Dengue fever is a high-burden mosquito-borne disease endemic in tropical and subtropical regions of the world. It is caused by four antigenically distinct dengue viruses (DENV-1 to DENV-4) that are transmitted to humans by bites from mosquitoes belonging to the genus Aedes. Dengue virus (DENV) infections can be asymptomatic or manifest as flu-like symptoms, but occasionally develop into a severe, hemorrhagic form that can lead to severe shock syndrome. DENV is an 11-kb, positive-sense, single-stranded RNA virus belonging to the Flavivirus genus. The four distinct serotypes share around 65% nucleotide identity and do not induce cross-protective immunity (1). Within each serotype, multiple genotype lineages exist, with differing global distributions, as classified by Weaver and Vasilakis in 2009 (2).

Laboratory-confirmed DENV outbreaks have been reported in Africa since 1968 (3), predominantly caused by DENV-1 and DENV-2. Despite major improvements in the diagnosis and surveillance of infectious diseases in Africa during the last decade, dengue fever is widely considered to be underdiagnosed (4, 5), and laboratory-confirmed cases remain rare.

DENV-1 was first isolated in Africa from a febrile patient in Nigeria in 1968 (3). Of the five recognized DENV-1 genotypes (2), two have been reported to circulate in Africa, with distinct geographic distributions. The African lineage of genotype I (GI) has been reported solely in East Africa: in Djibouti in 1998, in Eritrea in 2010, and in Somalia in 2011 (6–8). The African lineage of genotype V (GV) was originally identified in West Africa, specifically in Nigeria in 1968 and in Côte d’Ivoire in 1985 and 1998 (3, 9, 10). However, recent observations from Gabon in 2012 and Angola in 2013 indicate that the GV African lineage is also present in Central Africa (11, 12). In addition to these DENV-1 lineages endemic in Africa, strains phylogenetically related to the Asian lineage of GV have recently been detected in travel-acquired infections from East and West Africa (13–16) and in outbreaks in West Africa (17, 18).

In the present study, we report laboratory-confirmed human DENV-1 infections thought to have been acquired by French patients during a stay in West and Central Africa between 2013 and 2019. We provide molecular characterization of the detected strains in aid of an improved understanding of DENV-1 distribution and circulation in Africa.

Cases from our dataset of travelers have revealed the presence of DENV-1 in countries where the serotype and/or virus had yet to be reported ( Figure 3 ).

We report two cases of DENV-1 acquired in the Sahel, a region where DENV circulation is rarely documented. The Sahelian climate is characterized by an absence of rainfall for several months of the year, with no natural water surfaces for mosquito breeding. However, Aedes species have been reported across the entire Sahel region (28–31). Indeed, anthropophilic mosquitoes such as Aedes species can thrive in anthropogenic water sources, especially during the moderate rainy season, from July to September. Interestingly, some laboratory-confirmed DENV infections have been detected in Sahelian countries, such as Chad, Sudan, Mali, and Mauritania, since 1986 (6, 32–37) ( Figure 3 ). In Niger, Aedes vectors are established in dwellings from the south to the central regions (28). The first reported case of dengue fever was an imported case from Cuba in August 2022 of unknown serotype (38); however, our observation in case number 47248 suggests that DENV was present in the country prior to this, at least as early as 2018. Whether it resulted from a recent viral introduction or had been in undetected circulation for years remains unknown. The infection occurred in March during the 8-month-long dry season (i.e., the period of no rainfall from October to April), suggesting the presence of competent vectors outside the rainy season. Among other Sahelian countries, DENV-1 had previously been found in travelers returning from Eritrea (7, 15), Chad (35), and Sudan (39), and it caused an outbreak in Bamako, the capital city of Mali, in 2006 (32) ( Figure 3 ). The DENV-1 infection of case number 30078 also places this serotype in Mauritania, 1 year after the description of the establishment of Ae. aegypti in the capital city, Nouakchott (40).

In West and Central Africa, substantial rainfall all year round, the tropical climate, and forests provide a suitable environment for annual vector proliferation, with sylvatic and urban Aedes species, including Ae. aegypti and Ae. albopictus, being broadly established (40). Consequently, DENV (and DENV-1 in particular) has been reported in most countries of the region since 1968, mostly in travelers returning from the region and in the form of localized outbreaks (3, 6, 9, 10, 13–15, 18, 33, 39, 41–54). However, no official reports of DENV in the Republic of the Congo have been published, despite the presence of abundant competent vectors and known recurrent arboviral outbreaks (55, 56). The DENV-1 infection of case number 38640 was acquired during a stay to the Republic of the Congo, indicating that DENV was present in the country at least as early as 2016, and its occurrence at several time points in all neighboring countries (51, 53, 54, 57) suggests that it has probably been in undetected circulation for years.

