Background:  Sooty mangabeys harbor simian immunodeficiency viruses (SIV_sm) that have crossed to humans on at least 7 occasions to yield the diverse groups of HIV-2 (A-G). However, data on SIV_sm genetic diversity in wild SM populations is limited, and it is unclear how SIV_sm infection is maintained in its natural host. In 1996, a single troop of ~120 sooty mangabeys was habituated in the Tai National Forest, Ivory Coast, where behavioral studies revealed a stable and linear female dominance hierarchy.

Methods:  Fecal samples (n = 39) were collected from 36 wild-living sooty mangabeys (28 females, 8 males) from June to September 2002. Total RNA was extracted and subjected to RT-PCR using conserved primers in gag (800 bp) and gp43 (430 bp) regions. PCR products were sequenced and resulting sequences utilized for phylogenetic tree constructions. Prevalence calculations utilized a previously reported fecal viral RNA (vRNA) detection sensitivity of 50%.

Results: Of the total, 12 sooty mangabeys (9 females, 3 males) were fecal vRNA-positive, suggesting a 64% (95% CI: 0.39 to 0.93) SIV_sm prevalence in this troop. Phylogenetic analysis of amplified sequences identified 6 new SIV_sm lineages that varied by 8 to 17% and 7 to 25% in gag and gp43 nucleotide sequence, respectively. Two mother-infant pairs had nearly identical sequences, varying by 0.3 to 1.2% in gag and gp43. One strain exhibited discordant branching orders in gag and gp43, suggestive of recombination. Interestingly, there was an excess of higher ranking females among those that were found to be fecal vRNA positive (c² = 4.1, p <0.05). While highly divergent, the new SIV_sm strains formed a monophyletic clade with 2 previously described viruses from Ivory Coast, and the 2 major groups of HIV-2 (A and B) were more closely related to this clade than to SIVsm from other regions. 

Conclusions:  These findings document for the first time a high prevalence of SIV_sm infection in a wild SM population and reveal a considerable level of genetic diversity within a single sooty mangabey troop. The detection of epidemiologically linked SIV_sm strains in 2 mother/infant pairs provides the first evidence of vertical SIV transmission in the wild. The observation of an excess of higher ranking females among those that are vRNA positive lends support to the conclusion that SIV_sm infection is generally harmless in its natural host. Finally, the phylogenetic clustering of HIV-2 groups A and B with the Ivory Coast SIV_sm strains may point to the geographic origin of these human infections.

Keywords: SIV; sooty mangabey; genetic diversity