Simian Immunodeficiency Viruses (SIVs) are the direct precursors of Human Immunodeficiency Viruses (HIVs) that have caused the HIV/AIDS pandemic in the human population [1, 2]. Although the conditions and circumstances of cross-species transmission of SIVs from primates to humans remain unknown, human exposure to blood or other secretions of infected primates (chimpanzees, gorillas, sooty mangabeys) through hunting and butchering of primate bushmeat, represents the most plausible source for human infection [1–6]. Currently, serological evidence of SIV infection has been shown for more than 40 different primate species and SIV infection has been confirmed by sequence analysis in the majority of them. The routes of SIV transmission within and between host species are not fully known, however, sexual contact and biting within one species, and biting and blood-to-blood/mucosa contact (mainly observed in hunter - prey relationships) among different species provide possible infection routes for the virus [7, 8]. A high genetic diversity is observed among the different SIVs, but generally each primate species is infected with a species-specific virus, which forms monophyletic lineages in phylogenetic trees. There are many examples of co-evolution between viruses and their hosts, but also cross-species transmission and recombination between distant SIVs seems not exceptional and one species can even harbour two different SIVs. The chimpanzee SIV (SIVcpz) is for example the result of cross-species transmissions as this virus is a mosaic of SIVs infecting other African primates. The genome of the virus consists partly of nucleic acid sequences from red capped mangabey SIV (SIVrcm), and partly of sequences from the ancestor of SIVs infecting greater spot-nosed (SIVgsn), mona (SIVmon) or mustached monkey (SIVmus) [9–11]. Chimpanzees are known to hunt monkeys for food, and most probably, the recombination of these monkey viruses occurred within chimpanzees and gave rise to the common ancestor of today's SIVcpz lineages, which were subsequently transmitted to gorillas [5].

Despite the increasing number of SIV lineages that have been described recently, our knowledge on SIV in their natural hosts still remains limited. This is because only few viruses have been characterized for each species and there is a major bias in geographical sampling. By studying SIVs in wild primates in their natural habitat we can better understand the circulation and transmission of these viruses within and between different primate species and perhaps identify factors that play a role in viral adaptation to new hosts among different primate species [12–14].

Of the four chimpanzee subspecies, only Pan troglodytes troglodytes and Pan troglodytes schweinfurthii in Central/East Africa have been shown to harbour SIVcpz [1, 11, 15–18]. The two West African chimpanzee subspecies, Pan troglodytes ellioti and Pan troglodytes verus, appear to be free from SIVcpz infection. Therefore it is hypothesized that this virus was introduced after the evolutionary divergence and geographical separation of the West African subspecies from the Central/East subspecies [11, 15]. To test for SIVcpz in P. t. verus, more than 1500 captive chimpanzees of this subspecies have been screened for this virus. However, these chimpanzees do not represent the wild population since only 447 were wild-born and have mainly been captured as infants, when they are less likely to be infected [15, 19]. Therefore, it remains important to continue to collect data on wild living chimpanzees from this subspecies. To date, the only study on wild living P. t. verus has been based on 28 faecal samples from a population in Taï National Park, Côte d'Ivoire [16].

The chimpanzees of Taï National Park have been under human observation for more than 30 years [20] and are known to hunt and consume monkeys frequently. When hunting, the chimpanzees bite their prey and are sometimes bitten in return. The prey is consumed almost entirely, which means that many bones are crushed which could cause lesions in the oral cavity and result in direct blood to blood contact. They hunt weekly throughout the year and usually every day in the hunting season from September to November, and 80% of their prey consist of western red colobus monkeys (Piliocolobus badius badius) [20]. These red colobus monkeys harbour high levels of their own species specific strain of SIV (SIVwrc) as well as two other retroviruses; Simian T-cell Lymphotrophic Virus type 1 (STLV-1wrc) and Simian Foamy Virus (SFVwrc) [21–25].

