Physical characteristics of the larval breeding sites in Djilakh village.
A total of nine larval breeding sites, all of which were natural, were identified and repeatedly surveyed in Djilakh between 2013 and 2017, with three outings per year from June to November (Table 1). These temporary, sunny pools were located between 100 meters and 1 km from human dwellings and were filled by rainwater. The soils found in the pools were clay and sandy-clay, accounting for 77.78% (7/9) and 22.22% (2/9) respectively (Table 1).
Table 1
A description of the larval habitats of Anopheles mosquitoes.
Name of the breeding site | Nature and characteristics | Origin of water | Proximity to dwellings (in meters) | Type | Type of soil | Sunshine |
Ngass Ndeb | Natural/temporary | Rain | 500 | Natural water pool | Clay | Yes |
Route de REVA | Natural/temporary | Rain | 1000 | Natural water pool | Clay | Yes |
Falar | Natural/temporary | Rain | 300 | Natural water pool | Clay | Yes |
Cele | Natural/temporary | Rain | 400 | Natural water pool | Clay | Yes |
Mboudaye | Natural/temporary | Rain | 100 | Natural water pool | Sandy-clay | Yes |
Sassar | Natural/temporary | Rain | 800 | Natural water pool | Sandy-clay | Yes |
Nakhane | Natural/temporary | Rain | 150 | Natural water pool | Clay | Yes |
Ngass Mbaal | Natural/temporary | Rain | 300 | Natural water pool | Clay | Yes |
Mbel Ngagane | Natural/temporary | Rain | 100 | Natural water pool | Clay | Yes |
Relationship between malaria morbidity, breeding sites positivity and larval density from 2013 to 2017
At the beginning of the rainy season (June-July), the positivity rate of Anopheles breeding sites was 16.66%. This increased to 72.22% in the middle of the rainy season (August-September) and then reached 55.55% at the end of the season (October-November). The average surface area of larval sites increased from 168.05 m2 in June-July to 592.77 m2 in August-September, during the middle of the rainy season. At the end of the season (October-November), some sites dried up, and the amount of water in the remaining sites decreased significantly, with an average of 287.08 m2 (Table 2). The highest larval densities were recorded during this latter period, with an average of 2.11 larvae per liter in 2013 and 3.22 larvae per liter in 2015. This period also saw the most malaria cases, with more than half of all cases (69%). In the same years, the mean density was 0.61 larvae/liter and 0.06 larvae/liter, respectively in 2013 and 2015, for the period between August and September. Over June-July 2013, the survey revealed a larval density of 0.04 larvae per liter. In 2015, the density was zero. A similar result was observed in October and November 2017, as well as in June and July. In August-September, the density was 1.16 larvae per liter. The water depth remained consistently below 50 cm throughout the study period, with the exception of three sites where it exceeded 50 cm during the overwintering period (August-September). The water in the larval breeding sites was characterized by low turbidity at the beginning and end of the rainy season, while in the middle of the season it was all clear. Vegetation was present at some point in all the breeding sites and reached its peak in the middle of the rainy season (p < 0.001).
Table 2
Cumulative average of larval breeding sites parameters during the annual study periods from 2013 to 2017
Name of the breeding site | Periods | Average water surface area (m2) | Water height (in meters) | Turbidity | Positivity in Anopheles larvae | Presence or absence of vegetation |
Ngass Ndeb | June-July | 537,5 | -50 cm | +/- clear | - | Presence |
Route de REVA | 140 | -50 cm | +/- clear | - | Presence |
Falar | 62,5 | -50 cm | +/- clear | + | Presence |
Cele | 327,5 | -50 cm | +/- clear | + | Presence |
Mboudaye | 136,25 | -50 cm | +/- clear | - | Presence |
Sassar | 110 | -50 cm | +/- clear | - | Presence |
Nakhane | 103,73 | -50 cm | +/- clear | + | Presence |
Ngass Mbaal | 52,5 | -50 cm | +/- clear | - | Presence |
Mbel Ngagane | 42,5 | -50 cm | +/- clear | + | Presence |
Total average | | 168,05 | -50 cm | +/- clear | + | Presence |
Name of the breeding site | Periods | Average surface area (m2) | Water height (in meters) | Turbidity | Positivity in Anopheles larvae | Presence or absence of vegetation |
Ngass Ndeb | August-september | 195 | -50 cm | clear | + | Presence |
Route de REVA | 1115 | -50 cm | clear | + | Presence |
Falar | 387,5 | + 50 cm | clear | + | Presence |
Cele | 2425 | + 50 cm | clear | + | Presence |
Mbpudaye | 350 | + 50 cm | clear | + | Presence |
Sassar | 195 | -50 cm | clear | + | Presence |
Nakhane | 212,5 | -50 cm | clear | + | Presence |
Ngass Mbaal | 257,5 | -50 cm | clear | + | Presence |
Mbel Ngagane | 197,5 | -50 cm | clear | + | Presence |
Total average | | 592,77 | -50 cm | clear | + | Presence |
