Belief that polio vaccine can spread AIDS has hampered the World Health Organization's efforts to stamp out polio. In Nigeria, several states recently banned use of the vaccine. Nigeria now has the highest number of polio cases in the world.
Although scientists agree that HIV comes from a chimpanzee simian immunodeficiency virus (SIV) that infected humans, when and how SIV jumped the species barrier has not been definitively determined.
The researchers found and sequenced genetic material from a new strain of SIV present in wild chimpanzees in the Kisangani region of the Democratic Republic of Congo (DRC).
The new SIV is not the one from which HIV-1, the virus responsible for the human AIDS pandemic, originated. The research shows that the new SIV resides on a different part of the SIV family tree.
One theory about the origin of HIV-AIDS alleges that tissues from chimps from the vicinity of Kisangani were used in preparing oral polio vaccine, thus letting the vaccine be contaminated by SIV.
"We found a new simian immunodeficiency virus, SIVcpzDRC1," said Michael Worobey, an assistant professor of ecology and evolutionary biology at the University of Arizona in Tucson. "It's a new variant of SIV, and it's not closely related to HIV-1." Finding the new strain of SIV in chimpanzees near Kisangani shows that HIV-1 did not originate from polio vaccines that were tested in that area in the 1950s.
The team's research article will be published in the April 22 issue of the journal Nature.
Worobey's coauthors are Beatrice H. Hahn, George M. Shaw, Mario L. Santiago, and Brandon F. Keele of the University of Alabama in Birmingham, Jean-Bosco N. Ndjango, Bernard L. Labama and Benoît D. Dhed'a of the University of Kisangani in the Democratic Republic of Congo, Jeffrey B. Joy of Simon Fraser University in Burnaby, British Columbia, and Andrew Rambaut of the University of Oxford and Paul M. Sharp of the University of Nottingham in England.
The research was funded in part by the Wellcome Trust, St. John's College Oxford, the Rhodes Trust, the Royal Society, the National Institutes of Health and the Howard Hughes Medical Institute.
Previous studies have linked all variants of HIV to the chimpanzee subspecies native to the countries to the west of the Democratic Republic of Congo (DRC).
However, genetic diversity of the pandemic strain of HIV, known as HIV-1 group M, is highest in the DRC.
So the DRC is an important place to look for clues about the origins of AIDS and specifically to test the oral polio vaccine/AIDS theory.
"I wanted not just to test the OPV/AIDS theory, but also to collect fundamental data from a crucial area," Worobey said.
Easier said than done.
Although recent technological advances have made it possible to test feces for RNA, the genetic material of SIV and HIV, one still has to collect the specimens.
Despite civil war in the region, Worobey traveled to the Democratic Republic of Congo in 2000 and in 2003 to collect fresh samples from wild chimps in the Kisangani area.
"The fresher, the better," he said. "Basically you want to pick it up the same morning they let it go."
He worked with a team of hunters that track the chimps at night using their calls. The team carefully stored small samples of feces in little screw-capped vials filled with a preservative.
The 2000 trip met with some calamities. Worobey contracted blood poisoning from a rattan palm spike that stabbed his hand. He hiked out of the forest to seek medical attention. His two companions, Jeffrey Joy and the well-known evolutionary biologist W.D. (Bill) Hamilton, came down with malaria. Hamilton later died from complications.
The feces collected on that trip didn't yield any SIV genetic material, although tests of chimpanzee urine suggested that virus from the SIVcpz family was present in some of the chimps.
On his 2003 trip, Worobey was luckier. Although the work was strenuous -- he hiked about 200 miles through the rainforest collecting samples -- health wasn't an issue. And he met with the head of security from one of the rebel groups, who gave Worobey "a very nice letter that was very handy indeed."
Even so, out of 97 different samples collected, only one contained any SIV. But that was enough -- the analysis revealed a brand-new strain, which Worobey and his colleagues named SIVcpzDRC1.
"Bill (Hamilton) died after that first trip, and that's why it was so nice to go back and finish what we started," Worobey said. "I was surprised that we finally got something after so much blood and toil and tears and sweat -- but not surprised where the virus fell on the phylogenetic tree."
Having the sequence of the new SIV's genetic material definitively shows that HIV-1 didn't come from chimpanzees in that area, he said.
His next step is learning more about SIV from that region.
"This is one of only a handful of places in the world where SIVs are known to be circulating in chimpanzees. We know HIV-1 comes from chimpanzees, but we know little beyond this," he said. "I want to study this geographic area -- get more samples, understand how the virus is maintained and transmitted in chimps, and understand why the virus is so pathogenic in humans but not chimpanzees."