Based on the SIVwrc prevalence data from this red colobus population (50 to 82% of the population is positive [21]) and based on hunting data from the Taї Chimpanzee Project [20], we estimate that adult male chimpanzees are yearly exposed to approximately 45 kilograms of SIV infected red colobus tissue. Therefore the chimpanzees are exposed to high levels of SIVwrc through biting, blood-to-blood/mucosa contact and ingestion of their prey. This may provide possible infection routes for the virus, although the modes of SIV transmission are not fully known [7, 8]. It has already been documented that the other two retroviruses harboured by the red colobus monkeys in Taї National Park; STLV-1wrc and SFVwrc, are transmitted to the Taї chimpanzee population (individuals are included in the present study) most likely through hunting and meat consumption [22, 23]. Further, in chimpanzee subspecies where the chimpanzee lentivirus, SIVcpz, has been documented, it is believed that this mosaic virus was initially acquired through hunting and consumption of infected monkey prey species [9–11]. Such evidence of retroviral transmissions through hunting and consumption of primates makes it possible that also SIVwrc from red colobus monkeys can be transmitted to the P. t. verus chimpanzees. The aim of this study was therefore to investigate the SIV status of these wild chimpanzees, and to determine if transmission of SIVwrc occurs from red colobus monkeys to the chimpanzees through their natural and frequent hunting and meat-eating behaviour.

There are a number of interesting questions regarding the transmission and natural history of SIV infections in wild chimpanzees; an infection which entered into and adapted to the human population and caused the global AIDS pandemic [2]. It is presumed that the chimpanzees first acquired the infection through hunting and consumption of monkey prey infected each with their own species specific strains of SIV, which at some point in time recombined and persisted in the chimpanzee host [9–11]. To date, only this recombinant strain of SIV, known as SIVcpz, has been detected in wild chimpanzees [29] and one question that arises is: How easily are individual SIV strains from monkeys transmitted to chimpanzee populations, irrespective of subspecies, and do such infections persist?

We investigated this question through studying the natural hunter-prey relationship between wild chimpanzees (P. t. verus) and highly SIV-infected red colobus monkeys (P. b. badius) in the tropical rainforest of Taï National Park in Côte d'Ivoire, West Africa [21, 30]. Eight other diurnal monkey species live in this forest, including olive colobus monkeys (Procolobus verus), great spot-nosed monkeys (Cercopithecus nictitans) and sooty mangabeys (Cercocebus atys) which are also known to harbour species-specific SIVs: SIVolc, SIVgsn and SIVsmm, respectively [4, 24, 31]. However, according to more than 30 years of behavioural observations, red colobus is the preferred prey of the chimpanzees, whereas capture of greater spot-nosed monkeys has not been observed and olive colobus and sooty mangabeys are hunted extremely rarely. For example, over a twelve year period, the chimpanzees were seen to capture only six olive colobus and one sooty mangabey, while red colobus monkeys were captured 215 times [20]. Therefore, the exposure to these respective SIV strains through hunting is very low in comparison to the exposure to the SIVwrc strain carried by the red colobus monkeys, which the chimpanzees are frequently in close contact with. In addition, the prevalence of SIV in this monkey species in Taï National Park is among one of the highest documented in wild primates to date. Western red colobus represent a substantial reservoir to which chimpanzees, as well as human bushmeat hunters, are exposed [21]. As an example, we calculated that Sagu, an adult male in our study who was approximately 20 years old when he died, could have eaten nearly 250 kilograms of SIVwrc infected red colobus tissue in his life.