Name of the breeding site | Periods | Average surface area (m2) | Water height (in meters) | Turbidity | Positivity in Anopheles larvae | Presence or absence of vegetation |
Ngass Ndeb | October-november | 92,5 | -50 cm | +/- clear | + | Presence |
Route de REVA | 500 | -50 cm | +/- clear | + | Presence |
Falar | 237,5 | -50 cm | +/- clear | + | Presence |
Cele | 1140 | -50 cm | +/- clear | + | Presence |
Mboudaye | 200 | -50 cm | +/- clear | + | Presence |
Sassar | 80 | -50 cm | +/- clear | + | Presence |
Nakhane | 90 | -50 cm | +/- clear | + | Presence |
Ngass Mbaal | 151,25 | -50 cm | +/- clear | + | Presence |
Mbel Ngagane | 92,5 | -50 cm | +/- clear | + | Presence |
Totale average | | 287,08 | -50 cm | +/- clear | + | Presence |
Table 3
Variation in the average larval density of breeding sites over the study periods
Name of the breeding site | Periods | Year |
2013 | 2014 | 2015 | 2016 | 2017 |
Ngass Ndeb | June-July | 0 | Not available | 0 | Not available | Not available |
Route REVA | 0 | Not available | 0 | Not available | Not available |
Falar | 0 | Not available | 0 | Not available | Not available |
Cele | 0,08 larvae/liter | Not available | 0 | Not available | Not available |
Mboudaye | 0 | Not available | 0 | Not available | Not available |
Sassar | 0 | Not available | 0 | Not available | Not available |
Nakhane | 0 | Not available | 0 | Not available | Not available |
Ngass Mbaal | 0 | Not available | 0 | Not available | Not available |
Mbel Ngagane | 0,33 larvae/liter | Not available | 0 | Not available | Not available |
Total average | | 0,04 larvae/liter | Not available | 0 | Not available | Not available |
Name of the breeding site | Periods | Year |
2013 | 2014 | 2015 | 2016 | 2017 |
Ngass Ndeb | August-september | 1 larvae/liter | Not available | 1 larvae/liter | Not available | 1 larvae/liter |
Route REVA | 0 | Not available | 0,06 larvae/liter | Not available | 0,5 larvae/liter |
Falar | 2 larvae/liter | Not available | 0,3 larvae/liter | Not available | 2 larvae/liter |
Cele | 0 | Not available | 0,01 larvae/liter | Not available | 3 larvae/liter |
Mboudaye | 1 larvae/liter | Not available | 0,04 larvae/liter | Not available | 1 larvae/liter |
Sassar | 0,5 larvae/liter | Not available | 0,05 larvae/liter | Not available | 1 larvae/liter |
Nakhane | 0 | Not available | 0,08 larvae/liter | Not available | 0 larvae/liter |
Ngass Mbaal | 0 | Not available | 0,06 larvae/liter | Not available | 0 larvae/liter |
Mbel Ngagane | 1 larvae/liter | Not available | 0 | Not available | 2 larvae/liter |
Total average | | 0,61 larvae/liter | Not available | 0,06 larvae/liter | Not available | 1,16 larvae/liter |
Name of the breeding site | Periods | Year |
2013 | 2014 | 2015 | 2016 | 2017 |
Ngass Ndeb | October-november | 2 larvae/liter | Not available | 3 larvae/liter | Not available | 0 |
Route REVA | 1 larvae/liter | Not available | 2 larvae/liter | Not available | 0 |
Falar | 3 larvae/liter | Not available | 5 larvae/liter | Not available | 0 |
Cele | 5 larvae/liter | Not available | 8 larvae/liter | Not available | 0 |
Mboudaye | 2 larvae/liter | Not available | 2 larvae/liter | Not available | 0 |
Sassar | 1 larvae/liter | Not available | 2 larvae/liter | Not available | 0 |
Nakhane | 2 larvae/liter | Not available | 1 larvae/liter | Not available | 0 |
Ngass Mbaal | 1 larvae/liter | Not available | 1 larves/litre | Not available | 0 |
Mbel Ngagane | 2 larvae/liter | Not available | 5 larvae/liter | Not available | 0 |
Total average | | 2,11 larvae/liter | Not available | 3,22 larvae/liter | Not available | 0 |
NB
For the years 2014 and 2016, data were not available because there was no survey of the breeding sites at these periods.
Assessment of rainfall variability on malaria transmission
A critical examination of meteorological data and malaria morbidity statistics indicates that malaria transmission is influenced by rainfall. The graph of annual malaria morbidity from 2013 to 2017 demonstrates a similar trend to that of rainfall over the same period, with the exception of 2017 (see Fig. 2). The years 2013 and 2015 corresponded with the highest recorded levels of rainfall, which coincided with peaks in malaria morbidity rates. In 2014 and 2016, the levels of rainfall were lower, with fewer malaria cases recorded in these two years. It was only in 2017 that there was a relatively high level of rainfall with a low morbidity rate (see Fig. 2). Figure 3 illustrates a gradual increase in malaria cases from July each year, coinciding with the onset of the rainy season. The peak of incidence is observed in October, followed by a decline until December. Concurrently, the distribution of rainfall exhibits a unimodal pattern, with a peak in August, two months prior to the peak of malaria morbidity observed in October.
