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European Journal of Applied Sciences – Vol. 10, No. 4
Publication Date: August 25, 2022
DOI:10.14738/aivp.104.12241. Mbang Nguema, O. A., Obiang Ndong, G. P., M’Bondoukwe, N. P., Ndong, Ngomo, J. M., Koumba Lengongo, J. V., Pongui Ngondza,
B., Batchy Ognagosso, B. F., Effame Eya, E., Masiulis, E., Mawili-Mboumba, D. P., & Bouyou-Akotet, M. K. (2022). Prevalence of
Asymptomatic Malaria in Urban and Semi-Urban Areas in 2016 and 2019, to Gabon. European Journal of Applied Sciences, 10(4).
62-72.
Services for Science and Education – United Kingdom
Prevalence of Asymptomatic Malaria in Urban and Semi-Urban
Areas in 2016 and 2019, to Gabon
Mbang Nguema O. A.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Obiang Ndong G. P.
Ministère de la Santé publique, BP: 50, Libreville, Gabon
M’Bondoukwe N. P.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Ndong Ngomo J. M.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Koumba Lengongo J. V.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Pongui Ngondza B.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Batchy Ognagosso B. F.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Effame Eya E.
Croix – Rouge Gabonaise, BP : 2 274, Libreville, Gabon
Masiulis E.
Croix – Rouge Gabonaise, BP : 2 274, Libreville, Gabon
Mawili-Mboumba D. P.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
Bouyou-Akotet M. K.
Department of Parasitology-Mycology, Faculty of Medicine
Université des Sciences de la Santé, BP 4009, Libreville, Gabon
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Mbang Nguema, O. A., Obiang Ndong, G. P., M’Bondoukwe, N. P., Ndong, Ngomo, J. M., Koumba Lengongo, J. V., Pongui Ngondza, B., Batchy
Ognagosso, B. F., Effame Eya, E., Masiulis, E., Mawili-Mboumba, D. P., & Bouyou-Akotet, M. K. (2022). Prevalence of Asymptomatic Malaria in Urban
and Semi-Urban Areas in 2016 and 2019, to Gabon. European Journal of Applied Sciences, 10(4). 62-72.
URL: http://dx.doi.org/10.14738/aivp.104.12241
ABSTRACT
Malaria remains a major cause of mortality and morbidity in sub saharian Africa.
Asymptomatic carriers of the parasite contribute to the parasite reservoir and
allow the maintenance of malaria transmission. The aim of this study was to assess
the evolution of the prevalence of asymptomatic malaria in 2016 and 2019 in
Libreville and surrounding areas in Gabon. Thick- and thin-blood smears for
malaria diagnosis were performed inpeople, showing no signs suggestive of malaria
and having completed a questionnaire during the prospective, cross-sectional
studies carried out in July 2016 and April 2019 in urban (Libreville) and semi-urban
(Owendo ) in Gabon. A total of 724 participants were included over the two study
years; 337 in 2016 and 384 in 2019. The prevalence of asymptomatic malaria was
less frequent in 2019: 5.9% vs 12.8% in 2016 (p<0.01). Asymptomatic carriage was
more frequent in men regardless of the study period In 2016, under 5 years old, was
the most infected population compared to other age groups and in 2018, it was
higher among individuals aged 5-15 years old of 7.4%. These populations may
contribute to the reservoirs for Plasmodium and thus may strongly contribute to the
anchoring of malaria in favorable eco-epidemiological zones. These data highlight
the need use of molecular tools to detect submicroscopic infections in
asymptomatic individuals.
Keywords : Asymptomatic, malaria, urban area, Gabon.
INTRODUCTION
In areas with high malaria transmission level, asymptomatic individuals are frequent and
contribute to maintain the disease. Nevertheless, these Plasmodium spp. asymptomatic carriers
are not consider as targets for national strategies implemented to reduce malaria burden in
endemic areas, while, they can represent a persistent reservoir favorating the parasite
development [1]. Indeed, some authors reported a higher sensitivity to the vectors of the
parasites infecting asymptomatic than those found in symptomatic [2, 3]. As pointed out by
Laishram et al., [4], asymptomatic infections often go undetected, resulting in a major source of
gametocytes for local mosquito vectors. Such silent infections may contribute to the spread of
artemisinin combination therapy (ACT) resistance [5].
