot prevent its spread (James Lowe, 2014, Olanratmanee, E.O.,
2010). Although vaccines prepared with G1a subtype strains have been
developed and promoted for decades, it is precisely because of long-term
use that its shortcomings are exposed. The amino acid mutation rate of
the immunogen S gene of epidemic strains and vaccine strains has reached
as high as 10% (Seong-Hee Kim, 2015). The rapid development and
promotion of new vaccines based on G2b subtype antigens have effectively
prevented the spread of PEDV in China in the past 10 years. At present,
all the genotypes of PEDV exist in China.
Owing to the pressure of herd immunity, the S gene of the coronavirus
has frequently mutated, and changes in some amino acids have caused
antigenic drift and escape from the protection of existing vaccines
(Chun-Ming Lin, 2016). Therefore, periodic update of coronavirus vaccine
strains becomes urgent. However, even the latest PED variant vaccine
strain was isolated in 2011. For example, the antigenic difference has
been confirmed in the virus neutralization experiment between the
epidemic strain and vaccine strain of bovine coronavirus (Toru Kanno,
2013). Neutralization of the virus with hyperimmune serum showed that
there is only one serotype of PEDV. However, an increasing number of
field surveys show that a mismatch between virus sequence analysis and
serum neutralization test indicates a big difference between the
antigenicity of PEDV epidemic strain and the vaccine strain. To date,
the attenuated vaccine of the mutant strain continues to play a partial
protective role (Dongbo SUN, 2016). However, an increasing number of pig
farms, which have undergone normal immunization procedures continue to
have outbreaks of PEDV, and the isolated strains have confirmed it to be
highly pathogenic epidemic strain.
Following the occurrence of PED in the pig farm, the outbreak has spread
fast and has incurred huge losses; thus, a vaccine strain that matches
the that of the epidemic in the pig farm must be selected for the
immunization program. At present, there are only few PED vaccine strains
available in China, and pig farms will most probably use autogenous
inactivated vaccines for emergency immunization. Inactivated vaccines
mainly act through humoral immunity and can produce a large amount of
IgA. IgA antibodies are considered the most important protective
antibodies in pig colostrum and provide a protective effect for piglets
(En Zhang, 2020). Our research results show that although the vaccine
could produce high titers of IgA antibody, some pigs still shed virus in
the milk. This shows that the existing vaccine may not completely
neutralize the PEDV epidemic strain. PEDV is mainly transmitted through
feces and mouth, and the viral load carried in feces is very high. Data
show that 1 g of feces dissolved in 100 L of water is highly infectious
to pigs. Our continuous monitoring of the fecal virus shedding of
diseased sows found that they could intermittently shed virus for more
than two weeks; thus, it is particularly important to strengthen
disinfection and adopt biosafety measures. From the perspective of the
hazards of PEDV, before 2015, PEDV caused fulminant epidemics
nationwide. Since 2016, as China has gradually gained a clear
understanding of the prevention and control of PED, acute fulminant
epidemics have decreased significantly. It is worth noting that the
pathogenicity of highly pathogenic strains has not decreased.
To summarize, PEDV outbreaks on large-scale pig farms occurred from
regularly in the second half of 2020. The protective effects of
inactivated and attenuated live PEDV vaccines available on the market
are not satisfactory. The disease may develop despite elevated antibody
titers following immunization with vaccines, which indicates that the
existing vaccines may not provide adequate immune protection against
epidemic strains. This may be related to the overall health of the pig
herd and weak cross-protection between strains. The farm described in
this report, where the infection occurred, could finally control the
epidemic through immunization with autogenous inactivated vaccines,
biosafety measures, and strict cleaning and disinfection. Therefore, the
prevention and control of porcine epidemic diarrhea warrant
comprehensive prevention and control measures. By appropriate selection
of a suitable vaccine and effective management measures ideal prevention
and control may be achieved.