Background
Dengue is an arbovirus infection caused by a Flaviviridae family’s virus, of which there are four serotypes (Dengue Virus 1, 2, 3 and 4). It is transmitted to humans by the bite of infected female mosquitoes of the genus Aedes (
Aedes aegypti and
Aedes albopictus) [
1]. The World Health Organization distinguishes symptomatic forms of dengue into dengue with or without warning signs, a mild and common form, and severe dengue with bleeding complications, signs of shock or visceral failure [
2]. Classified as a neglected tropical disease, dengue has been experiencing epidemic resurgence in recent years and is present in more than 128 countries [
1,
3]. Its social and economic impact is increasingly important.
The first case of dengue (probably base on dengue-like syndrome) in Burkina Faso was reported in 1925. The country experienced epidemics in 1982 with patients positive for specific anti-Dengue IgM antibodies and virus isolation in cell culture, 2013, 2016 and then 2017 [
4‐
6].
Dengue was then included in the country’s list of priority diseases in 2016, and integrated into the list of diseases under epidemiological surveillance. Various intervention programs ranging from diagnostic capacity building to community-based interventions have been implemented to control dengue [
7‐
9].
Previous studies have shown that the spatial distribution of dengue can be quite heterogeneous within a country and even at subnational scales. Seasonality of dengue transmission mainly influenced by climatic factors has also been reported [
10‐
13].
Knowledge of the spatial and temporal epidemiology of dengue over time in Burkina Faso are essential for adapting dengue control programs. To our knowledge, there are no studies that have explored this. The objective of the present study is to assess and map the spatio-temporal trends of dengue cases in Burkina Faso at the health district level.
Discussion
In this study, we mapped the crude incidence, excess risk, and smoothed incidence of dengue at the health district level in Burkina Faso. We also identified significant spatial and temporal groupings of dengue cases.
To our knowledge, this is the first application of such a methodology to dengue data in Burkina Faso. This methodological approach is a strength of this study, as it is adapted for the analysis of aggregated data of a disease with low incidence or when the analysis involves spatial aggregation units of very heterogeneous size, small in places [
10,
16]. Smoothing highlights unexpected gradients and reduces outliers [
20] presenting a better incidence distribution mapping.
We used passive surveillance data for dengue in this study. The main limitations of this study are related to this. Indeed, this type of data is subject to under-reporting of cases. Also, aggregation at the district level, which corresponds to large geographical units, does not allow capturing heterogeneity of distribution at lower scales.
The results showed that the central region is the most vulnerable in the country. It covers the capital city, Ouagadougou, which is the most populated and urbanized city in the country. Dengue seems to be more urban because apart from the health district of Barsalgho, all dengue clusters are urban districts.
On one hand urbanization favors the development of artificial containers, development sites for Aedes aegypti dengue vectors. The high density of the population and their great mobility favor in turn the explosion of dengue cases. The high inter-mobility and contiguity with Ouagadougou could explain the excess of cases found in the health district of Ziniaré, which is located 35 km away and where different activities bring together actors from Ouagadougou. The same is true for the health districts of Sabou and Dédougou in relation with Koudougou.
On the other hand, dengue cases in non-urban areas are likely to be underestimated due not only to poor diagnostic accessibility but also to missed diagnoses. Indeed, many cases of fever are considered as malaria and treated as such in these areas without biologic confirmation.
The peak of cases has always been observed between October and November with a relative stability outside this period. This period follows the rainy season (May - October) in Burkina Faso. This seasonality in dengue transmission has been reported by several authors. With a time lag, there is a significant link between dengue incidence and climatic factors such as temperature and rainfall. In the life cycle of
Aedes Aegypti, the main dengue vector in Africa, studies have shown that at optimal average temperatures (< 18 °C) an increase in temperature increases the incidence of dengue by shortening the development period of
Aedes aegypti larvae and improving their blood feeding and oviposition, whereas at high average temperatures (> 18 °C) an increase in temperature will reduce the survival of
Aedes and thus the transmission of dengue [
21‐
23].
Large amounts of rainfall may result in the short-term removal of
Aedes aegypti eggs and larvae from potential containers, but residual water may create longer-term breeding habitats [
24‐
26].
The results of this study suggest strengthening dengue control interventions in urban settings and in the last quarter of the year. A study in the city of Ouagadougou at the level of health facility coverage areas and administrative sectors, exploring other risk factors for dengue, will complement the present study. Active surveillance in rural areas may provide a more reliable estimate of the burden of dengue in these areas.
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