Human infection with SFV was first identified in 1971 in a Kenyan with nasopharyngeal carcinoma and since then has been found in other populations around the world who are exposed to NHPs, but without overt evidence of disease
. To date about 136 persons have been identified with SFV infection in 11 different countries on four continents
[6, 11, 12]. However, given the high prevalence of SFV in NHPs, the frequent exposure to NHPs by persons who handle and consume NHPs, and the increasing demand for bushmeat in Africa and globally, models have predicted an even higher potential prevalence of SFV among humans
. Thus, to better assess the public health implications of this novel human infection requires a more comprehensive understanding of the epidemiology of SFV, including its geographical distribution, the evolutionary history of SFV in humans and NHPs, determination of exposure risks, and a thorough evaluation of secondary transmissibility and pathogenicity of SFV in humans and NHPs.
DRC is an ideal location for increasing our knowledge about emerging zoonotic infections since the majority of the population lives in rural forested regions where NHP density is greatest and who rely on bushmeat as a major food and income source. Our finding of 16 SFV WB-positive persons increases the total number of known global infections to 153, with 17 in DRC, including one woman who was previously found co-infected with HIV-1 and SFV in Kinshasa in 1985
. These discoveries support the presence of SFV in humans in DRC for at least two decades in both urban and rural settings. Given the wide exposure to NHPs across DRC, our results suggest that SFV infection may have a wider geographic distribution within DRC. Our results also reinforce the potential for an increased SFV prevalence throughout the forested regions of Africa where humans and simians co-exist. Also, since SIV and STLV are highly prevalent in NHPs, rural populations in close contact with NHPs may also be at increased risk for co-infection with other simian retroviruses. Indeed, novel STLVs have been reported recently in NHP hunter cohorts from Cameroon who were also infected with SFV
[8, 29–31] suggesting that zoonotic transmission of simian retroviruses is an ongoing phenomenon. The continued clearing of forests combined with economic declines in DRC resulting from recurrent civil war forces residents to rely on alternative protein sources such as primate bushmeat and exacerbates the potential for increased exposure to simian retroviruses. Our study documents an increased risk of SFV infection in persons preparing NHPs for consumption, or getting bitten and scratched by NHPs, or bringing dead NHPs from the forest, though there was insufficient evidence to conclude elevated risk for SFV infection from hunting NHPs after controlling for age and gender in the multivariate models. However, we also found that low exposure activities, such as entrance into the forest daily, placed persons at elevated risk for SFV infection. SFV has been found in feces and urine and could be sources of infection in these frequent forest visits
[14, 32]. Nonetheless, direct evidence for such transmissions is not available. Human infection with SFV has been identified in animal handlers who did not report known mucocutaneous exposures to infected animals suggesting that less intimate contact with body fluids may possibly place persons at risk for infection
. While handling and processing of dead NHPs can lead to exposure to SFV and other simian retroviruses, contact with dead primates and other mammals in Africa has also resulted in fatal exposure and spread of the Ebola virus that causes hemorrhagic fever, raising public health concerns for this practice. Thus, increased public awareness is needed in these communities, including educational messages advising caution in primate hunting and handling, and forest safety, to prevent cross-species viral infections.
Curiously, SFV prevalence in our study population did not differ by gender. We have previously identified SFV antibodies in a female sex worker in DRC
. These results contrast with all other African studies where male hunters were more likely to be infected via more severe NHP contact, such as bite wounds
[6, 11, 12]. Men who work with captive animals are also more frequently infected with SFV than women
. However, the higher prevalence seen in men in Cameroon and Gabon may have been biased by the recruitment of mostly hunters, which traditionally is a male activity in those countries. Data from our questionnaire support the fact that men are the primary hunters in DRC with women more commonly performing food preparation tasks. Although the number of severe cutaneous exposures in our study was low, this exposure increased the risk for SFV infection. Women processed bushmeat for consumption more frequently than men in our study possibly resulting in more mucocutaneous exposures. The reasons for our inability to detect SFV sequences in eleven WB-positive persons, including seven females and four males, are unclear and may be due to infection with highly divergent SFVs not detected with the current assays, low proviral loads, abortive infections, nonspecific seroreactivity, or other reasons. While similar results have been previously reported in other African countries which also used very generic PCR assays
[11, 12, 24, 25], the WB assay used in the current study has been shown to be highly sensitive and specific for detecting SFV infection
. Testing of longitudinal specimens may be necessary to investigate persistent SFV infection in these WB-positive and PCR-negative persons.
