In the monkey house at the zoo, one inmate is a slender, long-limbed medium primate with a dark short coat, white collar, and a tail that arches over the full length of its back and head. The sign on its cage reads:

Sooty Mangabey monkey (Cercocebus ATMs)

Habitat: Rain forests of Equatorial West Africa

(Cameroon, Guinea, Gabon)

Lives mainly on leaves, seeds, twigs; preyed upon by leopards.

A much larger, tailless, longer-haired, hulking brute of a man-like ape is identified on its cage as:

Chimpanzee (Pan troglodytes)

Habitat: Savannas and forests across tropical

Africa, from west to east.

Lives mainly on fruits, leaves, seeds, meat of small animals; preyed upon by lions, leopards, hyenas, wild dogs.

Both of these zoological rap sheets are missing an all-important last word - man.

Homo sapiens is today the prime predator of both primate species. Ape meat commands premium prices in upscale restaurants all over central Africa. Sooty mangabeys are hunted not only for what's sold as "bushmeat," but as pets. Chimpanzees, slaughtered by the thousands for the meat trade, are threatened with extinction.

The hunters who butcher them in the wild, without benefit of sanitary inspection, face a form of poetic justice: They risk catching the viral infections carried by their prey's blood - namely, the HIV virus. Mangabeys harbor the human HIV-2 strain; chimps, HIV-1.

History is replete with examples of such zoonosis - animals transmitting their infections to humans.

Hence, when the global AIDS pandemic erupted in the early 1980s, field virologists and epidemiologists surmised that wild African primates, notably mangabeys and chimpanzees, were likely reservoirs of HIV, the human immunodeficiency virus.

Chimps Carry HIV-1; Mangabeys, HIV-2

Actually, the virus exists in two primary persuasions, HIV-1 and HIV-2. The former is the source of the explosive spread of the pandemic, which infects some 35 million human carriers worldwide today. The latter, HIV-2, affects a much more limited human population, mainly in the West African homeland of wild sooty mangabey monkeys. They are infected with a simian immunodeficiency virus (SIV), which virologist Beatrice Hahn, at the University of Alabama at Birmingham, recently linked to human HIV-2.

No such link has been found until now between the wild chimpanzee's SIV and the viral reservoir for HIV-1 it's suspected of being. What made this uncertainty more than academic, Hahn pointed out, is the fact that chimps don't come down with any AIDS-like symptoms. "How infected chimpanzees resist disease, she observed, may lead to new strategies for designing HIV drugs and vaccines," Hahn said.

Hahn - a pioneer explorer for HIV animal reservoirs in Africa - is senior author of a research article in this week's issue of Nature, dated Feb. 4, 1999. Its title: "Origin of HIV-1 in the chimpanzee Pan troglodites troglodites." Its data, her paper sums up, "indicate strongly that HIV-1 infection of humans occurred as a cross-species transmission of [chimpanzee] SIV from P. t. troglodites."

Nature advanced its media embargo on Hahn's article to coincide with her presentation of the study's data Sunday night, Jan. 31, at the opening session of the Sixth Conference on Retroviruses and Opportunistic Infections, in Chicago. She spoke on the theme: "The Origin of HIV-1: A Puzzle Solved?"

An accompanying "News & Views" commentary in Nature, aptly headed "From Pan to pandemic," remarked that "By 1996, 198 chimpanzees had been experimentally infected with HIV in six of the major research institutions in the U.S. ... Only three chimps infected with HIV-1-related viruses in the wild have been documented, one of them only weakly." Primates born in captivity, or captured early in life, before becoming sexually active, rarely harbor SIV infection.

Solved By A Chimp Named Marylyn

When Hahn and her international co-authors recently identified a fourth SIV-infected chimp, they decided to compare all four viruses with various HIV-1 isolates from human patients.

Three of the four primate hosts belonged to a West African subspecies. The third of these chimps, Noah by name, was infected with an oddball virus unlike HIV-1; he belonged to an East African subspecies, Pan troglodites schweinfurthii.

That fourth DNA specimen came from a female chimp named Marylyn. She was wild-caught in Africa circa 1960, and brought up from infancy at a U.S. primate-breeding center in New Mexico. Marylyn died in 1984, at age 26, from complications of giving birth to still-born twins. She had never showed symptoms of AIDS infection, but in an immunological survey of 98 chimpanzees in 1985, she alone carried antibodies strongly reactive with HIV-1.

PCR-reconstituted genomic fragments revealed a gene found only in HIV-1 and SIV-chimp viruses. Hahn's team dubbed their complete proviral genome SIVcpzUS. Mitochondrial analysis of this genomic strain enabled them to determine the subspecies identity of all known SIVcpz-infected chimpanzees.

"All HIV-1 strains known to infect man," Hahn told her Chicago audience, "are closely related to just one of these SIVcpz lineages - that found in the West African P. t. troglodites." And she drew the inference that P. t. troglodites is the natural host and reservoir for HIV-1.

What cinched this long-sought identification of the viral reservoir in the African wild, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, "is that the natural habitat of these chimpanzees directly coincides with the pattern of the HIV-1 epidemic in this area of Africa."

However, given the small size of the sample population studied, Hahn recommended that, "to understand the full extent of natural SIVcpz infection, and the frequency of zoonotic transmission to humans, it will be necessary to screen free-living adult chimpanzees of all four subspecies as well as human populations from corresponding geographic locales."

Which is precisely the project that she is now embarked upon, Hahn's administrative assistant, Jennifer Wilson, told BioWorld Today. "She is now trying to obtain samples from those areas through her collaborators, to identify animals in the wild that are HIV-1-antibody positive," Wilson said. n