Molecular Ecology and Natural History of Simian Foamy Virus Infection in Wild-Living Chimpanzees

Abstract

Identifying microbial pathogens with zoonotic potential in wild-living primates can be important to human health, as evidenced by human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2) and Ebola virus. Simian foamy viruses (SFVs) are ancient retroviruses that infect Old and New World monkeys and apes. Although not known to cause disease, these viruses are of public health interest because they have the potential to infect humans and thus provide a more general indication of zoonotic exposure risks. Surprisingly, no information exists concerning the prevalence, geographic distribution, and genetic diversity of SFVs in wild-living monkeys and apes. Here, we report the first comprehensive survey of SFVcpz infection in free-ranging chimpanzees (Pan troglodytes) using newly developed, fecal-based assays. Chimpanzee fecal samples (n = 724) were collected at 25 field sites throughout equatorial Africa and tested for SFVcpz-specific antibodies (n = 706) or viral nucleic acids (n = 392). SFVcpz infection was documented at all field sites, with prevalence rates ranging from 44% to 100%. In two habituated communities, adult chimpanzees had significantly higher SFVcpz infection rates than infants and juveniles, indicating predominantly horizontal rather than vertical transmission routes. Some chimpanzees were co-infected with simian immunodeficiency virus (SIVcpz); however, there was no evidence that SFVcpz and SIVcpz were epidemiologically linked. SFVcpz nucleic acids were recovered from 177 fecal samples, all of which contained SFVcpz RNA and not DNA. Phylogenetic analysis of partial gag (616 bp), pol-RT (717 bp), and pol-IN (425 bp) sequences identified a diverse group of viruses, which could be subdivided into four distinct SFVcpz lineages according to their chimpanzee subspecies of origin. Within these lineages, there was evidence of frequent superinfection and viral recombination. One chimpanzee was infected by a foamy virus from a Cercopithecus monkey species, indicating cross-species transmission of SFVs in the wild. These data indicate that SFVcpz (i) is widely distributed among all chimpanzee subspecies; (ii) is shed in fecal samples as viral RNA; (iii) is transmitted predominantly by horizontal routes; (iv) is prone to superinfection and recombination; (v) has co-evolved with its natural host; and (vi) represents a sensitive marker of population structure that may be useful for chimpanzee taxonomy and conservation strategies. Cross-species transmissions of infectious agents from primates to humans have led to major disease outbreaks, with AIDS representing a particularly serious example. It has recently been shown that humans who hunt primates frequently acquire simian foamy virus (SFV) infections. Thus, these viruses have been proposed as an “early warning system” of human exposure to wild primates. In this study, we have tested this concept by developing non-invasive methods to determine the extent to which wild chimpanzees are infected with SFV. We analyzed more than 700 fecal samples from 25 chimpanzee communities across sub-Saharan Africa and obtained viral sequences from a large number of these. SFV was widespread among all chimpanzee subspecies, with infection rates ranging from 44% to 100%. The new viruses formed subspecies-specific lineages consistent with virus/host co-evolution. We also found mosaic sequences due to recombination, indicating that chimpanzees can be infected with multiple viral strains. One chimpanzee harbored an SFV from a monkey species, indicating cross-species transmission in the wild. These data indicate that chimpanzees represent a substantial natural reservoir of SFV. Thus, monitoring humans for these viruses should identify locations where human/chimpanzee encounters are most frequent, and where additional transmissions of chimpanzee pathogens should be anticipated.