Heatwaves and Health Risks in the Northern Part of Senegal: Analysing the Distribution of Temperature Related Diseases and Associated Risk Factors

The Sahelian zone of Senegal is marked by heatwave events due to temperatures increase especially in 2013 exceeding 45 ° C with an impact on morbidity and mortality rise. In order to document health impacts of recurrent extreme temperatures in this part of the country, a study was carried out combining heatwaves detection, occurrence of climate-sensitive diseases and risk factors for exposure.


Introduction
In the context of global climate warming marked by temperatures increase, particularly in the Sahel strip, heat waves events are becoming a serious public health threat with the absence of effective adaptation measures (Huang,et al., 2010 ;Benmarhnia et al., 2014 ;Murari et al., 2015 ;Guo et al., 2016 ;Baaghideh & Mayvaneh, 2017). With a predicted rise in global temperature from 1 to 6°C by 2100, the risk of extreme weather events such as heat waves will increase, and greatly affect Sahel territories (Kovats & Kristie, 2006 ;Fouillet et al., 2006;Christensen et al., 2007;Smith et al., 2014;WMO, 2015). In contrast to developed countries, heat wave episodes and their impacts on population health are still poorly documented in this area where the number of hot days and nights is constantly increasing ( Heat waves events have been associated with excess of mortality and morbidity which often affect young children, people suffering from chronic diseases and the ageing population (Armstrong, 2006 ;Elliot et al., 2013;Zacharias et al., 2014;Auger, 2015;Chen, 2016). A historical review of some past events provides a good context of health risks associated with heat waves. In the city of Chicago in USA, a large part of the ageing population was affected by rising temperatures, with 365 deaths recorded during the summer of 1995 (Naumova et al., 2007). In France, the heat wave recorded during the summer of 1983 led to the death of several elderly people, with more than 300 deaths reported in the city of Marseille (Cadot, 2006;Fouillet, 2006). The terrible heat wave of 2003 was particularly lethal with more than 15,000 deaths reported in France (Besancenot, 2005;Vandentorren et al., 2003). The occurrence of heat waves in July 2018 in the northern hemisphere, which lasted seven (07) days, was marked by excess of mortality with 70 deaths in Canada and 35,000 people hospitalized and 80 deaths in Japan. In 2013, the Sahelian region located between Senegal, Mauritania and Mali experienced an exceptional heat wave with a temperature which exceeded 45°C and lasted from May 23 to 27 (i.e., 5 days). This event had a considerable impact on the population health. During this heat wave episode, localities in northern Senegal recorded 27 cases of death reported and con rmed by the health districts.
These observations show a generalization of heat wave events at the planetary level, hence an international mobilization on issues related to the temperatures increase is considered as a health priority issue. The major ndings based on peer review litterature dealing with heat wave events over America and Europe have demonstrated that the impact of extreme temperatures on human health is a reality in terms of excess of mortality and morbidity recorded. In Africa, however, the health impact of heat waves is still little known due to the lack of scienti c work on this phenomenon, while in some geographical areas such as the Sahel, high rates of extreme temperatures are often recorded (Musengimana et al., 2016). Thus, as part of a research project of Sahelian Heatwaves Alert and Health Impacts (ACASIS) funded by the French National Research Agency (ANR) and implemented by the French Research Institute for Development (IRD) and its scienti c partners, a series of eld surveys were carried out between 2016 and 2018 in departments of the northern part of Senegal located on the Sahelian belt which is more affected by heat wave episodes. The purpose of these studies was to scienti cally document the effects of the extreme temperatures recurrence on communities health in this part of the country, with a particular emphasis on the detection of heat wave indices, the evolution of linked climate diseases and climate vulnerability factors such as the practices and behaviours of populations facing abnormally high temperatures during the months of April-May-June and the socio-economic, environmental and cultural characteristics of households. Thus, the rising temperatures are a major driver of health status deterioration of communities; the relationships between (i) the recurrence of heat wave episodes, (ii) excess of morbidity/mortality and (iii) the exposure factors involved in modulating the vulnerability of populations against this type of health risk, were analyzed in the present study.

Study area
The study was implemented in the Northern and North-Eastern part of Senegal and covers the departments of Dagana, Podor, Louga, Linguère, Matam, Ranérou, Kanel and Bakel located in the regions of Saint-Louis, Louga, Matam and Tambacounda ( gure 1). This area considered is the most affected one by heat waves in Senegal has an estimated population of 1,928,962 inhabitants (ANSD, 2013). The averaged population density, often very low, is about 30 inhabitants per km 2 due to a strong migration towards coastal urban centres. The majority of the resident population are often elderly peoples, women and children.
The climate characteristic is continental Sahelian type with two seasons: a rainy season (July-September) and a dry season (October-June). After three decades of drought, wet periods are back in this area since the early 2000s, even if rainfall total is generally low, varying between 200 and 400 millimetres per year. Temperatures in this area are among the highest in Senegal, with annual averages around 30°C and maxima sometimes exceeding 45°C. These particular climatic conditions that characterise the study area do not enable the development of dense vegetation and the presence of permanent watercourses which can lead to a micro-climate conditions.

