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

Abstract

BackgroundLassa 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 FindingsSmall 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/SignificanceThe 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.

Lassa Virus Seroprevalence in Sibirilia Commune, Bougouni District, Southern Mali

“…The initial surveys found that 25% of M. natalensis rats had molecular evidence of active LASV infection, which was confirmed by virus isolation and sequence analysis ( 16 ). Similar studies conducted across Mali suggest that LASV is restricted to the southern tip of the country, in several villages near the border of Côte d’Ivoire ( 17 ). On average, 20% of peridomestic Mastomys rodents collected in these villages had serologic or molecular evidence of LASV infection, with peak prevalence rates >50%.…”

“…Soromba is the most likely exposure site of the only known case of LF in Mali and, along with Bamba, has the highest LASV prevalence documented in peridomestic rodents ( 16 , 17 ). Banzana is a nearby village with a low prevalence of LASV-infected rodents.…”

“…Branco, M. Boisen, unpub. data) and have previously been used to detect anti-LASV antibodies in rodents collected in southern Mali ( 17 ). Optimized LASV ELISA methods developed at Kenema Government Hospital were used for testing the samples.…”

Sequence Variability and Geographic Distribution of Lassa Virus, Sierra Leone

“…Lineages I, II, and III, and the greatest diversity of LASV strains, were found among isolates from Nigeria, whereas strains from Guinea, Sierra Leone, and Liberia seemed to be more closely related and belong exclusively to lineage IV. Sequence of the AV strain ( 21 ) and recently published sequences from rodent LASV isolates from Mali ( 18 ) suggest the existence of an additional clade (proposed as lineage V) ( 22 ). LASV sequences of isolates from humans and rodents are found interspersed throughout the phylogenetic tree, which is consistent with the notion that human cases typically result from transmission from rodents ( 17 ).…”

Annual Incidence of Lassa Virus Infection in Southern Mali

“…12,13 The results of the 2015 serosurvey demonstrated that village-specific prevalence rates in humans correlated with infection rates previously observed in rodent populations. In Banzana, where rodent prevalence rates were relatively low, 14.5% of study participants had detectable IgG antibodies against LASV, whereas in Soromba and Bamba, where up to 50% of sampled rodents had evidence of LASV infection, 41% and 44% of volunteers, respectively, were seropositive for LASV.…”