While the major route for SFV transmission in NHPs appears to be via saliva from bites by juvenile and adult animals
[5, 6], little is known about the transmission of SFV from mother-to-child. Nonetheless, vertical transmission should be considered, including during birth or through breast feeding since evidence for vertical transmission from an SFV-infected chimpanzee to a female offspring in captivity has been reported
. The youngest SFV-positive person in our study was an 11-year old girl who reported eating NHPs but not going into the forest, and her mother was not a study participant. Eating NHPs was not associated with WB-positivity in our study, thus, we are unable to assess the origin of her infection. Our study was primarily designed to enroll participants from each village in a serosurvey and thus included household contacts of persons found to be infected in each village, which facilitated our investigation of secondary SFV transmission. While we were unable to identify any serological evidence of SFV transmission to small numbers of close contacts tested in our study, the duration of the spousal relationships in this polygamous culture, the usage of barrier contraception that might prevent sexual transmission, and duration of breast feeding were not recorded to fully assess the effect of these factors on transmission. Proviral loads in one woman were high and were 12 times those reported in hunters from Cameroon, which ranged from undetectable to 145 copies/105 cells
. The increased viral loads seen in this woman may result in increased transmissibility and/or pathogenicity of her infection but longitudinal studies are required to investigate this hypothesis. Although both of her teenage children were serologically negative for SFV, it is not known when the mother became infected. The wife of an SFV-infected hunter in Cameroon was reported to be persistently WB-positive but PCR-negative, suggestive of a possible case of person-to-person transmission
. However, information regarding her primate exposure was not disclosed to exclude a zoonotic origin of the infection. Likewise, we identified an SFV-infected woman in the current study who did not report any specific primate exposures also suggestive of a human-to-human transmission, but whose contacts were not available for testing to evaluate this possibility. Alternatively, self-reporting and recall bias could account for these results. Data on SFV shedding in vaginal secretions, breast milk, or semen from infected persons may provide important information on mother-to-child or sexual transmission.
The lower SFV prevalence seen in DRC, and the 1% reported in our study in Cameroon, contrasts with the 10.5% – 18.6% infection rate seen in Cameroon and Gabon in populations enriched for cross-species infections by targeting persons with severe simian exposures, including bites and wounds. The primary focus of our study was to investigate the seroprevalence of monkeypox infection in selected health zones in the Sankuru district and included persons with or without any NHP and animal exposure. Our study population reported few severe exposures and thus the lower SFV prevalence (0.5%) seen in our study most likely better represents infection prevalence in the general population compared to the higher rate seen in highly exposed persons in previous studies
[11, 12, 24]. The lower SFV prevalence may also result from almost a third of our population being less than 10 years old, and younger children may have less overall NHP exposure. Interestingly, the SFV prevalence was about half that seen for HIV-1 (1.2%) in Central-south DRC reported from the 2007 Demographic and Health Survey (DHS) (
http://www.measuredhs.com). Unlike HIV-1, SFV is not screened for at blood banks and thus transmission via transfusion of contaminated blood products requires more thorough investigation. Blood-borne transmission has been reported in macaque models demonstrating that transmission via this route is possible
Limited information prevents us from drawing strong conclusions on the disease potential of SFV infection
. Clinical information was not collected during our cross-sectional study so we were unable to ascertain possible diseases in SFV-infected persons. Longitudinal follow-up studies of infected persons and close contacts are required to better understand the epidemiology of these novel human infections, and to evaluate the role of SFV in disease and transmissibility. In addition, testing of persons with various diseases like cancer and neurological disorders in endemic regions would be beneficial to ascertain possible disease associations. Our data may also be limited by recall bias or the possible inaccurate reporting of NHP species that participants remembered exposure to. Although NHP species were chosen by participants using NHP pictures with their common names it may be difficult to distinguish between some monkey species.
Previously, we have shown that SFVs have co-evolved with their primate hosts over millions of years such that phylogenetic analysis of simian and human SFV sequences together can be used to accurately identify the species origin of human infections
[3, 6]. Detailed phylogenetic analysis using the largest dataset of SFV pol sequences assembled to date, including new SFVs endemic to NHPs from Cameroon and DRC, allowed exceptional resolution of the primate origin of human infections in DRC and also of those reported in Cameroon and Gabon
[11, 12]. We identified infection in three women in DRC with novel SFVs not seen before in humans, including SFV from red-tailed guenons (Ce. ascanius) and Angolan colobus (Co. angolensis), both of which inhabit only forests in DRC and are frequently hunted. Statistical analysis inferred that SFV prevalence was elevated in persons reporting exposure to Angolan colobus. In addition, we show that six hunters from Cameroon previously suspected of being infected with SFV from Debrazza guenons (Ce. neglectus) are actually infected with SFV from greater spot-nosed (Ce. nictitans) or moustached (Ce. cephus) monkeys, which are hunted more frequently than Debrazza monkeys in Cameroon
. Combined with SFV infection originating from red-tailed guenons, mona monkey (Ce. mona), and Debrazzas in persons from DRC, Cameroon and/or Gabon, humans are susceptible to SFV variants from five different Cercopithecus monkeys. Colobus, gorilla, chimpanzee, mandrill, baboon, and macaque SFVs can also infect humans from both captive and/or wild animals
. This high level of SFV diversity that has crossed into humans demonstrates a broad susceptibility to SFV infection, which contrasts with the more restricted susceptibility to SIV, since of the 45 known SIV variants only three (SIVcpz, SIVgor, SIVsmm) have crossed into humans at varying levels, two of which spread globally as HIV-1 (SIVcpz) and HIV-2 (SIVsmm). It is uncertain if any of the SFV variants identified so far will spread in humans and become a bona fide human virus.