In vitro susceptibility of macrophages to porcine reproductive and respiratory syndrome virus varies between genetically diverse lines of pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be responsible for financial losses in the swine industry worldwide. It remains undetermined whether genetic variability of the host in susceptibility to PRRSV exists and if this variability can be exploited to help control this important disease. The objective of this study was to determine if an in vitro flow cytometry (FACS) assay that detects the percentage of monocyte-derived macrophages (MDM) infected with PRRSV could be utilized to demonstrate genetic variability in the susceptibility between distinct lines of pigs. Over 400 growing pigs from six genetic lines maintained in a single commercial breeding herd were screened using an in vitro FACS assay. From this initial screening, two genetically diverse lines of pigs that were also divergent in their FACS results were selected for further study. An additional 264 pigs from these two lines were subsequently tested for in vitro susceptibility to PRRSV. As in the preliminary screening, the Large White line had significantly higher average percent positive MDM over the Duroc-Pietrain synthetic line. This report suggests a genetic component for susceptibility to PRRSV exists and that the in vitro assay may be useful in predicting the relative susceptibility to PRRSV in large groups of animals.