Available genomic data from DENV-1 in Africa, supplemented with our sequences, reveal the current circulation of several distinct lineages.

All known Central African DENV-1 strains cluster within the GV African lineage. The oldest strains of this lineage were isolated in Nigeria in 1968 (AF425625) and in Côte d’Ivoire in 1985 (AF425620), indicating endemic circulation in sub-Saharan Africa ( Figure 2 ). The GV African lineage was implicated in a major urban outbreak in Angola in 2013 (11, 33, 58–60), thus demonstrating its epidemic potential on the continent. Viruses from this cluster have already been exported from Africa by travelers returning to Asia (53, 61), Europe (54, 59), and Israel (57).

DENV-1 strains currently detected in West Africa (including Mauritania) cluster in the GV Asian lineage, in a monophyletic group composed of strains detected in Africa since 2015. An introduction from Southeast Asia to Africa, probably through East African countries, could have occurred around 2012, as suggested by a recent Swedish study (15). This lineage was found to be responsible for sporadic cases in Senegal and Mali since 2015 and for significant outbreaks in the following years in Senegal (43). Genomic data from this study were not available at the time of writing and are therefore not included in our analysis. This Senegalese study suggested that DENV-1 was introduced to Senegal via Mali around 2014 (43), which is consistent with the timeline reported by Alfsnes et al. (15). Since its introduction to Africa, the GV Asian lineage has been detected in multiple countries throughout the continent, revealing rapid geographical spread: Eritrea in 2015 (15), Mauritania in 2015, Senegal from 2015 to 2019 (43), Kenya in 2016 (15), Mali in 2017 (43), Burkina Faso in 2017 (17), Benin in 2019 (13), and Côte d’Ivoire in 2019 (16). It is important to note that during this period no other DENV genotype was reported in West Africa. Based on current knowledge, it could be hypothesized that the GV Asian lineage of DENV-1 has replaced the historical African lineage that was last detected in West Africa in 1998 in Côte d’Ivoire (AF298807). However, the disparity in publicly available data limits our understanding of the DENV-1 dynamic within the continent. Outside Africa, viruses belonging to this monophyletic group according to the E-gene phylogenetic tree have been identified in travelers in China in 2016 and 2019 (KX372686 and MN921591, respectively), in Australia and Thailand in 2015, and in an outbreak in Malaysia in 2013 ( Figure 2 ). Due to the high nucleotide identity of the E-gene sequences within this group and the lack of full-genome sequences, care should be taken in the interpretation of phylogenetic trajectories. Nevertheless, these observations reveal the exchange of highly similar viruses on multiple continents over the last 10 years.

Increasing economic exchanges between Asia and Africa over the past decades have resulted in an influx of Asian workers and freight shipping through African ports, thus creating putative importation routes for infectious diseases from Asia to Africa and vice versa (62). Considering the presence of local competent vectors in sub-Saharan Africa and the endemicity of DENV in Asia, more introduction events are to be expected in the future.

Our findings from molecular analyses indicated active circulation of two distinct DENV-1 genetic lineages in Africa: (i) the African group of GV Asian lineage, imported from Asia probably around 2012, which is currently spreading in the Sahel and West Africa; and (ii) the historical GV African lineage detected in Central Africa. Our cases found in the Sahel suggest that DENV, alongside its anthropophilic vectors, is spreading outside its well-established tropical and subtropical environments into the arid deserts of the Sahel. Moreover, this study underlines the importance of surveillance of travel-acquired infections, as publicly shared epidemiological surveillance data can only improve our overall understanding of disease circulation, in this instance in remote locations with limited access to molecular analysis.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession numbers can be found in the article/ Supplementary Material .

Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements.

IL-G, XL, GG, and GD contributed to conception and design of the study. TF and GP performed laboratory analysis. GP and AD-P contributed to primer design. GP, FT, and TF performed bioinformatic treatment and analysis. TF wrote the first draft of the manuscript. All authors contributed to the article and approved the submitted version.