Nowadays, malaria transmission is observed in areas with different level of urbanization.
Although, rural areas conditions are more suitable for Anopheles mosquitoes breeding sites,
malaria transmission in urban areas is regularly reported [6, 7]. Several studies highlight a low
malaria transmission level in many African cities [8, 9, 10]. However, more than 50% of the
African population is exposed to malaria in urban areas and it is expected to concern at least
60% of their population in 2050 [9, 5].
In addition to factors such as, host susceptibility, vector abundance and environmental
conditions which contribute to malaria transmission, parasites reservoir need to be well
characterized. Hence, in urban areas, asymptomatic malaria cases should be identified as
possible parasites reservoir and included as target population in strategies of malaria control
and elimination [11, 5].
In Gabon, after the deployment of artemisinin-based combination therapies (ACTs), Insecticide
Treated Nets (ITNs) and the administration of intermittent preventive treatment with
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sulfadoxine-pyrimethamine (IPTp-SP) during pregnancy, in 2005 a rebound of malaria
morbidity was observed since 2011, in both rural and urban areas [12]. In several rural areas,
there was no evidence of a decline, and malaria prevalence remains high; ranging from 23% to
more than 50% [13, 14, 15, 16].
During the same period, an entomological survey, showed Libreville, the capital city of Gabon,
an annual EIR estimated at 33.9 infected bites per person per year [17, 18]. Since that time,
there has been few interventions (less than 20%) for Anopheles density reduction particularly
in 2016 and 2019 [20]. The resistance to ACTs, the main antimalarial drug used in the country
for uncomplicated malaria treatment, is not reported.
In tsome rural areas of Gabon, asymptomatic malaria infection was reported and its frequency
areas not negligible [14]. Data concerning this group are missing in the province of Estuaire,
which includes Libreville where almost 50% of the total population of the country lives. The
prevalence of malaria is around 35% in Libreville in patient consulting at hospital (unpublished
data). Likewise submicroscopic infection is also frequent in this febrile population [21].
The aim of this study was to compare the prevalence of asymptomatic Plasmodium in 2016 and
2019, in Libreville and surrounding areas, in order to determine the extent of the parasite
reservoir for a proper design of malaria transmission control plan and further the disease
elimination in Gabon.
PATIENTS AND METHODS
Study areas
The study was carried out in two urban areas in Gabon. Libreville, the capital city of Gabon
which is situated on the borders of the Guinea gulf the Komo River. Its population is estimated
at 873 418 inhabitants (2012). This city is divided into five districts. Owendo the semi -urban
area, is towns for from Libreville. In Gabon, the equatorial climate is characterized by a long
rainy season lasting eight months (interrupted by a short dry season) and a long dry season
lasting 4 months. Average temperatures remain relatively constant throughout the course of
the year, with an average of temperatures of 30°C.
Study design and population
Cross- sectional surveys were carried out in July 2016 and April 2019 In Libreville and Owendo,
a household survey was performed in five districts of under-integrated neighborhood under
the supervision of the Department of Parasitology of the Faculty of Medicine and the Red Cross.
A blood sample was collected by finger prick from participants of all age without suggestive
signs of malaria. All subjects were interviewed to obtain socio-demographic and clinical
information using a pre-tested standardized questionnaire. It included gender, age, place of
residency, history of recent malaria (clinical manifestations such as fever, headache, muscle
pain, weakness 72 hours before the survey), anti-malarial treatment, and household members.
Each questionnaire was labelled with a unique identifying number, which was allocated to
blood sample to provide de-identified data for analysis.
Malaria diagnosis
For each patient, matched thick and thin-blood smears were prepared and stained with 20%
Giemsa. Thick smears were screened for the presence of malaria parasites according to the
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Mbang Nguema, O. A., Obiang Ndong, G. P., M’Bondoukwe, N. P., Ndong, Ngomo, J. M., Koumba Lengongo, J. V., Pongui Ngondza, B., Batchy
Ognagosso, B. F., Effame Eya, E., Masiulis, E., Mawili-Mboumba, D. P., & Bouyou-Akotet, M. K. (2022). Prevalence of Asymptomatic Malaria in Urban
and Semi-Urban Areas in 2016 and 2019, to Gabon. European Journal of Applied Sciences, 10(4). 62-72.
