Occult Hepatitis B Virus Infection in Chacma Baboons, South Africa

Caroline Dickens, Michael C. Kew, Robert H. Purcell, and Anna Kramvis

Author affiliations: University of the Witwatersrand, Johannesburg, South Africa (C. Dickens, M.C. Kew, A. Kramvis); Groote Schuur Hospital, Cape Town, South Africa (M.C. Kew); National Institutes of Health, Bethesda, Maryland, USA (R.H. Purcell)

Abstract

During previous studies of susceptibility to hepatitis B virus (HBV) infection, HBV DNA was detected in 2/6 wild-caught baboons. In the present study, HBV DNA was amplified from 15/69 wild-caught baboons. All animals were negative for HBV surface antigen and antibody against HBV core antigen. Liver tissue from 1 baboon was immunohistochemically negative for HBV surface antigen but positive for HBV core antigen. The complete HBV genome of an isolate from this liver clustered with subgenotype A2. Reverse transcription PCR of liver RNA amplified virus precore and surface protein genes, indicating replication of virus in baboon liver tissue. Four experimentally naive baboons were injected with serum from HBV DNA–positive baboons. These 4 baboons showed transient seroconversion, and HBV DNA was amplified from serum at various times after infection. The presence of HBV DNA at relatively low levels and in the absence of serologic markers in the baboon, a nonhuman primate, indicates an occult infection.

Hepatitis B virus (HBV) is a 3.2-kb partially double-stranded virus belonging to the family Hepadnaviridae. The outcome of infection with this virus is determined mainly by the immune response of the host and can be acute, chronic, or occult. HBV is divided into 9 genotypes (A–I); an additional genotype, J, has also been proposed (1–3). Several genotypes are further divided into subgenotypes. In sub-Saharan Africa, subgenotypes A1 and D3 and genotype E circulate (4).
Hepadnaviruses can infect avian and mammalian hosts but have a limited host range, infecting only their natural hosts and a few closely related species. Naturally occurring infections have been found in several Old and New World nonhuman primates, such as chimpanzees (5), gorillas (6), gibbons (7), orangutans (8) and woolly monkeys (9). HBV, whose natural host is humans, also infects chimpanzees (10); Barbary macaques (11); and tree shrews (12), in addition to humans.
Baboons (Papio species) have been proposed as a possible animal model of HBV infection. Phylogenetically, baboons are close to humans, showing ≈96% homology at the DNA level, and they have an immune system similar to that of humans (13). Early studies involving injection of baboons with HBV-positive serum failed to detect any clinical or biochemical signs of infection in these primates, and initial serologic surveys failed to detect HBV surface antigen (HBsAg) in serum, leading to the conclusion that baboons were not susceptible to HBV infection (14). This supposed lack of susceptibility of baboons to infection with HBV, and the fact that unlike chimpanzees, baboons are not an endangered species, intimated that baboons were good candidates for sources of liver for xenotransplants. The use of xenotransplants from pigs and nonhuman primates to humans was considered to overcome the donor shortage and to bridge patients with terminal hepatic failure until a human donor organ became available (15).
To confirm that baboons were not susceptible to HBV infection, Kedda et al. injected 6 wild-caught Chacma baboons (Papio ursinus orientalis) with pooled HBV-positive serum and analyzed the baboons for 52 weeks by using sensitive molecular techniques to detect evidence of transmission (16). HBV DNA was detected by nested PCR in serum and liver of 4 of the baboons <52 weeks after injection. Liver function and histologic results were within reference ranges, and HBsAg was not detected in serum (16). However, during that study, HBV DNA was detected by using nested PCR in serum of 2 of the 6 baboons at baseline, before injection with HBV. The presence of HBV DNA was confirmed by retesting samples in independent laboratories. This finding raised the possibility that baboons were naturally infected with a hepadnavirus. The aims of the present study were to determine the prevalence of HBV in wild baboons, molecularly characterize the virus isolated from these baboons, determine whether the virus replicates in the baboon liver, and demonstrate viral transmission to experimentally naive baboons.