Identification of factors associated with virus level in tonsils of pigs experimentally infected with Porcine Reproductive and Respiratory Syndrome virus

Authors: HESS, ANDREW - Iowa State University; Lunney, Joan; ABRAMS, SAMUEL - Former ARS Employee; CHOI, IGSEO - Former ARS Employee; TRIBLE, BR - Kansas State University; HESS, MELANIE - Iowa State University; ROWLAND, RRR - Kansas State University; PLASTOW, G - University Of Alberta; DEKKERS, JCM - Iowa State University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2018
Publication Date: 6/14/2018
Citation: Hess, A.S., Lunney, J.K., Abrams, S., Choi, I., Trible, B., Hess, M.K., Rowland, R., Plastow, G., Dekkers, J. 2018. Identification of factors associated with virus level in tonsils of pigs experimentally infected with Porcine Reproductive and Respiratory Syndrome virus. Journal of Animal Science. 97:536-547. https://doi.org/10.1093/jas/sky446.
DOI: https://doi.org/10.1093/jas/sky446

Interpretive Summary: Porcine Reproductive and Respiratory Syndrome (PRRS) is a globally important swine disease from both an economic and animal welfare standpoint but it can be a persistent infection. Tonsils are a main site of persistence; PRRS virus can be detected in some pigs out to 150 days post infection (dpi). A means of preventing, or identifying, pigs with persistent infection is desperately needed. Unfortunately, no host genomic regions with major effects on tonsil virus level were identified in our study. The level of tonsil virus was found to be associated with traits related to viral clearance from serum: pigs with low tonsil virus levels had an earlier and faster rate of maximal serum viral clearance, lower total serum viral load, and lower viremia level at 35 or 42 dpi. Thus, selection for viral clearance traits in serum may reduce PRRSV persistence in the tonsil across PRRSV isolates.

Technical Abstract: Porcine Reproductive and Respiratory Syndrome (PRRS) is one of the most important global swine diseases from both an economic and animal welfare standpoint. PRRS has plagued the US swine industry for over 25 years and containment of PRRS virus (PRRSV) has been unsuccessful to date. The primary phase of PRRS, tracked by serum viremia, typically clears between 21 and 42 days post infection (dpi) but tonsils are a main site of PRRSV persistence and PRRSV can be detected in tonsils in excess of 150 dpi. Measuring tonsil virus levels at late stages of infection (6-7 weeks post infection) can be used to assess tonsil persistence, as levels of virus in tonsil at this time likely influence how long the virus will remain in the tissue. Tonsil virus levels were measured on pigs experimentally infected with either the NVSL-97-7895 (NVSL; n=524) or KS-2006-72109 (KS06; n=328) PRRSV type 2 isolates across five trials. The objectives of this study were to 1) estimate the heritability of tonsil virus levels at 35 or 42 dpi; 2) identify factors that affect tonsil virus level, including serum viremia; 3) identify genomic regions associated with tonsil virus level; and 4) compare results for the two PRRSV isolates. Tonsil virus level was lowly heritable for both isolates (NVSL: 0.05±0.06; KS06: 0.11±0.10). Level of tonsil virus was phenotypically associated with traits related to viral clearance from serum: pigs with low tonsil virus levels had an earlier and faster rate of maximal serum viral clearance, lower total serum viral load, and lower viremia level at 35 or 42 dpi. Although no genomic regions with major effects on tonsil virus level were identified, several showed some association (>0.1% of total genetic variance in the NVSL-infected dataset, the KS06-infected dataset, and the combined dataset). These regions contained the genes CCL1, CCL2, CCL8, HS3ST3B1, GALNT10, TCF7, C1QA/B/C, HPSE, G0S2 and CD34, which are involved in viral infiltration or replication, immune cell migration and viral clearance from tissue. Results were similar between the two PRRSV isolates. In conclusion, selection for viral clearance traits in serum may reduce PRRSV persistence in the tonsil across PRRSV isolates. However, genetic correlations need to be estimated to determine whether this will be successful.