Endemic foci of the tick-borne relapsing fever spirochete Borrelia crocidurae in Mali, West Africa, and the potential for human infection

Abstract

Background: Tick-borne relapsing fever spirochetes are maintained in endemic foci that involve a diversity of small mammals and argasid ticks in the genus Ornithodoros. Most epidemiological studies of tick-borne relapsing fever in West Africa caused by Borrelia crocidurae have been conducted in Senegal. The risk for humans to acquire relapsing fever in Mali is uncertain, as only a few human cases have been identified. Given the high incidence of malaria in Mali, and the potential to confuse the clinical diagnosis of these two diseases, we initiated studies to determine if there were endemic foci of relapsing fever spirochetes that could pose a risk for human infection.

Methodology/principal findings: We investigated 20 villages across southern Mali for the presence of relapsing fever spirochetes. Small mammals were captured, thin blood smears were examined microscopically for spirochetes, and serum samples were tested for antibodies to relapsing fever spirochetes. Ornithodoros sonrai ticks were collected and examined for spirochetal infection. In total, 11.0% of the 663 rodents and 14.3% of the 63 shrews tested were seropositive and 2.2% of the animals had active spirochete infections when captured. In the Bandiagara region, the prevalence of infection was higher with 35% of the animals seropositive and 10% infected. Here also Ornithodoros sonrai were abundant and 17.3% of 278 individual ticks tested were infected with Borrelia crocidurae. Fifteen isolates of B. crocidurae were established and characterized by multi-locus sequence typing.

Conclusions/significance: The potential for human tick-borne relapsing fever exists in many areas of southern Mali.

Figure 1. Map of Mali with the names and locations of the 20 villages investigated.

The location of Mali in the African continent is identified in grey.

Figure 2. Immunoblots of selected serum samples from Malian small mammals.

Representative samples are shown for positive and negative results for antibodies to relapsing fever spirochetes. The animal species and number are shown above with one experimentally infected laboratory mouse included as a positive control. Each panel has the borrelia whole-cell lysate on the left and purified GlpQ on the right. Molecular mass standards (MMS) are shown on the far left in kilodaltons. + = positive; − = negative.

Figure 3. A thin blood smear showing borrelia spirochetes.

This sample is from Mastomys natalensis #649 captured September 30, 2011, in Doucombo, Mali. Scale bar represents 20 µm.

Figure 4. Agarose gel showing plasmid content of 15 isolates of Borrelia crocidurae from Mali.

The isolate designations are shown above with the genomic groups determined by MLST analysis (genomic groups A–D). Stars on right are aligned with presumptive circular plasmids identified in 2-dimensional agarose gels (not shown). The plasmid types (I–VI) are on the bottom. Molecular size standards (MSS) are shown on left in base pairs.

Figure 5. Phylogram based on the intergenic spacer sequences (IGS) for relapsing fever spirochetes.

The Malian isolates, represented by the four IGS sequence types, group with Borrelia crocidurae from Mauritania (Achema) and Tunisia (#12T038 and #7-10T047). Borrelia burgdorferi B31 is the out-group. The scale bar represents the number of base substitutions per nucleotide.

Figure 6. Phylogram based on the concatenated DNA sequences of the 16S rDNA, flaB and glpQ loci.

The Malian isolates, represented by the six unique sequence types, group with Borrelia crocidurae from Mauritania (Achema). Scale bar represents the number of base substitutions per nucleotide.