Assessment of Porcine Reproductive and Respiratory Syndrome Virus RNA Load in Sera and Tissues during Acute Infection

Porcine reproductive and respiratory syndrome virus (PRRSV) RNA load in sera and tissues during acute phase of infection was evaluated using a PCR- based quantitative assay. More than 80% of infected pigs (21/25) showed the peak level of viral RNA concentrations in serum (up to 8.6 x 108 copies/ml) at day 5 postinfection (PI), and started to clear the virus from the systemic circulation thereafter. Regression analysis using the viral RNA concentrations in sera obtained from days 5 to 14 PI showed that the viral RNA was cleared at the rate of 0.37 log reduction in the number of PRRSV RNA copies per day. It was estimated to be day 27 PI when the viral RNA in the serum of infected pigs becomes undetectable. When correlation analysis was performed between the systemic clearance rate and viral RNA concentrations in tissues of 9 infected pigs obtained at day 14 PI, moderately strong negative correlation was observed in the thymus (r = - 0.62) and brain stem (r = - 0.48), suggesting the capability of host animal to clear PRRSV from the systemic circulation appears to be related to the viral activity in the thymus and brain stem.

Localization of Porcine Reproductive and Respiratory Syndrome Virus Infection in Boars by In Situ Riboprobe Hybridization

The capability of porcine reproductive and respiratory syndrome virus (PRRSV) to be shed in semen for extended periods of time has been suggested to be a principal factor for viral transmission via insemination. In attempts to gain insights into the mechanism of PRRSV persistence in boars, tissue distribution and sites of viral infection were investigated by in situ hybridization (ISH) using digoxigenin-labeled RNA probe and the ISH results were compared with those of reverse transcription-nested polymerase chain reaction (RT-nested PCR). Animals were intranasally inoculated with 104 median tissue culture infectious dose of PRRSV VR-2332 and tissues collected at different times were examined. At day 7 postinfection, limited number of hybridization positive signals was observed in cells within or between seminiferous tubules in the testis sections while relatively abundant hybridization positive signals were observed in the brain stem and tracheobronchial lymph node. At later days of infection, hybridization positive signals were observed in cells within seminiferous tubules with much reduced frequency. Lack of agreement with the RT-nested PCR assay results in testis tissues obtained at days 14, 28, and 59 postinfection suggested that PRRSV infection in the testis may be extremely restricted, and may not necessarily constitute a major viral source in semen during extended periods of seminal shedding.