Environmental Conditions and Risks of Emergence of Dengue in Douala, Cameroon

By

AMOUGOU Judith Virginie

Doctor Ph.D in geography, University of Yaoundé I, ajuvie@yahoo.fr

TCHOTSOUA Michel

Professor, University of Ngaoundéré, tchotsoua@yahoo.fr

FOUDA Martin

Lecturer, University of Douala, mfouda60@yahoo.fr

PAUPY Christophe

IRD medical entomologist, paupy@ird.fr

Abstract. In the domain of tropical diseases, environmental factors explain the greater or lesser presence of infectious pathogenic systems. In fact, the climatic conditions and the urban atmosphere in Douala constitute fertile ground for the emergence of a tropical arbovirus called “dengue”. In 2006, a team of researchers (epidemiologists, entomologists, virologists, socio-demographers and geographers) set out to study dengue in the changing environments of Bolivia and Cameroon, as part of project n ° 00119 05 entitled “Emergence of dengue in changing environments (EPI-DENGUE)”. Dengue is an essentially tropical viral disease transmitted by two mosquitoes: Aedes albopictus and Aedes aegypti.

Τhis paper, is about analyzing the climatic and urban environment, likely to favor the emergence of dengue fever in Douala. From surveys, field observations and data collection, we assessed the climatic conditions and the urban environment favorable to the vectors and virus of the disease. Dengue is therefore a disease linked to the climatic and urban environment. The vector populations involved in the transmission are domestic and / or peri-domestic.

Key Words.  Douala, Dengue, climatic conditions, urban atmosphere, vectors.

Introduction

In 2006, a group of researchers (epidemiologists, entomologists, virologists, socio-demographers and geographers) set out to study dengue in the changing environments of Bolivia and Cameroon (figure 1), within the framework of the project n ° 00119 05 entitled “Emergence of dengue in changing environments (EPI-DENGUE)”. Dengue is an essentially tropical viral disease, whose symptoms are similar to those of malaria and transmitted by two mosquitoes, Aedes albopictus and Aedes aegypti. It is reported in several tropical countries in the world, including in neighboring Nigeria. Even if it has not been officially declared in Cameroon yet, the presence of vectors, the urban and climatic environment justifies our questioning in this communication. Through a study of the urban atmosphere and the climatic conditions prevailing in Douala, it is about considering a cause and effect relationship between these environmental conditions and its exposure to the emergence of dengue fever through risk simulations. .

Figure 1. Country of study of the EPI-DENGUE program

1. Material and methods

The method inspired in this research is the cross-sectional survey method. It is about carrying out ad hoc studies at a given time based on morpho-hydrological maps, satellite images and field observations.

 The information extracted from the images and collected in the field are grouped together in GIS (Geographic Information System) in the form of layers of objects on which the alphanumeric data are attached.

 The databases thus have 2 groups of information relating to:

– land use (district, orchards, road networks, bare and / or waterproofed areas) extracted from satellite images supplemented by G.P.S. and field surveys;

– physical environment (relief, hydrographic network, ponds and flood zones) extracted from the topographic map completed by G.P.S.

– – A procedure that led to the design of the cards was carried out. Indeed, a protocol has been set up to conduct surveys in the city of Douala (Figure 2)

To implement the protocol, a drawing of lots of clusters, a total of 75 per city, was carried out. A cluster is a pair of GPS coordinates that refers the starting point for surveys on the city’s surface.

From a cluster, we sought at least four concessions to the north of the cluster to conduct the surveys.

 As the study is multidisciplinary (geography, epidemiology, entomology, demography, etc.), the survey questionnaire was distributed according to each discipline. The investigation was conducted in a very specific order. To start it, participants had to agree to have 10 ml of blood drawn. This blood was to be used to look for traces of antibodies left by the disease. With the agreement of the participants, the order of administration of the questionnaires was as follows: Socio-demography, epidemiology entomology and geography. It is important to note that certain disciplines (geography and entomology) in addition to the interviews, collected information related to the respondent’s immediate environment: characteristic of the habitat and characteristics of mosquito breeding sites. The geographic coordinates of all the concessions surveyed during the study were taken by GPS.

