Malaria remains a major cause of human morbidity and mortality in  many  countries  of  the  world.  The  disease represents a complex public health problem in Africa where 94% of the global cases and deaths  occur  (WHO, 2020). The infection is widespread in tropical and subtropical regions with the most intense transmission occurring in sub-Saharan Africa. Therefore, it is important to understand temporal changes in malaria transmission, to determine the factors that contributed to these changes, and best monitor tools to design future control interventions (malERA, 2017). A number of studies have shown that Plasmodium infections are influenced by environmental factors such as temperature, rainfall, humidity and altitude (Cohen et al., 2008; Hay et al., 2002). These factors indirectly or directly influence and affect the geographical distribution of malaria. Urbanization profoundly changes the epidemiology of malaria, causing a decrease in risk, when compared with rural areas (Robert et al., 2003; Keiser et al., 2004; Hay et al., 2005). For long, malaria was considered as a rural disease in Africa and therethore most malaria research were conducted in rural settings (Kaiser et al., 2004; Donnelly et al., 2005).

In Côte d'Ivoire, malaria is the major reason for consultations in heath centers, representing 30 to 40% of all consultations (Yavo et al., 2002; Wang et al., 2006), Few epidemiological studies involving malaria have been conducted in Côte d’Ivoire after the political crisis with almost all of them done in Abidjan, the economic capital city of the country. In central Bouake city (the second largest city of the country) where the political crisis initialy triggered, previous report highlighted the relationship between peri-urban development and malaria transmission (Dossou-yovo et al., 1994).

This socio-political crisis which lasted more than 9 years (2002-2011) placed this city at disadvantage since malaria control provision by the National Malaria Control Programme (NMCP) through the Global Fund could not access the city due to insecurity (Bonfoh et al., 2011). After such a long period of time, it was critical to provide detailed information on malaria epidemiology in this area at the end of the crisis.

The aim of this paper is to update malaria transmission indicators, including the prevalence of Plasmodium falciparum infection and the level of the disease in rural and urban areas within Bouake city after the armed crisis ended.

Study area and human population

A cross-sectional study was carried out in Bouake (lat. 7°69'N and long. 5°03'W), part of the Gbêkè health regional in central Côte d'Ivoire (Figure 1), in August 2014 and then March-April 2015. It is the second largest city of Côte d'Ivoire, with a population size 680,694 inhabitants according to the national  census  conducted in 2014. The local climate is of a humid tropical type encompassing two seasons: a rainy season from April to October and a dry season from November to March. The average annual rainfall was 1,100 mm and temperature fluctuating between 24 and 34°C throughout the year.

Three sanitary districts within Bouake (Kennedy, Dar Es Salam and Ngattakro) from urban and two villages (Allocokro and Petessou) from rural were selected for the study. Participants were children aged 6 months to 14 years old, whose parents or legally acceptable representative provided informed consent prior to the study.

Size

The sample size was determined using the standard statistical formula:

N= E². (PQ) / i²

Where N = the sample size required, E = 1.96: confidence level test statistic at the desired level of significant, P = 50%: Plasmodium falciparum malaria infection prevalence, Q = (1-P): proportion of plasmodium malaria negative, and I = acceptable error willing to be committed.

For each of the study settings (rural and urban), the total number of people needed was calculated based on a parasite prevalence of 50%. This yielded 384 as the minimum number of individuals aged 6 months - 14 years required per setting.

Data collection procedures

Cross-sectional surveys (CSS) were conducted during each monitoring periods in all selected sites and the prevalence of malaria was measured during the rainy and the dry seasons to capture any seasonal variation.

An experienced health team, including a physician, a nurse, a laboratory technician and a local guide visited each village to enroll eligible children.