Correlation between the presence of Anopheles larvae and breeding sites characteristics.
The association between the presence of Anopheles larvae and breeding site characteristics was assessed using logistic regression (Table 4). The odds of finding Anopheles larvae in breeding sites were significantly associated with the size of the cottage (p < 0.05). Specifically, for each unit increase in the size of the cottage, the odds of finding Anopheles larvae decreased by approximately 0.2%. Additionally, breeding sites located near homes were strongly associated with the presence of Anopheles larvae (p < 0.05). The odds of finding Anopheles larvae in breeding sites located near housings (≤ 500 m) were 27 times higher compared to those located more than 500 m from housings. Similarly, the presence of vegetation in breeding sites was highly significant (p < 0.001), with sites having vegetation being 63 times more likely to be positive for Anopheles larvae compared to sites without vegetation. There was also a significant seasonal effect, with breeding sites being less likely to contain Anopheles larvae during June-July compared to August-September (p < 0.05), and no significant difference between October-November and the reference period (Table 4).
Table 4
Correlation between presence of Anopheles larvae and different characteristics of breeding sites. Results of logistic regression: regression coefficients, standard error, odds ratio and p-value.
Variable | Reference | Coefficient | Standard error | Odds ratio | P-value | Signif. |
Intercept | - | 57,153 | 386,700 | - | 0,883 | |
Distance from housing | - | 0,000 | 0,001 | 1,000 | 0,642 | |
Size of the breeding site | - | -0,002 | 0,001 | 0,998 | 0,019 | * |
Breeding site near housings | Breeding site far from housings | 3,296 | 1,292 | 27,006 | 0,011 | * |
Presence of the vegetation | Absence of vegetation | 4,147 | 1,078 | 63,214 | 0,000 | *** |
Height over 50 cm | Height below 50 cm | 2,393 | 1,422 | 10,950 | 0,092 | . |
Sandy-clay soils | Clay soils | -0,612 | 0,782 | 0,542 | 0,433 | |
June-July | August-September | -1,700 | 0,773 | 0,183 | 0,028 | * |
October-November | 1,262 | 0,915 | 3,531 | 0,168 | |
Year | - | -0,030 | 0,192 | 0,971 | 0,877 | |
Notes. The meaning of the codes are as follows : ‘***’ for p < 0,001, ‘**’ for p < 0,01, ‘*’ for p < 0,05, ‘.’ for p < 0,1. |
Correlation between malaria morbidity and breeding sites positivity from 2013 to 2017
The correlation between malaria morbidity and the positivity of breeding sites varied throughout the rainy season, depending on the size and stability of the breeding sites. Overall, there was a positive, highly significant correlation between malaria morbidity and the percentage of positive breeding sites (p < 0.001) (Table 5). In addition, the number of malaria cases decreased significantly over the years (p < 0.05). For each unit increase in the year, the expected number of malaria cases decreased by approximately 20.0%, after adjusting for other variables. There was also a seasonal pattern, as there was an important decrease in the expected number of malaria cases during June-July compared to August-September. Conversely, there was a significant increase in the expected number of malaria cases during October-November (p < 0.05), representing an increase of approximately 81.3% compared to the reference period (Table 5).
Table 5
Association between the number of malaria cases and the presence of anopheline larvae inside breeding sites Poisson regression results: regression coefficients, standard error, incidence rate ratio and p-value .
Variable | Reference | Coefficient | Standard error | Incidence rate ratio | P-value | Signif. |
Intercept | - | 448,949 | 175,957 | - | 0,011 | * |
Percentage of positive breeding sites | - | 0,020 | 0,005 | 1,020 | 2,35e-05 | *** |
June-July | August-September | -0,970 | 0,590 | 0,379 | 0,100 | |
October-November | 0,595 | 0,232 | 1,813 | 0,010 | * |
Year | - | -0,223 | 0,087 | 0,800 | 0,011 | * |
Notes. The meaning codes are as follows : ‘***’ for p < 0,001, ‘**’ for p < 0,01, ‘*’ for p < 0,05, ‘.’ for p < 0,1. |
Variation in malaria morbidity in hamlets according to the proximity of breeding sites.
A significant number of malaria cases were observed in the hamlets in close proximity to the larval habitats (less than 500m), with 82.4% (103/125) being recorded. This was followed by hamlets in a moderately isolated location (between 500-1,000m) with 17.6% (22/125) of the total cases, and finally, villages in the most remote location (more than 1,000m) with no recorded cases. Overall, the number of malaria cases decreased from the eastern part of the village, where the larval habitats were concentrated, to the western part, where no larval habitats were observed. With the exception of the eastern part of the village, the remaining area is covered by tropical ferruginous soils with minimal or no leaching (soils designated as "dior"). The soils in question have a sandy texture and are highly permeable, which leads to a low capacity of water retention. In this context, the risk of malaria transmission is relatively low, given that the types of breeding sites identified in the area are all naturally occurring. Consequently, the nature of the morbidity data allows us to divide the village into three geographical zones at risk, taking into account the position of the larval habitats in relation to the hamlets (Fig. 4).