PRRSV is an infectious disease that has long threatened China's pig industry and caused the greatest loss to China's pig industry. Large-scale pig farms can prevent and control the disease through comprehensive prevention and control measures combined with immunization, while small and medium-sized farms can only rely more on immunization. The biggest challenge in PRRSV immunity is that the disease mutates frequently, and the cross-protection between different strains is not ideal.
Table 1: The main PRRSV strains currently circulating in China.
According to the phylogenetic relationship of the ORF5 region, the PRRSV typing method based on phylogenetic lineage:
The PRRSV-1 strain is divided into four subtypes, subtype I (global), subtype I (Russia), subtype II and III).
The PRRSV-2 strain is divided into nine clades (L1-L9).
PRRSV-2 is currently widely prevalent and has a high degree of genetic diversity, and the 9 clades can be further divided into several subclades.
Among them, the L1 branch of PRRSV-2 is the most prevalent and diversified in the swine industry in the United States and Asia, with a global trend.
United States:In 2021, the L1 clade (NaDC30-Like and NaDC34-Like) remains the most prevalent and diverse lineage.
South Korea and Japan:L1 branch (18 sublineage;NADC30-like) is the second largest group of PRRSVs.
China:The L1 clade has become the dominant circulating strain in China (64%).
The pathogenicity of the L1 clade NADC30-like strains varies greatly, and some NADC30-like strains in China show high pathogenicity.
Currently, vaccines are widely used for the prevention and control of PRRSV.
In China, there are currently two types of PRRS vaccines: live attenuated vaccines and inactivated virus vaccines.
There are currently nine commercial vaccines used in China for the control and prevention of PRRSV infection, including live L8 clad vaccines (CH-1R, JXA1-P80, HUN4-F112, TJM-F92, GDR180, and PC) and live L5 clad vaccines (Respprrs MLV Ingelvac PRRS). MLV) and L8 branch inactivated vaccine (CH-1A strain), the application of these vaccines has played an important role in the prevention and control of PRRS in China, but there is no target for it in the worldNADC30 PRRSV-like L1 branch PRRSVof effective vaccines.
Harbin Veterinary Research Institute, Chinese Academy of Agricultural SciencesThe Swine Disease Integrated Prevention and Control Innovation Team and the Porcine PRRS and Pseudorabies Research Innovation Team were the first in the world to developL1 clade PRRSV vaccine candidate sd-rand evaluated the protective effect of the strain against homologous and heterologous strains.
The researchers measured a large number of NaDC30-like PRRSV whole genome sequencesThe analysis showed that NSP9 and GP2-3 were hotspots for NADC30-like PRRSV recombination in ChinaSubsequently, the NadC30 PRRSV-like SD strain, which had a small fragment recombination at two recombination hotspots and mild pathogenicity, was screened and weakened by subpassage, and the cytotoxicity of stable passage on MARC-145 cells was obtained (SD-R strain, F125 generation). Compared with the parent strain, the SD-R vaccine candidate produced a total of 31 stable mutations in amino acids and 4 nucleotides in the untranslated region
Due to the low genome-wide homology and complex recombination pattern among different Nadc30-like PRRSVs, the NADC30-like PRRSV HLJWK108-1711 strain was selected in this study (the recombination pattern is complex, and the genome-wide nucleotide homology with the SD strain is 89.).9%) for the evaluation of cross-protection effect.
High-dose immunization of the SD-R vaccine candidate is safe for piglets
Serum samples and 10 organ tissues were assessed by qPCR:
High-dose group: Serum samples were collected on the day after vaccination.
Repeat dose group: Serum samples were collected after vaccination and at 28 days.
Low levels of viral copies were detected only in serum samples 7 or 14 days of age after vaccination. In addition, low levels of copies of the virus have been detected in the lungs, tonsils, lymph nodes, and a small number of tissues(Figure 1).
Figure 1 Viral load in ten tissues or sera in the high-dose group (a) and (b) and the repeat dose (c) groupVaccine strain immunization does not cause viremia and significantly reduces viral load in blood and tissues compared with the control group after challenge
No viremia was detected in the immunized pigs after vaccination with the SD-R vaccine candidate, and the viral load in the blood was significantly reduced in the immunized group compared with the challenged control group after challenge, and the viral load in the tissues was significantly reduced (Fig. 2).
Fig. 2 Changes in viremia and tissue viral load in each group after challenge by NADC30 PRRSV SD strain and HLJWK108-1711 strainCompared with the control group, challenge after SD-R immunization could significantly improve the elevated body temperature and clinical manifestations
There was only a brief increase in body temperature in the immune-challenged group, which was significantly lower than that in the control challenged group. The unimmunized challenged group experienced a prolonged increase in body temperature (Figure 3).
Both the SD strain and the HLJWK108-1711 strain showed varying degrees of loss of appetite (daily weight loss) in the challenged group, while there was no change in the daily weight gain in the immune challenge group (Fig. 3).
Fig. 3 Changes in body temperature and body weight of NADC30 PRRSV SD strain and HLJWK108-1711 strain after challengeN protein antibody (mean SP less than 2.) was produced 7-14 days after SD-R immunization0), 7-10 days after challenge, the antibody level of piglets increased
Fig. 4 PRRSV antibody levels after challenge from NADC-like 30 PRRSV SD strain and HLJWK108-1711 strainPathological analysis showed that the lungs showed typical patchy consolidation and lymph node hemorrhage in the control group after the challenge, while there were no obvious pathological changes in the immune challenge group (Fig. 5).
Fig. 5 Pathological and histopathological changes of NADC30 PRRSV SD strain and HLJWK108-1711 strain after challenge.
Unlike CH-1A or HP-PRRSV, NADC30-like PRRSVs have low genome-wide similarity, and the original vaccine is not as effective as protecting against this strain.
Almost all NADC30-like PRRSVs are recombinant viruses. Although the recombination location was relatively random, statistical analysis showed thatThe nucleotide positions of the recombination hotspots are approximately 7900 to 8100 (NSP9) and 12400 to 13500 (GP2-GP3).。The researchers selected NADC30-like PRRSV SD with the two recombinant regions described above (7365-7661 in the NSP9 region and 12305-12773 in the GP2-GP3 region) to be subpassaged in MARC-145 cellsThe vaccine candidate strains have the following characteristics
Good stability;It is protective against different NADC30 PRRSV-like strains;
High dose and repeat dose trials based on SD-R are safe for all piglets, including no fever, no clinical signs, and only a small number of immunized piglets detected low levels of viremia in the lungs, tonsils, and lymph nodes;
Good protection.
The vaccine candidate has the potential to develop a commercial vaccine for the prevention and control of NADC30-like PRRSV.
The company has developed fluorescent PCR reagents and antibody detection reagents for pig diseases, poultry diseases, ruminants and aquatic animals, with supporting fluorescence quantitative PCR instruments, nucleic acid automatic extractors, microplate readers and other equipment, which can accurately diagnose animal infectious diseases and guide disease prevention and control.