The Effect of Highly Virulent PRRSV-2 (L1C.5) Infection on Calcium Homeostasis and Bone Changes in Pigs Fed with Various Levels of Dietary Vitamin D

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major cause of disease in swine production, resulting in a high economic impact that has been exacerbated by the recent North American outbreaks of highly virulent PRRSV-2 strain (L1C.5). However, there is limited knowledge about how underlying systemic infections, particularly with this emergent PRRSV strain, affect calcium regulation and bone in pigs fed varying levels of vitamin D. To address this, the goal of this study was to determine the effect of different dietary vitamin D levels and PRRSV infection on calcium regulation and phenotypic changes in bone. Three-week-old pigs (n = 42) were assigned to four treatment groups: marginal dietary vitamin D3 (200 IU/kg) + PRRSV (n = 12), industry standard dietary vitamin D3 (1500 IU/kg) + PRRSV (n = 12), industry standard dietary vitamin D3 (1500 IU/kg) supplemented with 25-hydroxy-vitamin D3 (25-OH D3) (2000 IU/kg) + PRRSV (n = 12), or marginal dietary vitamin D3 without PRRSV inoculation (200 IU/kg) (control, n = 6). Following 26 days of dietary acclimation, assigned treatment groups were inoculated with a PRRSV-2 L1C.5 isolate. Blood samples were collected to evaluate the ionized calcium, 25-OH D3, calcium, phosphorus, and parathyroid hormone levels. The 2nd and 10th ribs were collected at 14 days post challenge for bone ash and density analysis, as well as examination of microscopic changes and scoring of the physis. High mortality was noted in all pigs infected with the virus, regardless of the vitamin D diet. Additionally, a significant depletion of serum calcium was observed at 7 DPC in infected animals, suggesting a high calcium demand at early stages of PRRSV infection. No significant differences in serum calcium, phosphorus, or ionized calcium concentrations were observed between dietary groups during the first 14 days of PRRSV-2 L1C infection. In pigs that succumbed to PRRSV at the early stage of infection, microscopic lesions of multifocal myelonecrosis were noted. This study provides the first report of microscopic changes of bone marrow necrosis and inflammation associated with PRRSV infection and demonstrates calcium dysregulation at the early stage of infection by this highly virulent PRRSV strain.