Treponema pallidum infection in the wild baboons of East Africa: distribution and genetic characterization of the strains responsible

Abstract

It has been known for decades that wild baboons are naturally infected with Treponema pallidum, the bacterium that causes the diseases syphilis (subsp. pallidum), yaws (subsp. pertenue), and bejel (subsp. endemicum) in humans. Recently, a form of T. pallidum infection associated with severe genital lesions has been described in wild baboons at Lake Manyara National Park in Tanzania. In this study, we investigated ten additional sites in Tanzania and Kenya using a combination of macroscopic observation and serology, in order to determine whether the infection was present in each area. In addition, we obtained genetic sequence data from six polymorphic regions using T. pallidum strains collected from baboons at two different Tanzanian sites. We report that lesions consistent with T. pallidum infection were present at four of the five Tanzanian sites examined, and serology was used to confirm treponemal infection at three of these. By contrast, no signs of treponemal infection were observed at the six Kenyan sites, and serology indicated T. pallidum was present at only one of them. A survey of sexually mature baboons at Lake Manyara National Park in 2006 carried out as part of this study indicated that roughly ten percent displayed T. pallidum-associated lesions severe enough to cause major structural damage to the genitalia. Finally, we found that T. pallidum strains from Lake Manyara National Park and Serengeti National Park were genetically distinct, and a phylogeny suggested that baboon strains may have diverged prior to the clade containing human strains. We conclude that T. pallidum infection associated with genital lesions appears to be common in the wild baboons of the regions studied in Tanzania. Further study is needed to elucidate the infection's transmission mode, its associated morbidity and mortality, and the relationship between baboon and human strains.

Figure 1. Gross pathology of olive baboons (P. anubis) with genital and circum-anal ulceration caused by T. pallidum at Lake Manyara National Park, Tanzania (2007).

A. Adult female, severely affected, with massive destruction of the outer genitalia. The granulated tissue is fragile and bleeds easily on contact. The lesion is chronic and active. B. Adult female, severely affected, with massive destruction of the outer genitalia. The lesion is characterized by progressive scarification that has led to the vagina and anus being in a permanently open state. C. Sub-adult male, in an early stage of clinical infection. The corpus penis shows multiple erosions of the epidermis. D. Adult male with severe phimosis and genital ulceration. The prepuce is filled with smegma. The lesion is chronic and active.

Figure 2. A phylogeny demonstrates that T. pallidum strains infecting baboons in Serengeti National Park and Lake Manyara National Park are genetically distinct from one another.

Phylogenies were constructed using both Maximum Parsimony and Maximum Likelihood methods to analyze 25 polymorphisms in six concatenated regions of the Treponema genome. The phylogenies were congruent and a Maximum Parsimony tree was chosen for display, with bootstrap support displayed at all nodes that received greater than 50% using both methods.

Figure 3. Geographic location of African sites where baboons have tested seropositive for T. pallidum antibodies.

This map is based on the results presented in this paper (East African sites) as well as the results in , (West African sites). Inset: East African sites examined in this paper, with circles proportional to the number of animals tested serologically at each site. Years of serum collection ranged from 1977–2006, as described in Table 1.