For all participants structured questionnaires were administered to record age, sex, household characteristics, malaria prevention methods including ownership and use of ITN.  Any history of fever, malaria treatment, recent illness or chronic diseases was recorded. Secondly, for each child, axillary temperature was recorded, a rapid diagnostic test (RDT SD Bioline, one step Malaria Pf HRP II Antigen Rapide TEST, Standard Diagnostics, Inc.) was performed in each participant who had a history of fever within 24 h or an axillary temperature ≥ 37.5°C. A finger prick blood was collected in all children for thick films and was examined posteriori. Individuals with symptoms suggestive of malaria, including fever, associated with positive RDT was recorded as a confirmed case of malaria and was immediately treated as per the national malaria control programme guideline. In the event of fever with negative RDT, children were treatedaccording to the clinical diagnosis as assessed by the physician.The presence of the parasite was confirmed posteriori in the laboratory, by microscopy examination.

Laboratory procedures

The RDT   SD  Bioline   Malaria   Antigen  P.f.  test  kit  is  a  rapid, qualitative test for the detection of histidine-rich protein II (HRP-II) antigen of malaria Plasmodium  falciparum in human whole blood. This RDT was shown to have 99.0% sensitivity and 97.8% specificity of P. falciparum (Tadesse et al., 2016) and the results from these RDT kit in the present study helped guide treatment in the field.

Parasite was identified by examining thick blood smear. Each slide was air-dried and stained with 10% Giemsa for 22 min. Parasitemia was determined by counting the number of asexual parasites and number of leucocytes in 200 high-powered fields based on a putative count of 8,000 leucocytes/µL of blood (Assi et al., 2017) at Pierre Richet Institute in Bouake by experienced technicians under the supervision of a parasitologist.

The slide was considered positive if malaria parasites were detected in the blood smear. A blood smear was considered negative if no parasite was detected after the examination of 200 oil-immersion fields on the thick smear. The count was made by species (P. falciparum, P. malariae, Plasmodium vivax, or P  ovale). Cross-check quality control was done on a randomly selected sample representing 10% of all thick smears by independent microscopists.

Data analysis

Demographic, clinical and parasitological data were entered into Excel, coded and were analyzed using Stata software 14.1, College Station, Texas. The four dependent variables (prevalence rate of P. falciparum infection among asymptomatic and symptomatic children, prevalence of illness, prevalence of confirmed malaria cases with thick blood and with RDT) were analyzed according to demographic (age groups 6 months-4 years; 5-9; 10-14 years and sex), environmental (season and area) and LLIN actual use (never and always) variables. Microscopy results were used to determine malaria endemicity in children aged 2 to 9 years old, the prevalence of Plasmodium species and P. falciparum infection in every asymptomatic and symptomatic children. The prevalence rate of P. falciparum infection also termed as plasmodial index was the percentage of children whose thick blood smear were positive for P. falciparum (Number of children positive by microscopy divided by the number of children tested, according to the age group, seasons, area and use of LLINs). Malaria attack was defined as the presence of fever (axillary temperature ≥37.5°C or history of fever within 24 h) associated with any parasitaemia (malaria cases with thick blood) or with RDT positive (malaria cases with RDT). The prevalence of malaria cases was the prevalence of confirmed cases of malaria (number of malaria confirmed cases by RDT or by microscopy divided by the number of children examined according to the age group, sex, season, area and use of LLINs). Unadjusted odds ratios (OR) with their corresponding 95% confidence intervals (CI) were calculated using bivariate logistic regression analysis.

Ethical considerations

All data and blood samples were collected after informed consent from each participant through their parent/guardian. Ethical approval for the study was granted by the National Ethical Committee in Côte d'Ivoire (Ref: letter N°41 / MSLS /CNER-dkn, of 25 June 2014 Session).

Demographic characteristics of study population

Characteristics  of  study   participants   are  presented  in Table 1. A total of 859 children were included in the study. The sex ratio was almost even (0.93), in favor of women though (51.33%). The majority (56%) of them were urban dwellers and the mean age of the study subjects was 6.97 (6.72-7.22) years old (Table 1). Age ranged 6 months - 14 years old with the age group of 5-9 years old the most represented (39.46%), followed by the age groups of 6 months-4 years old (30.73%) and 10-14 years old (29.8%). There were fewer children living in rural areas (44%) than in urban areas (56%). The mean percentage usage of LLIN was 34.10% and varied from 46.08% (95% CI 40.26-51.96) in rural areas to 53.92% (95% CI 48.03-59.73) in urban areas.

