R.R.R. Rowland, J. Nietfeld, R. Molina, J. Hermann, J. Zimmerman
PRRSV infections broadly impact the swine production system, contributing to a variety of syndromes, such as PRDC and PMWS. One outcome is the appearance of pigs that grow relatively slowly, so-called “poor-doers.” The objective of this study was to evaluate the effect of PRRSV infection in a relatively large population of experimentally infected pigs over a period of 200 days. The study was initiated by infecting 109 2-week-old pigs with VR-2332; 56 age-matched animals served as un-inoculated controls. Pigs were randomly assigned to either infected or control groups. At 2 week intervals, a subset of pigs was randomly removed, weighed, and necropsied. Serology and RT-PCR were used to monitor infection. Serology was performed on sera from control pigs. Results showed that all PRRS pigs were productively infected and that the control groups remained PRRS free throughout the study period. Within about two weeks after infection, a distinct subpopulation of pigs emerged within the infected group. Approximately 15% of pigs appeared to be smaller and had acquired a “rough” appearance. Weights at the time of necropsy showed a high degree of variability in the PRRSV group, but not in controls. For example, at day 133 PID, the range in weights was 242-271 lbs (mean=258, n=10) for the control group versus 150-251 (mean 217, n=5) for the PRRSV group. Microhistology of lymphoid and non-lymphoid tissues, serology, and PCR showed that all pigs were negative for circovirus infection.
These results describe a subpopulation of PRRSV-infected pigs that show defects related to growth. This effect was not observed in control pigs and could not be attributed to a secondary infection.