American Society for Microbiology

ASM News, April 2005, Volume 71, Number 4, Page 157


Origin of AIDS Revisited

The origin of the AIDS pandemic remains a mystery (ASM News, January
2005, p. 15). One theory holds that human immunodeficiency virus type 1
(HIV-1) evolved from a chimpanzee retrovirus, SIVcpz, that crossed the
species barrier in an unknown manner (F. Gao, E. Bailes, D. L.
Robertson, et al., Nature 397:436-441, 1999). This theory is based
primarily on the discovery of a chimp named Marilyn who was found to be
infected with a retrovirus that closely resembled HIV-1 (R. V. Gilden,
L. O. Arthur, W. G. Robey, et al., Lancet i:678-679, 1986). Extensive
testing in other chimps has so far failed to reveal similar infection in
more than a handful of animals, and the origin of SIVcpz itself remains
a mystery.

We have examined new information about chimps such as Marilyn who were
used in animal experiments or followed in the wild in the 1960s (W. D.
Hillis , Am. J. Hyg. 73:316-328, 1961; B. W. Allmond, Jr., J. E.
Froeschle, and N. B. Guilloud, Am. J. Epidemiol. 85:229-239, 1967;
J. Goodall, The Chimpanzees of Gombe: Patterns of Behaviour, Harvard
University Press, Cambridge, 1986; J. van Lawick-Goodall, In the Shadow
of Man, Houghton Mifflin Company, Boston, 1971). Chimps from African
vendors were sometimes inoculated intraperitoneally with pooled human
whole blood in order to "protect them from human diseases." This
practice was thought to be the cause of an infectious hepatitis outbreak
among chimp handlers in 1960 at Holloman Air Force Base in New Mexico,
the same location where Marilyn was housed. Thus inoculation with human
blood in Africa could have been the source of Marilyn's retroviral
infection. Furthermore, chimps in Africa and the United States suffered
from outbreaks of poliomyelitis, and these animals received an oral
polio vaccine derived from human diploid cell strains on at least one
occasion in 1967. Thus, chimps were exposed to blood products and
vaccines that could have transmitted an HIV-1 precursor virus that
evolved into SIVcpz (S. Corbet, M. C. Muller-Trutwin, P. Versmisse, et
al., J. Virol. 74:529-534, 2000). This virus would have been difficult
to transmit among chimps, as HIV-1 is not readily transmitted from
infected to uninfected animals (P. N. Fultz, C. Greene, W. Switzer,
B. Swenson, et al., J. Med. Primatol. 16:341-347, 1987). Hence the small
number of SIVcpz cases.

Sporadic exposure of chimps to human blood products and vaccines in the
1960s suggests that SIVcpz evolved in parallel with HIV-1, rather than
being its evolutionary parent. The common source of the human and chimp
retrovirus that spawned the AIDS pandemic remains to be elucidated
(B. Goldberg and R. B. Stricker, J. Theor. Biol. 204:497-503, 2000).

Billi Goldberg
International DNCB Study Group
San Francisco, Calif.

Raphael B. Stricker
California Pacific Medical Center
San Francisco, Calif.