Temperature data
The temperature data were obtained from ANACIM weather stations in the departments of Louga, Linguère, Podor, Matam and Bakel. These are ambient temperatures in the areas polarized by health structures in the study area. These data were supplemented by data from climate model outputs for the departments of Dagana, Ranérou and Kanel without meteorological stations. The variables used to analyse the impact of heat wave episodes on population health in the study are relative to the maximum, average and minimum temperatures during the hottest months of April, May and June for the period 2009 to 2019. The choice of these variables for the correlation analysis between heat wave episodes and health risks was motivated by the scienti c need to document and analyse the relationship between heat wave episodes and health risks in terms of excess of morbidity and mortality.

Epidemiological data from health facilities
Health data were collected on the basis of the prior de nition of a list of diseases considered as linked to climate-variability (i.e., arterial hypertension, diarrhoea, asthma, colds and coughs, acute respiratory infection, diabete, kidney problems, heart problems, joint pain and skin irritations). This list was de ned through the precise and clear identi cation of clinical signs and symptoms (fevers, headaches, body pain, heat stroke, exhaustion, dehydration, syncope and hyperthermia) associated with the physiological state of the patients in consultation during periods of rising temperatures.
The health data collection was carried out for the months considered to be the hottest (April-May-June) from 2009 to 2019.
The collected data comes from two sources: (i) the National Health Information System « SNIS » via the DHSI2 (District Health Information Software) platform of the Ministry of Health and Social Action and (ii) the use of information from patient consultation registers using a collection form provided by the health centres of the departments of Louga, Linguère, Dagana, Podor, Ranérou, Matam, Kanel and Bakel. The variables recorded were the date of consultation, age, sex, place of residence, clinical signs or symptoms, the pathology diagnosed, the treatment prescribed, etc. These informations were then used to determine the patient's health status and the cause of consultation.

Socio-economic, environmental and health surveys among households
These quantitative and qualitative cross-sectional surveys were carried out with the help of the Health Districts professionals in the target departments between 2015 and 2017. These household surveys were designed to highlight the factors of exposure and vulnerability to the heat waves health impacts. They involved households in the departments that agreed to participate by signing the informed consent form. The information collected by means of a questionnaire covered the following topics: demographic and socio-economic characteristics, knowledge of climate change and climate risks, identi cation of clinical signs or symptoms associated with climate-sensitive diseases, morbidity and mortality of the target pathologies, access to health care, factors of exposure and vulnerability to the health impacts of heat waves, adaptation behaviours and practices.
The sample size was determined using the National Agency for Statistics and Demography (ANSD) database, which provides estimates of 13,306 households in the study area. By applying a con dence level of 90% and a margin of error of 5%, the sample size was 1,331 households to be surveyed for the entire target area. For the sample spatial distribution, a two-stage strati ed survey was carried out based on a strati cation of the different departments according to speci c demographic, socio-economic and environmental characteristics. In each stratum of departments, ve villages were selected, three of which were in urban areas and two in rural areas. In the selected villages, the proportion of households to be interviewed in the overall sample was calculated in proportion to the percentage of the number of households present in the locality. The method used to select the households to be surveyed was done randomly with a time step of ten (10) concessions. The questionnaire was submitted to the head of household or persons designated by him to answer the questions. Out of an initial planned sample of 1,331 households targeted for the study, only 1,246 were surveyed due to different reasons such as the inaccessibility of the village and unavailability of respondents.

