Researcher identified candidate genes to address PRRSV infiltration, replication and tissue clearance
This Checkoff-funded project proposed to identify pigs that have persistent porcine reproductive and respiratory syndrome virus (PRRSV) infections (aka. carrier pigs), which pose a continuing threat to production units. Viral survival is maintained because in some pigs the virus persists in lymphoid tissues (tonsil, lymph nodes) and can be shed occasionally due to stress, diseases or other factors. This shed virus then infects the remaining pigs, many of which are PRRSV-naïve and thus susceptible. Currently there is no way to accurately identify PRRSV-carrier pigs, nor are there procedures to treat pigs to eliminate persistent virus from their tissues. This proposal determined the frequency of pigs with persistent PRRSV by quantitating viral RNA levels in tonsil as a surrogate measure of viral persistence.
To perform this, researchers used the repository of samples collected through the Checkoff-funded PRRS Host Genetics Consortium (PHGC). Each PHGC weaned pig came from current commercial breeding stocks, was infected with PRRSV and followed for 42 days post infection (dpi). Tonsil tissue was archived for every pig that survived to 42 dpi. All data is preserved in the PHGC database, including the pig’s pedigree, response to PRRSV infection (serum viral and antibody levels and weight gain data), along with extensive genotypic information. 
Overall, nursery pigs infected with a less pathogenic PRRSV isolate, or that have higher tonsil viral level, have a stronger tonsil immune response. Mapping studies revealed several genomic regions that explained a proportion of genetic variance for tonsil viral levels. Several strong candidate genes were identified and may be involved with the host’s ability to control viral infiltration/replication and the ability to clear infected cells from tissue. These are useful targets for gene-expression analyses on tonsil tissue currently underway. These data may provide insight into alternative genes involved in host genetic control of tonsil virus levels and viral persistence.
“These findings contribute to our understanding of the mechanisms involved in tonsil pathology induced by PRRSV infection in pigs,” the researchers noted. “Based on this, efforts can be planned to selectively breed for pigs with lower tissue persistence or, alternately, to identify means of stimulating anti-viral responses in pigs with persistent PRRSV infections.” Click here for more details.
Unlocking answers to PRRSV-carrier pigs