URL: http://dx.doi.org/10.14738/aivp.104.12241
Lambarene’s method [20]. Parasitemia was expressed as the number of parasites per microliter
of blood (p/μL) and parasite species were identified in the matched thin-blood smears. Thick
smears were considered negative if no asexual blood stages of parasites were seen in 100 oil- immersion fields. Quality control of the blood smears was realized by a microscopist who was
unaware of the results of the first reading. In case of discrepancy, slides were controlled by a
third reader, and the average of the two closest parasitaemia was taken. The case definition of
plasmodial infection was a slide-confirmed parasites. Asymptomatic malaria was defined as the
presence of plasmodial infection and an axillary temperature < 37.5°C, for over a week.
Ethical Considerations
This study was nested in a survey estimating the burden of microscopic asymptomatic malaria
which was approved by the Ministry of Health. Participation was voluntary and all study
subjects provided written informed consent. Parents or legal guardians signed on behalf of
children. Malaria diagnosis was free of charge and infected patients were treated according to
the national policy guidelines.
Data analysis
Data were entered in Microsoft Office Excel Work sheet and analyzed using Statview (Version
5.0). Chi-square test was used for the analysis of prevalence of asymptomatic malaria infection
according to year, gender and age. The value of p <0.05 was considered statistically significant.
RESULTS
Characteristic of study population
A total of 724 individuals participated to the study and answered the survey questionnaire :
340 in 2016 and 384 in 2019. More than 59% of the study population were women in 2016.
They were 51% in 2019. The sex ratio was 0.8 in 2016 and 0.9 in 2019. In Libreville and
Owendo, more than 80% (n= 626) of the population was less than 15 years old. The majority
(n=510 ; 70%) lived near a swampy. (Table 1).
Prevalence of asymptomatic malaria according to the year and site
The overall prevalence of asymptomatic malaria cases was 12.6% (N=43/340) in 2016 and
5.9% (N=23/384) in 2019, being twofold higher in 2016, (p= 0. 06). It was higher in 2016 in
Owendo, the semi-urban area: 16% vs 11.6% in Libreville, in 2016 (p= 0.0002). Inversely, the
frequency of asymptomatic malaria was it was in 7.3% and 2.7% in Owendo in 2019), (p=0.04).
Two species of Plasmodium were detected in 2016, P. falciparum in 97.8% of the participants
and P. malariae in one patient (2.2%). In 2019, only P. falciparum was identified in all samples.
Overall, the median parasite density was comparable according to the study period: 40 [7-
1867] p/μL in 2019 and 38 [7- 3340] p/μL in 2016. About two-thirds of the population had a
parasite density of less than 100 p/μL of blood.
In 2016, the prevalence of Plasmodium infection was the highest (20.8%) in the population over
15 years without significant difference compared to other age groups (p = 0.07). In 2019, it was
higher in participants whose age varied between 5 years and 15 years (p<0.0002). In 2016,
Plasmodium infection was frequent in people living far from swampy areas, unlike the results
obtained in 2019 (table 2). Asymptomatic malaria tend to be more frequent in men regardless
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of the study period in 2016 (14% vs 11.6% in women) and in 2019 (6.9% vs 5.1% in women)
(Table 2).
Likewise, the overall prevalence of asymptomatic plasmodial infection was higher in men in
Owendo (17.9% vs 14.3%) and in Libreville (12.2% vs 10.8%) in 2016. In 2018, Children aged
5 to from 15 yrs were more frequently infected in Owendo and adults in Libreville in 2016
(Table 3). The prevalence of Plasmodium infection was at least five fold more frequent in living
near of swampy areas. In Owendo in 2016 compared to 2019 (Table 3). Such difference was
not found in Libreville 2019 (8.8% vs 9.7%).
DISCUSSION
The present study evaluated the prevalence of asymptomatic carriage of Plasmodium spp in
urban areas of Libreville and semi-urban area of Owendo, in Gabon. These data showed that the
prevalence of asymptomatic plasmodial infection is not negligible in Owendo and Libreville, in
2016 and 2019, although a decrease was observed. Nevertheless, it is comparable to the
frequency of asymptomatic malaria found in rural areas. Indeed, Nkoghe et al., in a study
carried in out in Gabon, reported a prevalence of 6. 3% of asymptomatic carriers in 2008. It is
known that in rural areas of Gabon, malaria transmission is intense [21]. In Côte d’Ivoire, higher
proportion, 12%, of asymptomatic malaria cases has been found, in urban area [1]. In addition,
a study conducted in rural Cameroon a neighboring country of showed a much higher
prevalence (> 60%) of asymptomatic plasmodial infection using PCR technique [22].