At the end of the surveys, each discipline retrieved its part of the questionnaire and entered the information gathered. After the respective entries, it was necessary to reintegrate all of the data thus distributed.

             Figure 3. Distributed data integration process. 

The result of this operation is a spatially referenced data file. 

Based on surveys, field observations and data collection, we assessed an environment of causes grouping together the climatic conditions and the urban environment favorable to the vectors and the virus of the disease (Amougou, 2009). The environment of consequences includes the presence of dengue antibodies.

 2. Results and discussion

2.1. Urban atmosphere favorable to the emergence of dengue fever

Numerous breeding sites of mosquitoes, potential vectors of dengue, were found in concessions in the city of Douala in 2006 (Figure 4).

Figure 4. Comparative sketch of the presence of individuals Aedes albopictus and Aedes aegypti in the same concessions of health areas in the city of Douala.

According to the entomologists of the Epi-dengue team, the number of breeding sites positive for Aedes aegypti individuals in the concessions ranged from 1 to 66 while that of Aedes albopictus individuals in these same concessions ranged from 1 to 54.

Analyzes of surveys carried out by epidemiologists of the Epi-dengue team in Douala in the concessions in October 2006 seemed to indicate a prevalence of IgG antibodies (old testifying to an infection dating beyond three months) raised to more than 60% out of 730 people sampled voluntarily. Out of 299 concessions surveyed in the city, 233 were positive, which is 77.92%. 

These results attest that the virus circulated well in the city of Douala. Based on these antibodies, the prevalence of dengue was clearly presented in Figure 5. The prevalence index ranges from 0.1 to 0.9. The prevalence was very low in the health area of Nguele, low in the health areas of Nyala, Bonendale and Sodiko, low in those of the Cité des Palmiers, Bonamoussadi, Makepe – Logpom, Cité – Sic, Bonaminkano, Mabanda , Nkomba, Youpwe – Airport, Akwa 1, Nylon, Ndogpassi 1 and 2, higher in those of Bonassama, Bilingue, Akwa Nord, Logbessou, Bepanda, Logbaba, Makea, Bonadiwoto, and Ndogpassi Centre. This prevalence implied the presence of vector agents, in particular the Aedes aegypti and Aedes albopictus mosquitoes which proliferated in this city (Figure 3).

Figure 5. Spatialization test of the prevalence of dengue fever by health area in the city of Douala

In Figures 4 and 5, the cluster represented the point of inquiry within which the concessions were located.

Douala is characterized by increasing urbanization. According to the Urban Community of Douala, in 1982, this city had approximately 6,500 ha of urbanized area with a population of approximately 650,000 inhabitants. According to the same source, in 2007, the urbanized area was estimated at around 275 km² with a population of at least 2,000,000 inhabitants. The population density exceeds 7000 inhabitants per km².

According to the Douala Urban Master Plan for 2025, the urbanized area in 2012 is estimated at 16,300 ha, which means an average of 326 ha per year since 1960. The trend hypothesis provides for an average of 27,000 ha of urbanized areas for the period 2020/2025 with a population of around 5,000,000 inhabitants. It is one of the great metropolises of Africa and crystallizes the main economic functions including commerce, transport, banks, industries and the labor market. This economic hub is a point of attraction for the populations who flock from all parts of the country and from outside. The high density of the road network testifies to the extent of urbanization with the presence of numerous paved and unpaved roads.

According to the Urban Community of Douala, in 2004, the city already had 118 districts, the main ones being: administrative (Bonanjo); industrial (Bassa, Bonabéri); commercial (Akwa, Nkololoun …); residential (Bonapriso, Bonadoumbe, Kotto…); popular housing (Nkongmondo, Akwa, New-Bell, Nkololoun, Kassalafam, Km 5, Nkolminta, Nylon, Oyak, Madagascar, Ndogpassi, Bepanda, Ndoghem, Ngodi, Yabassi …). However, a very strong promiscuity reigns in the popular housing districts, since the houses are grafted to each other. These same neighborhoods are the stronghold of insalubrity with a proliferation of waste likely to promote the development of mosquitoes.