Despite the enormous infection pressure, we could not detect SIVwrc (or any other strain of SIV) in blood and tissue samples from the chimpanzees. Theoretically, these chimpanzees could carry a SIV strain which is not detectable by the PCR methods used in this study. Alternatively, the level of SIVwrc viraemia is so low that it can not be detected by the PCR methods used. This could be in particular true for the 2 chimpanzees for which only samples of muscle were available. However, as no SIV-specific antibodies were detected with the Luminex test it is more plausible that no persistent SIV infection exists in these chimpanzees, although about half of the chimpanzees showed some cross-reactions to the HIV-antigens on the INNO-LIA HIVI/II Score kit. The strongest reactions were observed in samples from Leo and Olduvai, and their test results were HIV positive, according to the test manufacturer's criteria (two or more bands stronger than the minimum control band [the +/- band]). For another chimpanzee, Dorry, the result was indeterminate (one band stronger than the minimum control band). For other chimpanzees where weak reactions were seen, the results are considered negative for this HIV-test. It could be that there is a difference in sensitivity and specificity of HIV antibody detection of the Luminex and INNO-LIA tests, but it is also likely that the reactivity to HIV antigens in the INNO-LIA test was due to false positive cross-reaction phenomena due to other causes than HIV/SIV infection, such as observed in human HIV testing, especially in Africa [32]. It has been shown that the INNO-LIA test produces false positive results also in other primate species. In C. nictitans and C. cephus the estimated prevalence based on INNO-LIA results is higher than that estimated using lineage specific antigens, and samples from C. pogonias, L. albigena and C. agilis, that were cross-reacting with some HIV antigens on the INNO- LIA test, were negative with SIV lineage ELISAs and PCR [33, 34]. Therefore the reactivity we observed with the INNO-LIA testing of the chimpanzee samples is most likely a false positive non-specific cross-reactivity, as no specific antibody reaction to SIVwrc, or any other known SIV and HIV strain, could be detected by SIV/HIV lineage specific Luminex EIAs.

It was not surprising that the P. t. verus chimpanzees were negative for SIVcpz, as this virus is believed to have been introduced into two other, Central/East chimpanzee subspecies (P. t. troglodytes and P. t. schweinfurthii) after the evolutionary split from the Western chimpanzee subspecies [15]. It was however interesting that we could not detect any SIVwrc infection, considering the high exposure of this virus.

Our results show that there was no detectable specific antibody reaction to, or infection with, SIV from red colobus monkeys (or any other primates), despite high and frequent exposure to the virus. This seemingly resistance of the chimpanzees to SIVwrc could be due to immunological factors or mechanisms, or lack of these, which are important for the recognition and subsequent establishment or rejection of immunodeficiency viruses [35–38]. HIV research is much focused on these mechanisms, especially in certain individuals that remain persistently seronegative despite known exposure to HIV [39]. P. t. verus chimpanzees are however not totally resistant to immunodeficiency virus infections in general, as susceptibility of captive chimpanzees of this subspecies to HIV, SIV, and co-infections of the two viruses, has been documented [7]. In wild chimpanzees (P. t. troglodytes and P. t. schweinfurthii) no other SIV strain than the chimpanzee specific SIVcpz has been detected to date [1, 5, 18], which suggests that the chimpanzees' susceptibility to individual SIV strains from monkeys is low. SIVcpz is a mosaic consisting partly of SIV from red capped mangabey and partly of one of the SIV strains in greater spot-nosed monkey, mona monkey or mustached monkey [9, 10]. Only one of these species, the greater spot-nosed monkey (C. nictitants), lives in the Taï forest. These monkeys are however rare in this forest, the chimpanzees have never been observed to hunt them, and there is also no evidence yet that they are SIV infected, although only few animals have been tested [20, 31]. Interestingly and comparably to what we report about the chimpanzees, no SIVwrc infections have so far been documented in humans, who also frequently hunt red colobus monkeys [40]. We could also speculate whether the SIV status of the chimpanzees in the Taï National Park would be different had they hunted sooty mangabeys more frequently. The sooty mangabey population from this national park harbours the sooty mangabey strain of SIV (SIVsmm) which crossed the species barrier at least 8 times and infected humans through bushmeat hunting, and then became HIV-2 [4]. The genetic and physiologic similarities between humans and chimpanzees and also the similar susceptibility to specific infections, suggest that such transmission could also occur from sooty mangabeys to chimpanzees, if an efficient transmission pathway existed.

We could not detect any conclusive sign of infection with SIVwrc in the P. t. verus chimpanzees in Taï National Park, despite exposure of highly infected red colobus. However, the frequent hunting and consumption of red colobus by the chimpanzees represents a transmission pathway for other simian retroviruses between these two host species. It remains to be determined which factors that seemingly protect these chimpanzees from infection, and whether the local human population, frequently exposed to meat and organs of the red colobus in this region, is free of SIVwrc infections. This knowledge on natural SIV infections in wild primate populations might benefit human HIV/AIDS research [13, 29].