Study area
This survey was accomplished at the Child Welfare Clinic and the Pediatric Ward of the Agogo Presbyterian Hospital, Asante Akim North District (Ashanti Region) in central Ghana, West Africa. Recruitment area included 14 villages which were summarized into 4 cluster ("Greater Agogo" (Agogo city and Hwidiem), "Greater Konongo" (Konongo and Odumasi), "West of Agogo" (Akutuase, Amantena and Wioso) and "Near Street" (Domeabra, Juansa, Kyekyebiase, Nyaboo, Obenimase, Patriensah and Pekyerekye) (Figure 1). The study area covers ~345 km2; the coverage population of the study hospital was 61,346 inhabitants (census data 2004) where the population ranged from 890 inhabitants in the smallest village to 15,383 in the largest.
The vegetation of the study area is mainly semi-deciduous forest with major vegetation types of open forest, closed forest and wooded savannah. The climate is tropical with a mean annual temperature of 26°C and two rainy seasons: a first rainy season from May to July and a second from September to November. The dry harmattan season occurs between December and April and is associated with drought conditions. The topography of the district is generally undulating and the altitude variation is 226 m between the lowest (227 m) and the highest (453 m) village included in our study. Agriculture is the predominant major occupation among people; main staple food crops produced in the district include maize, cassava, plantain, cocoyam and yam [9]. The principal malaria vectors are mosquitoes of the Anopheles gambiae complex and Anopheles funestus. Malaria is hyper-/holoendemic in this area with intense perennial transmission and seasonal peaks and the predominant Plasmodium species is Plasmodium falciparum (> 90%) [10]. Entomological evaluation during the study period indicated ~400 infective bites per person-year (EIR) (unpublished data). Subsidized insecticide-treated bed nets were available, and their use was encouraged.
The study was carried out between May 2007 and August 2009 (duration 26 months). Diagnostic assessments were integrated into the hospital routine. In total, 1,496 children up to 14 years of age, who visited the hospital for medical care, were included in the study. The case definition for malaria was fulfilled if the axillary temperature was ≥ 37.5°C and a P. falciparum parasitaemia count of > 0 parasites/μL was detected in the thick or thin smears. Parasite examination was done according to quality-controlled standardized procedures described elsewhere [11].
Data collection
Information on personal or family characteristics with a possible influence on malaria (sex, ethnic group, age, mother's age, use of protective measures [usage of bed net, window net, other or no protection], number of children and place of residence) and information about factors indicating the family's financial situation (living in a brick or wood/mud house, existence of electricity, water supply, mother's education and profession, father's education and profession, indoor toilet and use of freezing as measure of conservation, income management, existence of a relative abroad for possible financial support and membership in the national Health Insurance Scheme [NHIS]) was collected through interviewing a parent or the guardian who accompanied the child to the hospital using a structured interview with a questionnaire in English or if necessary in the local language, Twi. The question sheet was composed according to standard questionnaires adjusted to local requirements and appropriateness. Data from questionnaires and forms were double entered after case closed, plausibility checked, and cleaned before the database was locked. All information on participants and their parents was treated confidentially. Only children who were examined for malaria and where information about the sociodemographic and socioeconomic situation was available were included in the analysis (n = 1496).
The study was approved by the Committee on Human Research, Publications, and Ethics, School of Medical Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana.
Data analysis
Participants were allocated into one of the four village clusters described above (Greater Agogo, Greater Konongo, West of Agogo and Near Street), according to their place of residence. Additionally, all participants were classified into two ethnic groups according to their tribal background: the Akan and those who are the natives of the area and the Northeners who have a migratory background but are now permanent residents of the area. It was hypothesized that children between the ages of 1 to 5 years are at highest risk of acquiring malaria; hence we stratified for age (≤ 1 year, > 1 to ≤ 5 years and > 5 years). It was also suggested that the mother's age might be of importance for the child and its risk for malaria; all mothers were stratified and grouped to young mothers (≤ 30 years) and older mothers (> 30 years). High numbers of children living in a household were assumed as an influence factor (two groups: ≤ 4 children and > 4 children). Additionally, it was asked in the interview whether a family used protective measures such as bed nets or window-screens. Individuals with missing values on any of these variables (n = 18) were excluded from the analysis (n = 1478).
To classify the family's economic status, the following socioeconomic indicator variables were considered: mother's and father's profession (employed/unemployed) and education (ability to read and write: yes/no), type of house the family is living in (cement/brick house or mud/wood house), water supply (open water source/closed water source), existence of an indoor kitchen (Yes/No), electricity (Yes/No), indoor toilet (Yes/No), use of freezing as measure of conservation (Yes/No), existence of a relative abroad who might financially support the family (Yes/No), the self-rated ability to manage with the available monthly income (difficult or not difficult) as well as the membership in the health insurance (Yes/No). All socioeconomic and sociodemographic data including information on protective measures based on self-reports of the mothers or guardians and were not confirmed by direct observations during household visits.
For the sake of the multivariable analysis, a principal component analysis (PCA) was applied to those socioeconomic indicator variables, which showed relevant contributions (> 10%) to the combined socioeconomic status score factor [12]. The factor of the PCA with the highest eigenvalue was used as the variable, which describes sufficiently the socioeconomic status of a household. The respective factor scores were categorized in terciles and used in the regression analysis. The lowest 33% of households according to the economic status variable were classified as poor, the highest 33% as rich and the rest as average economic status [13].
For the PCA, missing values of distinct binary variables were replaced by the means of all summarized "0" values (asset not present) and "1" values (asset present) of this variable (n = 1496) [12]. This approach may have reduced variation among households and may have increased the potential for clumping and truncation [12, 14]. In the presented study population, the percentage of households with missing values was, however, small (< 1%) and such a bias might be negligible.
For each potential risk factor of malaria, the odds ratio (OR) was calculated and the significance level was tested by the chi-square test. Adjusted ORs were estimated by multivariate logistic regression. Confounding was determined as a relative difference of 15% between crude odds ratios and odds ratios adjusted for predefined covariates without signs of effect modification. All covariables in the multivariate regression model were examined for possible effect modification by Wald tests and preference of the model with interaction by log-likelihood tests (both p < 0.05).