Endemic Foci of the Tick-Borne Relapsing Fever Spirochete Borrelia crocidurae in Mali,West Africa,and the Potential for Human Infection

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.     

A molecular map of chloroquine resistance in Mali

Plasmodium falciparum chloroquine resistance (CQR) transporter point mutation (PfCRT 76T) is known to be the key determinant of CQR. Molecular detection of PfCRT 76T in field samples may be used for the surveillance of CQR in malaria-endemic countries. The genotype-resistance index (GRI), which is obtained as the ratio of the prevalence of PfCRT 76T to the incidence of CQR in a clinical trial, was proposed as a simple and practical molecular-based addition to the tools currently available for monitoring CQR in the field. In order to validate the GRI model across populations, time, and resistance patterns, we compiled data from the literature and generated new data from 12 sites across Mali. We found a mean PfCRT 76T mutation prevalence of 84.5% (range 60.9–95.1%) across all sites. CQR rates predicted from the GRI model were extrapolated onto a map of Mali to show the patterns of resistance throughout the participating regions. We present a comprehensive map of CQR in Mali, which strongly supports recent changes in drug policy away from chloroquine.     

Geographic Distribution and Genetic Characterization of Lassa Virus in Sub-Saharan Mali

Background

Lassa fever is an acute viral illness characterized by multi-organ failure and hemorrhagic manifestations. Lassa fever is most frequently diagnosed in Nigeria, Sierra Leone, Liberia, and Guinea, although sporadic cases have been recorded in other West African countries, including Mali. The etiological agent of Lassa fever is Lassa virus (LASV), an Arenavirus which is maintained in nature and frequently transmitted to humans by Mastomys natalensis. The purpose of this study was to better define the geographic distribution of LASV-infected rodents in sub-Saharan Mali.

Methodologies/Principal Findings

Small mammals were live-trapped at various locations across Mali for the purpose of identifying potential zoonotic pathogens. Serological and molecular assays were employed and determined LASV infected rodents were exclusively found in the southern Mali near the border of Côte d''Ivoire. Overall, 19.4% of Mastomys natalensis sampled in this region had evidence of LASV infection, with prevalence rates for individual villages ranging from 0 to 52%. Full-length genomic sequences were determined using high throughput sequencing methodologies for LASV isolates generated from tissue samples of rodents collected in four villages and confirmed the phylogenetic clustering of Malian LASV with strain AV.

Conclusions/Significance

The risk of human infections with LASV is greatest in villages in southern Mali. Lassa fever should be considered in the differential diagnosis for febrile individuals and appropriate diagnostic techniques need to be established to determine the incidence of infection and disease in these regions.     

Co-infection of Long-Term Carriers of Plasmodium falciparum with Schistosoma haematobium Enhances Protection from Febrile Malaria: A Prospective Cohort Study in Mali

Background

Malaria and schistosomiasis often overlap in tropical and subtropical countries and impose tremendous disease burdens; however, the extent to which schistosomiasis modifies the risk of febrile malaria remains unclear.

Methods

We evaluated the effect of baseline S. haematobium mono-infection, baseline P. falciparum mono-infection, and co-infection with both parasites on the risk of febrile malaria in a prospective cohort study of 616 children and adults living in Kalifabougou, Mali. Individuals with S. haematobium were treated with praziquantel within 6 weeks of enrollment. Malaria episodes were detected by weekly physical examination and self-referral for 7 months. The primary outcome was time to first or only malaria episode defined as fever (≥37.5°C) and parasitemia (≥2500 asexual parasites/µl). Secondary definitions of malaria using different parasite densities were also explored.

Results

After adjusting for age, anemia status, sickle cell trait, distance from home to river, residence within a cluster of high S. haematobium transmission, and housing type, baseline P. falciparum mono-infection (n = 254) and co-infection (n = 39) were significantly associated with protection from febrile malaria by Cox regression (hazard ratios 0.71 and 0.44; P = 0.01 and 0.02; reference group: uninfected at baseline). Baseline S. haematobium mono-infection (n = 23) did not associate with malaria protection in the adjusted analysis, but this may be due to lack of statistical power. Anemia significantly interacted with co-infection (P = 0.009), and the malaria-protective effect of co-infection was strongest in non-anemic individuals. Co-infection was an independent negative predictor of lower parasite density at the first febrile malaria episode.

Conclusions

Co-infection with S. haematobium and P. falciparum is significantly associated with reduced risk of febrile malaria in long-term asymptomatic carriers of P. falciparum. Future studies are needed to determine whether co-infection induces immunomodulatory mechanisms that protect against febrile malaria or whether genetic, behavioral, or environmental factors not accounted for here explain these findings.     

Population Size and Migration of Anopheles gambiae in the Bancoumana Region of Mali and Their Significance for Efficient Vector Control

We present results of two intensive mark-release-recapture surveys conducted during the wet and dry seasons of 2008 in the villages of Fourda and Kenieroba, Mali. The former is a small fishing village by the Niger River with a moderate to high densities of Anopheles gambiae Giles s.s. (Diptera: Culicidae) throughout the year, while the latter is a large agricultural community 2 km inland that experiences strong seasonal fluctuation in An. gambiae densities. We estimate the population size of female An. gambiae in Fourda to be in less than 3,000 during the dry season. We found evidence of large population size and migration from Fourda in Kenieroba during the wet season, but very low numbers and no sign of migrants during the dry season. We suggest that malaria vector control measures aimed at adult mosquitoes might be made more efficient in this region and other seasonal riparian habitats by targeting disruption of mosquito populations by the river during the dry season. This would decrease the size of an already small population, and would be likely to delay the explosive growth in vector numbers in the larger inland villages as rainfall increases.     

Seasonal climate effects anemotaxis in newly emerged adult Anopheles gambiae Giles in Mali, West Africa

The direction and magnitude of movement by the malaria vector Anopheles gambiae Giles has been of great interest to medical entomologists for over 70 years. This direction of movement is likely to be affected by many factors, from environmental conditions and stage of life history of the mosquito to the existence of attractants in the vicinity. We report here the direction of movement of newly emerged An. gambiae in nature, around the village of Donéguébougou, Mali. We assessed the direction of movement for individual mosquitoes by placing them in a novel enclosure with exit traps oriented in the direction of the cardinal and intermediate points of the compass. We consistently found predominantly Southward directions of movement during 2009 and 2010, with an additional Eastward component during the dry season and a Westward one during the wet season. Our data indicate that wind has an important effect on the direction of movement, but that this effect varied by season: Average directions of movement were downwind during the dry season and upwind during the wet season. A switch in anemotactic response suggests that the direction of movement of An. gambiae relative to the wind immediately after emergence under varying conditions of humidity should be further investigated under controlled conditions.