Abstract
Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is one of the most economically important diseases, that has significantly impacted the global pork industry for over three decades, since it was first recognized in the United States in the late 1980s. Attributed to the PRRSV extensive genetic and antigenic variation and rapid mutability and evolution, nearly worldwide epidemics have been sustained by a set of emerging and re-emerging virus strains. Since the first modified live virus (MLV) vaccine was commercially available, it has been widely used for more than 20 years, for preventing and controlling PRRS. On the one hand, MLV can induce a protective immune response against homologous viruses by lightening the clinical signs of pigs and reducing the virus transmission in the affected herd, as well as helping to cost-effectively increase the production performance on pig farms affected by heterologous viruses. On the other hand, MLV can still replicate in the host, inducing viremia and virus shedding, and it fails to confer sterilizing immunity against PRRSV infection, that may accelerate viral mutation or recombination to adapt the host and to escape from the immune response, raising the risk of reversion to virulence. The unsatisfied heterologous cross-protection and safety issue of MLV are two debatable characterizations, which raise the concerns that whether it is necessary or valuable to use this leaky vaccine to protect the field viruses with a high probability of being heterologous. To provide better insights into the immune protection and safety related to MLV, recent advances and opinions on PRRSV attenuation, protection efficacy, immunosuppression, recombination, and reversion to virulence are reviewed here, hoping to give a more comprehensive recognition on MLV and to motivate scientific inspiration on novel strategies and approaches of developing the next generation of PRRS vaccine.
Highlights
Porcine reproductive and respiratory syndrome (PRRS), characterized as reproductive failure in breeding pigs and respiratory distress in pigs of all age, is one of the costliest diseases disturbing the global swine industry [1,2]
Since the replication of PRRS modified live virus (MLV) mimics that of wild-type virus, the host immune response resembles what occurs after a viral natural infection
In our previous experiment on evaluating the protection efficacy of MLV against the NADC30like virus, viremia of all three vaccinated groups were still detectable at 28 dpv (0 dpi), with the highest mean titer to 10−4 TCID50 /mL in the group of JXA1-R, an attenuated strain derived from HP-PRRS virus (PRRSV) [107]
Summary
Porcine reproductive and respiratory syndrome (PRRS), characterized as reproductive failure in breeding pigs and respiratory distress in pigs of all age, is one of the costliest diseases disturbing the global swine industry [1,2]. The PRRS MLV vaccine has been widely used for almost three decades (Table 1), and it is the major commercial vaccine that can successfully induce a protective immune response against the homologous virus and help in reducing the clinical sign and virus shedding during the heterologous viruses infection. It fails to confer sterilizing immunity against various field viruses and cannot provide solid protection against heterologous field strains [1,11,24,25]. Note: * A chimeric virus between the classical malicious PTK strain of PRRSV and HP-PRRSV strain, constructed by reverse genetic operation
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