Data processing and analysis
The processing of the collected data led to the identi cation of indicators related to heat waves associated to health effects, the epidemiology of climate-sensitive diseases and the associated risk factors. The temperature and health data collected were statistically analysed using different data processing software such as SPSS, R, Stata, XLStat and Arcgis. For the data analysis on perceived morbidity, it should be noted that only 1,119 questionnaires for which information on the symptoms described by the respondents corresponded to the clinical signs of climate-sensitive diseases were taken into account in the statistical analyses.
The temperature data (maximum, minimum and average) were analysed on the basis of the ANACIM 1971-2000 normal, the three-day temperature threshold exceedance of the 90th percentiles (IPCC, 2007) and the de nition of heat wave indices as part of ACASIS project. Daily maxima observation and daily mean maxima temperatures were considered to de ne a heat wave period that takes into account the threshold of the 97.5 and 81th percentile of the maximum temperature distribution (Huth, 2000). These indices of heat waves and temperature threshold exceedances (maximum, minimum and average) compared to the seasonal normal were used to identify trends in temperature anomalies and highlight the years during which the months of April, May and June are the warmest for the period 2009-2019 in this part of Senegal. These analyses provided a series of temperature curves and maps that enabled to select three years during which the months of April, May and June were warmer.
Epidemiological data on climate-sensitive diseases (diagnosed morbidity) were analysed by applying descriptive statistical methods (dynamic cross-tabulation) to determine their prevalence in overall morbidity and mortality but also their distribution according to the different departments, age and sex of patients affected by the pathologies concerned for the months of April, May and June of the period 2009-2019. Cross-analyses in the form of correlative evolution curves between the temperature variables (maxima, minima, averages) for the months of April, May and June and the evolution of the number of consultations and deaths for climate-sensitive diseases were carried out to highlight the association between heat waves and health risks. The results of these analyses are presented in the form of graphs and maps.
Data on the perceived morbidity and mortality of temperature related diseases from household surveys were rst analysed using descriptive statistical methods (dynamic cross-tabulation) to determine their prevalence in overall morbidity and mortality rate but also their distribution according to the different departments, age and sex of patients affected by the diseases concerned. Factorial and correlative statistical analyses were then carried out between the prevalence of perceived morbidity and the various risk factors identi ed (socio-demographic, economic, environmental and cultural variables) by carrying out Chi-2 (Fischer/Pearson) tests, logistic regression with Odds Ratios (OR) and 95% con dence intervals and a correlation matrix between different dependent variables.

Distribution of morbidity and mortality of heatwavesensitive diseases
The spatial distribution of morbidity diagnosed from 2009 to 2019 of diseases sensitive to the rise in temperatures for the months of April, May and June shows a higher number of consultation cases in the departments of Matam (44,514 cases), Kanel (41,655 cases) and Bakel (41,204 cases) while this proportion is lower in Linguère (10,588 cases), Louga (21,527 cases) and Dagana (23,476 consultation cases) with a predominance of the female gender ( gure 4). For the three months considered, the distribution according to type of disease shows that diarrhea is the most represented in global pathology.
The global distribution of the morbidity diagnosed nearly obeys the same dynamic as that reported for 2013, with more than 12.5% of the households surveyed declaring that they had experienced morbid episodes (769 cases) during the heat wave period ( gure 5). Indeed, households that declared cases of morbidity are more represented in the departments of Kanel (17.7%), Ranérou (16.1%), Matam (13.7%) and Bakel (13.7%), while this proportion is relatively lower in Linguère (7.8%) and Podor (8.5%). The gender distribution of affected population during the May 2013 heat wave shows that women (57%) were more affected than men (43%). The occurrence of heat waves in May 2013 was associated with cases of death reported by the households surveyed, and had particularly affected elderly women ( gure 6). However, reported mortality (con rmed by health professional) of 12.4% (119 cases) is also unevenly distributed across the departments with more households reporting deaths noted in Matam (25.2%), Bakel (23.5%), Dagana (22.7%) and Louga (10%) than in Linguère (9.2%), Podor (8.4%) and Kanel (0.8%).
The spatial distribution of reported morbidity is almost proportional to the distribution of reported mortality according to the departments except for Dagana where the rate of households that registered occurrence of morbid episodes is lower than the number of reported deaths.

Relationships between heatwaves and morbidity of temperature-sensitive diseases
The analysis results of the association between the temperatures evolution for the months of April, May and June and the cumulative of temperature related pathologies show a growing increase in consultations in health centres from the years during which heat waves began to be more recurrent.