Plasmodial formula or species

Three species of human malaria parasites (P. falciparum, P. malariae and P. ovale) were observed in the rural zone versus two species (P. falciparum and P. malariae) detected in the urban area. Nearly 100% of malaria parasite found in both settings was P. falciparum, with rare occasions where P. malariae and P. ovale (< 6%) were in sympatry with P. falciparum (Table 2). No cases of triparasitism (P. falciparum + P. malariae + P. ovale) were observed in the study (Table 2).

Prevalence of P. falciparum infection

Overall, a total of 859 thick smears collected  during the study in rural and urban areas were examined. Of these, 678 were carriers of malaria parasites and the mean plasmodic index was 78.92% (95%IC: 76.04-81.61). By setting, the plasmodic index was 78.30% (95%CI: 73.80-82.36) and 79.42% (95%CI: 75.52-82.94), respectively, in rural zone and in urban zone.The overall plasmodic index in children aged 2 to 9 years old was 77.42% (95%CI: 73.72-80.82), indicating a hyper-endemic statut of the Bouake area, regardless of the setting, P. falciparum infection prevalence was analyzed in 593 asymptomatic and 266 symptomatic children (Table 3). In the asymptomatic group, P. falciparum infection prevalence in rural zone (79.53%) was similar to the urban zone (78.19%) (p=0,683). There was no significant association between age groups, sex or season and the prevalence of infection in children (Table 3). Oddly, asymptomatic children who constantly reported sleeping under LLIN the night before had higher parasite prevalence (83.33%) than those who did not sleep under a net (76.13%) (p=0.017). The trend in the symptomatic children was similar to that observed in the asymptomatic group (Table 3).

Febrile episode prevalence

Overall, fever as defined by an axillary temperature ≥37.5 °C or history of fever within 24 hours, was detected in 30.96 % of the children with no significant difference between sex (p=0.342) (Table 4). Children from the urban area had 27% (OR=0.63; 95%CI: 0.47-0.84) lower prevalence of fever than those living in rural setting (p=0,002). Prevalence of fever in age group 5 - 9 years old but not above, was significantly higher than in those below five (OR: 2.04 95%CI: 1.43-2.91, p=0.0001) (Table 4). Usually declared users of LLINs were associated with reduced prevalence of fever (29%) compared to those who never slept under LLINs (p=0.033) (Table 4). There was significantly more fever in the dry season than in the raining season (p=0.034)  in  children  but  the  difference between sex was never significant (p=0.342).

Prevalence of malaria attacks

Confirmed malaria cases were determined from both the results of the thick blood and the RDT (Table 5). According to malaria definition with thick blood results (fevers with any parasitaemia), 209 (24.33%) cases of malaria attack were recorded in the 859 children. Overall, prevalence of malaria cases as detected with thick blood was associated with children location (p=0.032), those with age below 9 years old (p=0,001) and the season (p=0,037). Children in urban area had 28% lower prevalence of malaria cases than those in rural places.

Likewise, children between 5 and 9 years old had higher prevalence of malaria attack than those under 5 years but there was no significant difference between the age group > 9 years old and the kids below 5 years (p=0.141).  The  prevalence  of  malaria  cases was similar between male and female (22.97% versus 25.62% p= 0.365) and also between the group always under LLINs and none users (OR: 0.81; CI95% 0.59-1.13; p=0.222). There were more malaria cases in the end of dry season (28.52% (95%CI: 23.46-34.01)) than the rainy season (22.10% (95%CI: 18.73-25.77). In febrile children, 78.6% (209/266) had laboratory-confirmed malaria infection.

Regarding malaria definition with RDT results (fevers with RDT positive), 101 (11.76%) cases of malaria attack were recorded in the 859 children. In febrile children, 44.69% (101/266) had RDT-confirmed malaria cases.

