Yaws is a disease caused by the bacterium Treponema pallidum subsp. pertenue, which is believed to be an exclusively human pathogen (1). However, this bacterium has recently been identified in African nonhuman primates (NHPs) (2), raising concerns about a possible zoonotic reservoir for human infection. Kenya is 1 of 76 countries that the World Health Organization categorizes as previously endemic for yaws, but no current data support its presence or absence (http://apps.who.int/gho/data/node.main.NTDYAWSEND). However, sustainable yaws eradication will rely on information about transmission dynamics and potential links between human and NHP T. pallidum strains (3).

In the early 1960s, Fribourg-Blanc and Mollaret tested 150 serum samples from wild-caught baboons (Papio sp.) from Guinea and Kenya (4). Although 72 (65%) of 111 serum samples from Guinea were positive for T. pallidum antibodies, none of the samples from Kenya were positive. In subsequent years, an additional 276 serum samples from baboons in Kenya supported the absence of T. pallidum infection. However, a more recent study of baboon samples collected during 1977–1994 in Kenya reported serologic evidence of T. pallidum infection in Nanyuki, Laikipia County (prevalence 57.5%) (5). For our study, we hypothesized that 39 years after the first samples were positive for antibodies against T. pallidum in Nanyuki (5), infection is still present in the NHP population.

All animal protocols were approved by the Kenya Wildlife Service (permit #4004), the Institute of Primate Research Scientific and Ethics Review Committee, and the Smithsonian Institution Animal Use and Care Committee. In October 2016, we sampled 65 olive baboons (Papio anubis) and 2 vervet monkeys (Chlorocebus pygerythrus) at sites surrounding the Mpala Research Centre in Laikipia County, Kenya. We performed a preliminary serologic screening by using the immunochromatographic Dual Path Platform (DPP) HIV-Syphilis Assay (Chembio Diagnostic Systems, Inc., http://chembio.com) according to the manufacturer guidelines. This syphilis (T. pallidum) assay is a useful screening tool because antibodies against Treponema subspecies are cross-reactive (6). We tested 67 samples with the DPP assay; 49 were positive and 18 negative.

However, because this test is not certified for use with NHPs, we subsequently confirmed results by using the T. pallidum Particle Agglutination Assay (TPPA) (SERODIA TPPA, https://www.fujirebio-us.com), which has been validated for use in baboons (7). Of the 52 samples tested with the TPPA assay, there were 33 positive, 6 negative, and 13 inconclusive results. Inconclusive TPPA results indicate nonspecific antibodies reacting with nonsensitized particles. Because of limited sample material, we were unable to perform repeated testing with a preabsorption step to remove all nonspecific binding antibodies (as described in the assay manual) and therefore excluded the inconclusive TPPA results from our analysis.

If we defined seropositive monkeys as those with positive results for the TPPA or DPP, 1 of 2 vervet monkeys and 53 (85.5%) of 62 baboons were seropositive. Male baboons (90.4%, 38/42) had a relative seropositivity risk ratio of 1.3 (95% CI 0.984–1.858) when compared with female baboons (72.2%, 13/18); however, this difference was not significant (p = 0.111 by Fisher exact test). If we included age, in addition to sex, in the analysis, adult male and female baboons both showed 100% seropositivity (21/21 and 10/10, respectively). Subadult males and females also showed seropositivity of 100% (6/6 and 1/1, respectively). Juveniles had a combined seropositivity of 61.1%: a total of 81.8% (9/11) of males and 28.6% (2/7) of females were seropositive. Infants had the lowest seroprevalence rate (50%, 2/4) (Table).

None of the tested NHPs had overt clinical signs of infection, such as skin lesions, which might have contained T. pallidum DNA. However, several other studies found that NHPs are frequently seropositive for T. pallidum antibodies without clinical lesions (5,8,9). Because wild NHPs are not treated and bacterial clearance is unlikely, the absence of lesions presumably corresponds to the latency stage of infection, which is also a key characteristic of human treponematoses (10). Future molecular investigations should include nontreponemal tests to further support the assumption that animals are in the latency stage and should target the DNA of the pathogen, which would enable comparison of T. pallidum strains of NHP origin from Kenya with those infecting NHPs in neighboring countries and possibly humans. In Tanzania, a country that has a similar history of previous yaws endemicity in humans and lacks current prevalence data, clinical lesions have been documented in olive baboons, vervet monkeys, yellow baboons, and blue monkeys, in addition to widespread seroprevalence in NHPs closely matching previous human infection geographic distribution (9).

Our results suggest that evidence of Treponema exposure in NHPs continues to be present in Laikipia County almost 4 decades after it was first detected. Our data provide further evidence that, in East Africa, T. pallidum infection is endemic to NHPs and that multiple NHP taxa contain antibodies indicating latent infection. Providing reliable information on the epidemiology of treponematoses in humans and NHPs has major programmatic implications for yaws eradication. Under a One Health approach, we call for additional yaws surveillance in communities in Kenya, especially in regions where NHPs and humans coexist.