AIDS Weekly 2 April 2001

African green monkeys do not develop AIDS despite high levels of the simian immunodeficiency virus (SIV) in their blood.

This surprising finding challenges some commonly held beliefs about how the virus and its cousin, HIV, cause disease.

Researchers from the Southwest Foundation for Biomedical Research and Emory University reported their results in the March 2001 issue of the Journal of Virology.

SIV and HIV have long been thought to cause disease by replicating at a high rate inside immune cells, eventually overtaxing the immune system and causing it to fail. This model of AIDS is supported by evidence that progression to AIDS in humans is measured by a decline in CD4 immune cells, which is associated with high levels of the virus in the blood. Antiviral drug treatment, which reduces the level of virus in the blood, stops and most often reverses progression of the disease.

Scientists have known for years that African green monkeys can be infected with SIV and not develop AIDS but it was believed that this was due to effective immune system control of the virus, which would appear as a low viral load in the blood.

S. Broussard and colleagues, though, found high levels of the virus in the blood of all the infected monkeys ("Simian immunodeficiency virus replicates to high levels in naturally infected African green monkeys without inducing immunologic or neurologic disease," Journal of Virology, 2001;75:2262-2275).

They also found high levels of viral replication in the central nervous system, believed to be the primary cause of neurological wasting, without any symptoms in the monkeys.

"These data clearly indicate that high levels of viremia and high rates of virus replication do not necessarily lead to disease and that specific host factors or the particular nature of the host response plays a critical role in determining whether disease arises following infection," said the researchers.