Background:  Simian immunodeficiency virus (SIV) from West Central African chimpanzees (Pan t. troglodytes) and western gorillas (Gorilla g. gorilla) are the progenitors of HIV-1. HIV-1 M and N ancestors can be traced to distinct chimpanzee populations in southern Cameroon and gorillas from Cameroon are infected with SIV_gor, related to HIV-1 O. Here we report more in detail the geographic distribution of SIV_gor infections in wild gorillas.

Methods:  Fecal samples were collected at 1 field site in Democratic Republic of Congo (DRC) (n = 28) and 6 in Cameroon (n = 926), including 3 sites where SIV_gor⁺ gorillas were previously identified. These 3 sites are separated by 200 and 400 km and cover the G. g. gorilla distribution in Cameroon. Samples, preserved in RNA-later, were tested for SIV_gor antibodies with the INNO-LIA HIVI/II Score Assay. RNA was extracted from antibody positive samples and real-time polymerase  chain reaction (RT-PCR) using consensus env (gp41) and pol primers was attempted. Species identification was based on field data and confirmed by mtDNA analysis. Microsatellite analysis determined the number of infected individuals.

Results:  Among the 926 fecal samples from Cameroon, 158 were from chimpanzees and 764 from gorillas, all from the G. g. gorilla area. Only in 1 field site were new antibody-positive animals identified:  of 156 gorilla samples, 21 were from the area where 2 SIV_gor-infected gorillas had been previously reported, and of 21 chimpanzee samples, 2 came from the same area. Microsatellite analysis revealed that the 21 positive gorilla samples represented 8 different animals, all new individuals not previously collected. For 3 gorillas, 1 or more genomic fragments could be amplified. New SIV_gor sequences clustered with previously characterized SIV_gor strains from that area and are most closely related to SIV_gor-CP684. All new SIV_gor⁺ samples are collected in a small area (<10 km²), but additional field data on home ranges, nesting sites, and group sizes of the gorilla communities present in the area, show that SIV_gor circulates in at least 2 different troops. All samples from eastern gorillas (G. beringei), collected in the DRC, were antibody negative.

Conclusions:  New and previous field studies indicate an uneven distribution of SIV_gor in western lowland gorillas (G. g. gorilla) throughout southern Cameroon and suggest that ape reservoir(s) that gave rise to HIV-1 O could occur outside Cameroon. However, we identified an area of high SIV_gor endemicity in southwest Cameroon. More eastern gorillas need to be tested to allow significant conclusions about their SIVstatus.