Our data may be underestimated since in Gabon submicroscopic infections are frequent [21].
The technique of microscopy is known to be less sensitive than PCR for parasites detection in
man or vectors [23, 24, 25]. Thus, combining molecular tools and microscopy may be useful.
Taken together these data suggest that introduction of molecular tools such as LAMP (Loop
isothermal Amplification) in sentinel sites of malaria surveillance which are throughout the
country may be necessary. That may contribute to the decision-making processes, the design of
sustainable interventions with active community participation, and the implementation of
educational schemes.
Otherwise, the present study confirms the predominance of P. falciparum as previously
reported in symptomatic and asymptomatic individuals in Gabon [26]. This result is similar to
that obtained during a study carried out in Cameroon which shows that 95% consisted of
Plasmodium falciparum [22]. About two thirds of the people surveyed during this study lived
near marshy areas. Environmental management for vector control is one of the key elements of
the national strategy for the prevention and control of malaria (FMN, 2012). The study showed
that people who had standing water around the house and those who did not had a significantly
higher risk of being infected.
The prevalence rate of asymptomatic malaria in men is higher than in women. This observation
could be explained by specific gender behaviors. It has been suggested that men tend to be more
active outdoors in the evening than women, therefore at a higher risk of exposure to mosquito
bites [27]. Another plausible explanation is the fact that women are more likely to use
preventive measures against mosquito bites (e.g. long clothes, sleeping under LLINs). Women
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Mbang Nguema, O. A., Obiang Ndong, G. P., M’Bondoukwe, N. P., Ndong, Ngomo, J. M., Koumba Lengongo, J. V., Pongui Ngondza, B., Batchy
Ognagosso, B. F., Effame Eya, E., Masiulis, E., Mawili-Mboumba, D. P., & Bouyou-Akotet, M. K. (2022). Prevalence of Asymptomatic Malaria in Urban
and Semi-Urban Areas in 2016 and 2019, to Gabon. European Journal of Applied Sciences, 10(4). 62-72.
URL: http://dx.doi.org/10.14738/aivp.104.12241
may also use self medication treatment once the clinical and symptoms of malaria are suspected
[28, 29].
The age of asymptomatic individuals varied according to the year, but the older children (5 - 15
years) and adults had the highest prevalence of asymptomatic malaria. It has already been
shown that older children sleep less frequently under mosquito nets than children under the
age of 5. These results are similar to those of Gbalegba et al., [1]. Furthermore, high prevalence
of infection in adults indicates the development of partial immunity allowing the control of
parasitaemia and keeping it at low densities [30, 31].
The present data showed that most infections were of low to moderate parasitaemia and the
parasite density was comparable over the two study periods (less than 5000 parasites / μl of
blood). These results are similar to those of Ossinga Bassandja et al., who found parasite density
2000 parasite / μL in asymptomatic blood donors in Congo [32].
CONCLUSION
These data highlight a significant prevalence of asymptomatic carriers of Plasmodium in urban
and semi-urban areas in 2016 and 2019. This prevalence was higher in adults in 2016 and in
individuals whose age varied between 5 and 15 years in 2019. These populations may
contribute to the reservoirs for Plasmodium and thus may strongly contribute to the anchoring
of malaria in favorable eco-epidemiological zones. These data highlight the need to detect
submicroscopic plasmodial infection in asymptomatic individual.
