In vivo Replication of Porcine Reproductive and Respiratory Syndrome Virus in Swine Alveolar Macrophages and Change in the Cell Population in Bronchoalveolar Lavage Fluid after Infection.

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Abstract
Replication of porcine reproductive and respiratory syndrome (PRRS) virus in swine alveolar macrophages (AM) and cell population in broncho-alveolar lavage fluid (BALF) obtained from PRRS virus-infected pigs were investigated. BALF samples were periodically collected from 6 pigs infected with PRRS virus and 3 non-inoculated control pigs by means of fiber-optic bronchoscope between post-inoculation day (PID) 0 and 56. The mean ratio of macrophages in BALF collected from infected group was 92.7 +/- 3.2% before inoculation and gradually decreased from PID 14. On the other hand, the ratio of lymphocytes was 4.8 +/- 3.2% before inoculation and increased from PID 21 and indicated 41.8 +/- 9.1% on PID 28. After that, they decreased gradually and that of macrophages correspondingly increased. The ratio of neutrophils maintained between 0.7% and 5.1%. The ratios of macrophages, lymphocytes and neutrophils collected from control group were almost stable through the examination. Intracellular PRRS virus antigens in AM were detected from PID 2 by indirect immunofluorescence assay (IIFA). PRRS virus was first isolated from BALF samples collected from inoculated group between PID 2 and 49. From serum, virus was isolated between PID 2 and 21. Antibodies in sera measured by IIFA to PRRS virus were first detected on PID 14 and the antibody titer rose to 1:640 or 1:1,280. The results suggested that PRRS virus replicates in swine AM for a relatively long period.

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