Effects of low (modified-live virus vaccine) and high (VR-2385)-virulence strains of porcine reproductive and respiratory syndrome virus on pulmonary clearance of copper particles in pigs.

Seventy-five 3-week-old, crossbred pigs from a herd free of porcine reproductive and respiratory syndrome virus (PRSSV) were randomly assigned to three groups: uninfected controls, pigs inoculated intranasally with RespPRRS/Repro modified-live virus vaccine (RespPRRS), and pigs inoculated intranasally with a high-virulence strain of PRRSV (VR-2385). Pigs were intravenously infused with 3% copper phthalocyanine tetrasulfonic acid (0.2 ml/kg) in normal saline 30 minutes before necropsy, which was performed 3, 7, 10, 14, or 28 days postinoculation (DPI) with PRRSV. There were no differences in serum copper concentration in samples collected at 0, 15, or 30 minutes after infusion. Copper concentrations in the lungs of VR-2385-inoculated pigs were significantly lower than levels in the lungs of control and RespPRRS-inoculated pigs at 7, 10, and 14 DPI (P < 0.05). The greatest difference between the groups was observed at 10 DPI. Liver and spleen copper concentrations were slightly, but not significantly, higher in both PRRSV-infected groups. The percentage of lung affected by grossly visible pneumonia ranged from 0 to 5.6% in the RespPRRS-inoculated group and from 15.2 to 46.4% in the VR-2385-inoculated group, with lesions peaking at 7 and 10 DPI, respectively. PRRSV antigen was demonstrated in both pulmonary alveolar macrophages (PAMs) and pulmonary intravascular macrophages (PIMs) by immunohistochemical methods. Copper particles were demonstrated in the PIMs by light microscopy. PRRSV was isolated from bronchoalveolar lavage fluid of VR-2385-infected pigs from 3 to 28 DPI and from RespPRRS-inoculated pigs from 7 to 28 DPI. No PRRSV, PRRSV antibodies, or PRRSV-induced pneumonia was detected in the control group. These results suggest that 1) PRRSV has a detrimental effect on the uptake of copper particles by PIMs, 2) the severity of PRRSV-induced damage to PIMs differs among strains, and 3) demonstration of PRRSV-induced decreased pulmonary clearance supports the hypothesis that PRRSV infection may make pigs more susceptible to bacterial septicemia.