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.