Background:  Hepatitis B virus (HBV) is widely distributed among humans, but has also been found in wild-caught apes including chimpanzees, gorillas, orangutans, and gibbons. Previous phylogenetic analyses have placed the ape viruses within the radiation of human HBV genotypes, and a recent study has dated the common ancestor of all of these viruses at only 200 to 500 years ago. However, little is known about the prevalence and geographic distribution of HBV viruses in wild great apes. We have used a noninvasive approach to conduct the first comprehensive survey of HBV in wild-living African apes.

Methods:  Ape fecal samples (n = 2963) were collected from 68 different field sites throughout equatorial Africa. These were tested for HBV DNA, with full-length HBV sequences amplified from a subset of the positive samples.

Results:  We found that HBV DNA is readily detected in ape fecal samples collected under field conditions. Enzootic HBV infection in chimpanzees was documented at 32 field sites, and in gorillas at 8 field sites; no HBV infections were found in bonobos. Prevalence varied across sites and hosts: up to 100% of chimpanzee samples were positive at some sites, but no more than 20% of gorilla samples. Phylogenetic analyses of a genome fragment (1848 bp) encompassing the polymerase and preS1/S2 genes revealed that all newly identified HBV strains from chimpanzees (ChHBV, n = 68) clustered within four subspecies-specific clades, congruent with the mtDNA phylogeny of their respective hosts. Newly identified HBV strains from gorillas (GoHBV, n = 12) formed 3 clades, each falling within the radiation of viruses from the chimpanzee subspecies of the same region. These results were confirmed by analyses of full-length genomes from a subset (n = 30) of strains.

Conclusions:  These results indicate that HBV infection is enzootic in wild chimpanzees and gorillas at multiple field sites, but may be absent from bonobos. The ancestry of ChHBV strains is likely to date back many thousands of years, and may even reflect codivergence with chimpanzee subspecies over the past million years. GoHBV arose subsequently from chimpanzee-to-gorilla cross-species transmission, which has occurred on at least 3 separate occasions.