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Table 1 : Study population and sites characteristics
2015
2018
Libreville Owendo
Libreville Owendo
Characteristics n(%) n(%)
n(%) n(%)
Gender (N=724)
Male 111 (42.9) 42 (51.9)
121 (44.3) 66 (59.5)
Female 148 (57.1) 39 (48.1)
152 (55.7) 45 (40.5)
Age in years (N=724)
<5 yrs 114 (44) 37 (45,7)
106 (38.8) 27 (24.3)
5-15 years 112 (43.2) 29 (35,8) 148
(54.2) 53(47.8)
> 15 years 33 (12.8) 14 (18.5) 19
(7) 31 (27.9)
Existence of Swampy
area close to the house (N= 724)
Yes 216 (83.4) 77(95.1)
104(38.1) 109(98.2)
No 43 (16.6) 4 (4.9)
169(61.9) 2 (1.8)
Reported fever the last 24h (N=724)
Yes 15 (4.3) 13(16.1) 13
(4.8) 0 (0)
No 244 (95.7) 68(83.9)
260 (95.2) 111(100)
Reported fever the last week (N= 724)
Yes 86 (33.2) 31(38.3) 17
(6.3) 2 (1.8)
No 173 (66.8) 50(61.7)
256 (93.7) 109 (98.2)
Previous antimalarial treatment (N=724)
Yes 4 (1.5) 9 (11.1)
6 (2.2) 2 (1.8)
No 255 (98.5) 72 (88.9)
267 (97.8) 109(98.2)
Table 2: Prevalence of Plasmodium infection according to the years
Page 10 of 11
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Mbang Nguema, O. A., Obiang Ndong, G. P., M’Bondoukwe, N. P., Ndong, Ngomo, J. M., Koumba Lengongo, J. V., Pongui Ngondza, B., Batchy
Ognagosso, B. F., Effame Eya, E., Masiulis, E., Mawili-Mboumba, D. P., & Bouyou-Akotet, M. K. (2022). Prevalence of Asymptomatic Malaria in Urban
and Semi-Urban Areas in 2016 and 2019, to Gabon. European Journal of Applied Sciences, 10(4). 62-72.
URL: http://dx.doi.org/10.14738/aivp.104.12241
2015
2018
Prevalence of malaria N n (%) N
n (%)
P-value P-value
Sites 0.0002 -
Libreville 259 30 (11.6) 273
20 (7. 3)
Owendo 81 13 (16) 111
3 (2.7)
Gender 0.5 0.4
Male 150 21 (14) 187
13 (6.9)
Female 190 22 (11.6)
196 10 (5.1)
Age in years 0.07 <.0002
<5yrs 139 15 (10.8) 125
5 (4)
5 – 15 yrs 141 15 (10.6) 201
15 (7.5)
>15yrs 48 10 (20.8) 50
2 (4)
Swampy area 0.02
0.5
Yes 225 28 (12.4) 277
18 (6.5)
No 63 8 (16.7) 104
5 (4.8)
Reported fever the last 24h <.0002 -
Yes 28 7 (25) 13
1 (7.7)
No 312 36 (11.5) 371 22 (5.9)
Reported fever the last week 0.7
-
Yes 117 14 (12) 10
2 (20)
No 223 29 (13)
374 21(5.6)
Previous antimalarial treatment -
-
Yes 13 2 (15. 4) 8
0 (0)
No (n=) 327 41 (12.5) 376
23 (6.1)
Page 11 of 11
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European Journal of Applied Sciences (EJAS) Vol. 10, Issue 4, August-2022
Services for Science and Education – United Kingdom
Table 3: Factors associated with Plasmodium infection by site and years
2015
2018
Libreville Owendo P-value
Libreville Owendo
n (%) n (%) n
(%) n (%)
Sex
Male 14 (12.2) 7 (17.9) 0.4 12
(9.9) 1 (1.5)
Female 16 (10.8) 6 (14.3) 0.5 8 (5.3)
2 (4.5)
Age (years)
<5yrs 11 (10.6) 4 (11.4) - 3 (2.9) 2
(8. 3)
5 – 15yrs 9 (8.03) 6 (20.7) 0.04 14
(9.5) 1 (1.9)
>15yrs 9 (27.3) 1 (6.7) - 2 (10.5)
0 (0)
Swamps area
Yes 19 (9.7) 9 (15.3) 0. 2 15
(8.8) 3 (2.8)
No 7 (14.6) 1 (6.3) - 5
(4.9) 2 (0)
Reported fever the last 24h
Yes 3 (20) 4 (30.8) - 1 (7.7)-
No 27 (11.1) 9 (13.2) 0.6 19 (7.3) 3
(27)
Reported fever the last week
Yes 8 (9.3) 6 (19.4) 0.1 2 (25) 0 (0)
No 22 (100) 7 (14) - 18 (6. 8) 3
(100)
Previous antimalarial treatment
Yes 1 (25) 0 (0) - 6 (0) 2 (100)
No 29 (11. 4) 12 (100) - 20 (7.5) 3
(2.7)