What is the significance of these results for producers? First, the most likely primary site for replication of the PRRS virus is lymphoid tissue, either tonsil and/or lymph nodes. The early colonization of lymphoid tissue and the persistence of PRRS virus in lymphoid tissue indicate that PRRS virus has the potential to modulate the immune response early in infection. The persistence of PRRS virus in lymphoid tissues and tonsil provides a mechanism for shedding of this virus in bodily secretions such as saliva.
The PRRS virus can be detected in lymph nodes, tonsil, lung and other tissues during an acute infection virus isolation, IHC and RT-PCR. Overall, RT-PCR is the most reliable technique for detection of the presence of PRRS virus, because the viral nucleic acid can be consistently detected in both acute and persistent infections. Diagnostic laboratories should include tonsil and lymph nodes as part of the diagnostic sampling scheme in addition to lung. Since it is difficult to determine at what stage of infection pigs may be infected, the inclusion of tonsils and lymph nodes and RT-PCR in the diagnostic repertoire will markedly improve the chances of a positive diagnosis.
Finally, the results of this study indicate that variability in the ectodomain of ORF5 occurs in acute and persistent infections. It is possible that a particular quasispecies colonizes the pig early in infection and mutations accumulate over time in the ectodomain of ORF5. The mechanism of how PRRS established persistence remains to be determined, but the virus does change over time within individual pigs.
For additional information please contact David A. Benfield, at benfield.22osu.edu or phone 330-263-3703.