Images such as abandoned water vases, livestock farms, old tires scattered in, around and on the roofs of concessions, and abandoned concrete blocks, very recurrent in their landscape, constitute potential breeding grounds for Aedes, especially in the rainy season (Plate n ° 1). Around residential neighborhoods, spontaneous settlements with similar environmental implications rise anarchically. This urbanization promotes the fluidity of human-mosquito contacts, in this case the Aedes which are so-called domestic mosquitoes. In addition, the fluidity of contact between humans and mosquitoes in a high density urban environment, accompanied by promiscuity, favors the emergence of viral diseases (Saluzzo, Vidal and Gonzales, 2004).

Plate 1. Photos of an environment favorable to the reproduction of Aedes aegypti and Aedes albopictus mosquitoes, vectors of dengue fever

2.2. Climatic conditions favorable to the emergence of dengue fever in Douala

The climate of Douala is Cameroonian, mainly coastal type with two seasons: a rainy season (March to November) and a dry season (December to February). The Cameroonian appellation results from the abundance of rains that fall all year round due to the proximity of the coast.

2.2.1. Winds

Douala is a tropical metropolis located at the bottom of the ITF (Intertropical Front) in an area of​barometric tides with a low wind gradient, subject for most of the year to the influences of the Atlantic trade winds which are the monsoon and marginally influenced by the continental trade winds which are the harmattan (end of October, November, December and January). Winds are rarely violent in the city of Douala. Their average speed is less than or equal to 1 m / s. Their maximum speeds vary between 7 and 14 m / s. They have a main direction southwest (similar to that of the monsoon) in the rainy season and northeast (direction of the advance of the harmattan) in the dry season. These winds are favorable for the establishment of Aedes mosquitoes, especially since according to Beisel (2006), Aedes albopictus has a low active flight capacity. The low wind speed helps keep standing water in objects.

2.2.2. Rainfall

Due to the importance of the monsoon which stays permanently on the coastal zone of the Gulf of Guinea, Douala and its surroundings register heavy rainfall almost all the year, with a respite from December to February. The average annual rainfall is 4079 mm, for a daily rainfall of around 150 mm. These are mainly monsoon rains which, in addition to their occurrence over several consecutive days, fall throughout the day.

The table below shows the distribution of average monthly rainfall in Douala from 1973-2003.

Table I. Average monthly precipitation (in mm) in Douala of 1973- 2003

Source: Amougou based on data from the Directorate of National Meteorology

In terms of seasonal variations, the figure below shows the average monthly rainfall distribution at the Douala station. This figure perfectly highlights the absence of a real dry season in the Douala region. Indeed, the driest months (January and December) record a monthly average of around 50 mm, the maximum being able to exceed 200 mm. July and August are the rainiest months. They alone record nearly 1,500 mm of rainfall, which is Yaounde’s annual rain.

Source: Amougou based on data from the Directorate of National Meteorology

Figure 6. Average monthly rainfall histogram in Douala from 1973-2003

The rainfall pattern of the city of Douala can be explained by a unimodal histogram with a peak centered on the month of August in absolute frequency. Indeed, in the series of years of observations, the month of August dominates with an average monthly rainfall of more than 680 mm of water. This major rainfall is explained by:

• A significant and major penetration of the Atlantic monsoon which is currently thick and deep. Douala is mainly subject to the southwest trade winds from the Saint Helena high pressure system at maximum power with its isobar 1018 passing over the Cameroonian coast.
• The mechanism of upwellings (water rises) and downwellings (water sinks) present on the Gulf of Biafra in the Gulf of Guinea which maintains a permanent and massive humidity on the Cameroonian coasts according to the report of the Mission of Nashville and the National Meteorological Authority (Mbah and Fotso, 2009).