Exposure and vulnerability risk factors to health impacts of heat waves
Exposure and vulnerability to the health impacts of heatwaves are in uenced by a number of risk factors ( Table 1). The age of persons has been identi ed as an exposure risk factor to the effects of heat waves, which affect more people over 61 years old and children and adolescents under 20 years old. This risk factor is often aggravated by having a medical background including chronic diseases such as high blood pressure, epilepsy, diabetes, heart disease or asthma which more affect the elderly. The majority of those affected had symptoms associated with the onset of heat waves such as hyperthermia, headaches, tiredness body, fainting or dizziness. Access problem to safe drinking water was also analyzed as an exposure factor to health impacts of heat waves. Households with a tap at home are less exposed to the effects of increasing temperature than those who move to get water from standpipes (91%) or wells (86%) ( gure 8). The type of fuel used is also an exposure risk factor to temperature-sensitive diseases.
The results of the analyses show that people living in households using wood (85%) or charcoal (85%) are more affected than those using butane gas ( gure 4: b). The kitchen location was also an exposure factor to health risk related to heat waves. Indeed, women cooking open air are much more at exposure risk to effects of heat waves than those whose kitchen is located in housing inside ( gure 8). The type of housing and building materials were identi ed as risk factors for diseases related to heat waves. People living in huts and straw huts are less exposed to the temperature effects than those living in low-rise (76%) or multi-storey (80%) houses ( gure 8). For building materials, people living in straw or wooden houses are less exposed than those living in solid or banco-built housings ( gure 8). The availability of household equipment such as a fan or air conditioner was analysed as a mitigation factor of heat waves effects. The analyses demonstrated that people living in households with a fan or air conditioner are less exposed to heat effects compared to those without such equipment ( gure 8).  Thus, the occurrence frequency of heat waves is increasingly becoming a major public health issue in the Senegalese context, in particular since the results of the study showed that the gradual increase in temperatures up to around 39°C resulted in a surplus of 45 patients in health centres and an excess mortality of nearly 25 people during hot days, in particular in the departments of Matam, Bakel, Dagana and Louga (Diboulo, 2012;Petkova, 2013;Zhang, 2014). According to clinical examinations during the heat wave periods, the symptoms presented by the patients were hyperthermia, headaches, tiredness, fainting or dizziness, while the pathologies diagnosed were mainly chronic diseases such as high blood pressure, epilepsy, diabetes, cardiovascular diseases, respiratory infections, diarrhoea and asthma (Semenza et al., 1995 ;Pascal et al., 2013;Smith et al., 2016 ;Sanderson et al., 2017).
However, the distribution of heat-wave-related diseases varies widely by location, age, gender and risk factors. Indeed, the departments located in the North-Eastern part of the country, which are Matam, Ranérou, Kanel and Bakel, concentrate the populations that are most exposed to the health risks related to heat waves occurrence. As demonstrated in other countries in Europe, Asia and America, people over 61 years and the infant and child fringe are the most vulnerable to the health impacts related to the rise in temperatures in the most affected departments (Jones et al., 1982;Semenza et al. 1996;Hémon et al., 2003;Pascal et al., 2005;Hémon et al., 2005). Older people are exposed through the prism of co-morbidity to chronic diseases considered as the aggravating health background of heat waves effects (Nitschke, 2013). Regarding children, increasing temperatures amplify the spread of childhood illnesses, especially those that cause dehydration, in a group consisting mainly of schoolchildren who, in these localities, walk long distances to reach their schools, whose descent (12 to 13 hours) and recovery (14 to 15 hours) times correspond to period of extreme heat peaks (Xu et al., 2012). According to gender, women engaged in extra-domestic activities, such as fetching water, searching rewood and agro-pastoral activities that require them to travel long distances under the sun, are the most exposed to heat waves effects,

Conclusion
Minimum and maximum temperatures are on an increasing trend in the northern part of Senegal with the years 2010, 2013, 2017 and 2018 considered as very hot with a large number of days with heat waves observed. This temperature upward combined with the rise in relative humidity tends to increase the heat felt sensation and the heat waves effect on populations health.
The frequency of heat waves recurrence resulted in a resurgence of certain diseases sensitive to the temperature increase leading to excess of morbidity and mortality with high frequentation of health facilities in the North and North-East departments of Senegal. The diseases concerned are mainly chronic diseases such as heart disease, diabetes, arterial hypertension, asthma, respiratory affections, hyperthermia, rheumatism, etc., which affect more people over 60 years and children under 14 years. The highest morbidity and mortality rates were recorded especially among elderly women during the severe heat wave episodes experienced especially in May 2013.
While there are many risk factors associated with morbidity and mortality related to the impact of heat waves, including housing conditions, lifestyles, socio-economic comfort level and types of activities, populations with a health background are mainly most exposed and vulnerable to rising temperatures.
However, the high vulnerability of the populations noted in these localities is not just related to the occurrence of extreme temperatures but also to other factors such as adaptation strategies or the responses of communities to the health impact of heat waves. Thus, the prevention of health problems related to rising temperatures requires the development of an early warning system for heat waves in order to strengthen the resilience of populations and the health system to the impacts of climate change.

Con ict of interests
The authors declare that they have no con icts of interest to disclose.

Funding
This study was supported by funding from the Climate Research for Developpement (CR4D) from the African Academy of Sciences (AAS), the United Nations Economic Commission for Africa (UNECA) and the United Kingdom Aid (UKAID) through the Centre for Ecological Monitoring (CSE). The funder had no role in the study design, in the data collection and analyses or interpretation of the data, decision to publish or preparation of the manuscript.