Virus quantification and identification of cellular targets in the lungs and lymphoid tissues of pigs at different time intervals after inoculation with porcine reproductive and respiratory syndrome virus (PRRSV).

Sixteen 6 week old conventional pigs were inoculated by aerosol with a European strain of porcine reproductive and respiratory syndrome virus (PRRSV). Virus replication was followed by virus titration and immunofluorescence in the lungs and in associated and distant lymphoid tissues at 3, 14, 21, 35, 42 and 82 days post-inoculation (DPI). PRRSV replication was detected in alveolar macrophages, lungs, tonsils, spleen, retropharyngeal lymph nodes, bronchial lymph nodes and thoracic aortic lymph nodes at 3 DPI. The same tissues, except retropharyngeal and thoracic aortic lymph nodes, were PRRSV positive at 14 DPI. Lungs and alveolar macrophages were PRRSV positive until 35 DPI. PRRSV was not detected in heart, peripheral blood mononuclear cells and bone marrow cells. Viremia was detected from 3 to 28 DPI. Not more than 2% of alveolar macrophages were PRRSV positive even during the acute stage of infection. 80 to 94% of the PRRSV infected cells in the lungs and in lung lavaged cells were identified as macrophages using a porcine macrophage specific monoclonal antibodies. In the lymph nodes and spleen, 100% of the infected cells were macrophages. Anti-PRRSV antibodies were detected by a blocking ELISA as early as 7 DPI. the antibody titre gradually increased to reach a geometric mean titre (GMT) of 160 at 35 DPI. It remained at that level until the end of the study. These findings clearly demonstrate that PRRSV has a tropism for macrophages. PRRSV mainly replicates in macrophages of the lymphoid tissues and lungs in the acute phase of infection and persists in the lung macrophages.