These pluviogenic mechanisms result in high relative humidity. Rainfall leaves water deposits or stagnations in objects that can collect or hold back water, namely drums, cans, boxes, tires, leaf armpits, plastics, etc. These objects serve as breeding grounds for female Aedes, thus promoting the reproduction of mosquitoes.

2.2.3. Temperatures

The distribution of monthly average temperatures is given in the table below:      

Table II: Average monthly temperatures (° C) in Douala from 1973-2003

Source: Amougou based on data from the Directorate of National Meteorology

On average, temperatures vary between 25.2 and 28.5 ° C. The annual average is 26.8 ° C. Indeed, the deviated transcontinental trade wind that is the monsoon plunges Douala into an atmospheric ambiance of sweltering, dampness and bathed in a constant thermal regime above 25 ° C on average, with a strong summer cloudiness of a sky heavily covered and low, structured by stratiform and cumuliform clouds often coexisting in a sky loaded with thick and dense strato-cumulus clouds. Stratus clouds mainly generate drizzle (fine and persistent rainfall). This atmospheric ambiance of sweltering and dampness predisposes this urban city of more than 2 million inhabitants to pathologies of infectious origin, including dengue fever.

2.2.4. Relative Humidity

The monthly average values of relative humidity are reported in the table below:

Table III. Monthly average values of relative humidity in% in Douala from 1973-2003

Source: Amougou based on data from the Directorate of National Meteorology

Relative humidity is high in Douala. On average, it varies between 80.6% (January to March) and 90.4% (August), with an annual average of 84.8%. The low humidity values​coincide with the high temperature values​in the dry season while the high values relate to the low temperature values ​​recorded in the rainy season.

The influence of the monsoon and of the mechanism of upwellings and downwellings on the Cameroonian coast increases the hygrometric capacity of the air which permanently saturates the lower atmospheric layers up to 3500 mm in height. This climatic atmosphere of high summer humidity behaves like an ecological niche, a land of vectors of most tropical infectious pathologies, including dengue.

2.3. Risk analysis of dengue fever in Douala

The risk analysis of the emergence of dengue fever in Douala is done qualitatively on the one hand, through the preliminary risk / hazard analysis method.

Preliminary risk analysis (PRA) was first developed in the United States in the early 1960s in the aeronautical and military fields. Conceptual evolutions have made it possible to extend its application to several other fields, among others, the construction of the cartography of dangerous situations, the development of risk maps, the evaluation of the benefits / risks ratios and the achievements of preliminary safety allocations. (Desroches, Baudrin and Dadoun, 2009).

The preliminary risk / hazard analysis method consists of determining potential incidents likely to be caused by hazardous elements taken from a related list.

Table IV. Preliminary risk analysis of dengue fever in Douala

                   Source : Amougou Judith

According to the above table, the potential incidents of emergence of dengue fever, in particular contamination, can be caused by the environmental promiscuity and the proliferation of urban waste discussed in the second section.

The second potential incident of the emergence of dengue fever, namely the development of vector mosquitoes, may be caused by the permanent water deposition from rainfall described in the third section.

Conclusion

In short, the vector populations involved in the transmission of dengue fever in Douala are domestic and / or peri-domestic. The rhythm of winds, precipitation, temperature and humidity favor the development of the Aedes aegypti and Aedes albopictus mosquitoes in the city of Douala. The geographical setting does provide an environment favorable to the development of dengue vectors. The climatic conditions characterized by the influence of the monsoon, the mechanism of upwellings and down wellings and the urban atmosphere characterized by the proliferation of domestic and peri-domestic waste are the main culprits. The Preliminary Risk Analysis scenario is also an illustration of this. This is all the more important to note as the fluidity of contacts between populations, the fluidity of human mobility and goods favored by the development of techniques and means of transport, and urbanization favor the spread of viral diseases (Saluzzo, Vidal and Gonzales, 2004). Given the presence of the Aedes aegypti and Aedes albopictus mosquitoes, the prevalence of the climate and the urban environment, this paper remains relevant. The integration of dengue diagnoses into the Cameroonian health system could serve as a lever for research on the seasonal prevalence of dengue.

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