Equine infectious anemia virus is found in tissue macrophages during subclinical infection

The equine infectious anemia virus (EIAV) often results in lifelong subclinical infection following early episodes of clinical disease. To identify the cellular reservoirs of EIAV during subclinical infection, horses were infected with EIAV and allowed to develop subclinical infections. Horses with acute disease served as a basis for comparison. The tissue distribution, replication status, location of infected cells, and viral load were characterized by PCR for proviral DNA and reverse transcriptase PCR for viral RNA, in situ hybridization, and in situ PCR. Proviral DNA was widespread in tissues regardless of disease status. Viral gag and env RNAs were also detected in tissues of all horses regardless of disease status. Plasma viral RNA (viremia) could be detected in some, but not all, horses with subclinical infections. In situ assays determined that a primary cellular reservoir and site of viral replication during subclinical infection is the macrophage. During subclinical infection, viral load was decreased 4- to 733-fold and there was decreased viral RNA expression within infected cells. These data indicate that viral replication continues at all times, even in horses that are clinically quiescent. Moreover, restricted viral replication at the cellular level is associated with clinical remission.