The trend in the prevalence of malaria cases with RDT detection mirrored that with thick blood analysis for both setting and age groups (Table 5). No significant differences in prevalence of malaria cases were observed between dry and raining seasons (p=0.379) nor between female and male (p=0.546). The usual use of LLINs was associated with 44% reduction in prelance of malaria attack (p=0.021).

These observations highlight the importance, in malaria endemic, to diagnose all suspected malaria in patients with fever in health facility before the decision to treat the patient (Kabir et al., 2014). Such attitude will reduce (i) the risk of overdiagnosis of malaria when patients are treated presumptively, (ii) the risk of experiencing side effects and treatment cost, and (iii) the risk of drug resistance (Porras et al., 2014; Glinz et al., 2010). Also, diagnosing suspected malaria cases would allow clinicians to look for other causes of the patient's illness, thereby improving clinical outcomes when patients are properly managed (Becker et al., 2011; Righetti et al., 2012).

In the current study, the percentage of malaria attack (24.33%; 209/859) was relatively small because the majority (64.2%) of children with malaria parasites was asymptomatic, suggesting they may have developed anti-malarial immunity, although the anti-parasite immunity did not reach levels high enough to eliminate the infection (Greenwood et al., 1987).

Our results showed that the thick blood smear was reliable and was able to detect more malaria cases than the RDT kit. For example, the thick blood smear analysis detected 24.33% malaria cases compared to only 11.75% with the RDT. The higher sensitivity of the thick blood smear relative to the RDT and reading technique of the smears in microscopy based on 200 microscopic fields rather than 200 white blood cells could explain the difference in performance between the two malaria detection methods.

Malaria cases prevalence in children by microscopy was higher in rural than urban area (27.78% vs. 21.62%; p<0.032). This is in line with findings in the Hohoe municipality of Ghana, where the prevalence of malaria attack by microscopy was significantly higher in rural than urban zone (28.5% vs. 16.0%) (Margaret et al., 2017). Similarly, the study conducted in Southern Sudan reported higher prevalence of malaria cases by RDT in rural Sonsoro (41%) than in urban Gansosso (7.5%) (Govoetchan et al., 2014), probably due to the lack of health facilities in thsese areas (Wanji et al., 2012; Achidi et al., 2008).

The data showed that effective usage of LLIN in Boauke area was lower than the rate at national level, as per the demographic and health survey data gathered during 2010-2011 (EDS-CI, 2013). According to WHO, the minimum criteria for optimum community protection against malaria is an ownership rate of LLIN of at least 80% (WHO, 2010). Nervertheless, report indicated that effective use of LLIN is better than ownership and should be preferred as indicator of malaria risk (Moiroux et al., 2012). It is interesting to note that no clear impact derived from ITNs use was observed on the different parameter (P. falciparum infection prevalence, fever prevalence and malaria attacks prevalence) relative to none users. Surprisingly, children reported always sleeping under ITN were more infected with parasites than those who never used   them.  This    contradicts    other    studies   where substantial ITN use had a significant impact on malaria infection and incidence (Moiroux et al., 2012; Lengeler, 2004). Some and parents giving false information about their history of LLIN use during the survey might partly explain the observed differences.

This study, carried out after the military-crisis, showed that malaria was hyper-endemic in both urban and rural areas of Bouake, central Côte d’Ivoire. Parasitological indicators highlighted the fact that parasite transmission occurred equally in rural an urban areas, both in the dry and wet seasons. Confirmed malaria cases were higher in rural areas in children aged 5-9 years old and at the end of the dry season.

This high endemicity was partly linked to power access of families to health facilities due to the crisis. It is of paramount importance for government reforce local capacity including infrastructure, health facilities and commodities for a prompt management of malaria in remote areas affected by political crisis.

The authors have not declared any conflict of interests.

This research was funded by Institut de Recherche pour le Developpement via Jeune Chercheur Associés.