Infection of macaque monkeys with simian immunodeficiency virus (SIV) has been established as an excellent animal model system for studying the pathogenesis of an HIV-like virus and for evaluating newly developed antiretroviral drugs and
vaccines. Based on their genetic, antigenic, and
biologic properties, the simian immunodeficiency viruses are the closest known relatives of the human AIDS viruses, and experimental
infection of macaque monkeys results in a disease that is remarkably similar to human
AIDS. Infected macaques show
diarrhea,
weight loss, hematologic abnormalities including
lymphopenia and
thrombocytopenia,
lymphadenopathy/lymphoid
hyperplasia that progresses to lymphoid depletion, immunosuppression with marked reduction in CD4+ cells and in the CD4+/CD8+ cell ratio, and
opportunistic infections. A majority of such macaques die from an
AIDS-like disease within one to three years of
infection. An acutely lethal variant of SIV has been identified that results in death in susceptible macaques within 7-12 days of
infection. Preliminary prophylactic treatment trials with AZT in macaque monkeys exposed to the acutely lethal SIV variant indicate that some protection is provided when AZT treatment is initiated within 24 hours of virus exposure. Other studies with the more chronic SIV
infection model, however, failed to show any prophylactic efficacy of CS-87, AZT,
D4T, or FDT.