Urban malaria in the Sahel: prevalence and seasonality of presumptive malaria and parasitaemia at primary care level in Chad


Corresponding Author Kaspar Wyss, Swiss Centre for International Health, Swiss Tropical Institute, Socinstr.57, 4002 Basel. Tel.: +41-61-2848140; Fax: +41-61-2848103; E-mail: kaspar.wyss@unibas.ch


Objective To assess malaria prevalence rates and seasonal patterns among clinically diagnosed malaria cases at the level of primary care facilities in an urban Sahelian setting.

Method Screening all patients consulting two private and two governmental providers on a randomly selected weekday over a period of 9 months. Patients with presumptive malaria underwent a blood test.

Results Of 1658 patients included in the survey, 47% were clinically diagnosed and treated as malaria cases. Malaria was more often diagnosed by private providers. There were no clear seasonal patterns in presumptive malaria. A 30% of clinically diagnosed cases were positive for Plasmodium (all falciparum) by thick film examination. Thus, false positive cases constituted more than 70% of the clinically diagnosed malaria cases. The highest positive prevalence rates were found at the end and shortly after the rainy season (44%–47%) and the lowest during the dry season (2%).

Conclusions Clinical diagnosis of malaria has a very low positive predicted value in this low endemicity urban setting, and its low specificity leads to inappropriate care for a large proportion of patients. This has a major impact on economic costs for health services and households. In the Sahel, systematic use of microscopy-based diagnosis and/or rapid diagnostic tests should be considered to appropriately manage malaria and non-malaria cases.


Objectif  Evaluer les taux de prévalence et les profils saisonniers de la malaria cliniquement diagnostiquée dans les services de soins primaires dans un cadre urbain sahélien.

Méthode  Screener tous les patients consultant dans deux services privés et deux services gouvernementaux durant des jours sélectionnés aléatoirement sur 9 mois. Les patients avec une présomption de malaria ont été soumis à un test sanguin.

Résultats  Sur 1658 patients inclus dans l’étude, 47% ont èté cliniquement diagnostiqués et traités comme cas de malaria. La malaria était plus souvent diagnostiquée dans les services privés. Il n'y avait pas de profils saisonniers clairs dans les présomptions de malaria. 30% des cas diagnostiqués cliniquement étaient positifs à l'examen de goutte épaisse pour plasmodium (tous étaient falciparum). Des lors, les faux positifs constituaient plus de 70% des cas diagnostiqués cliniquement. Les valeurs prédictives positives les plus élevées ont été mesurées à la fin ou juste après la saison des pluies (44 à 47%) et les valeurs les plus basses, durant la saison sèche (2%).

Conclusion  Le diagnostic clinique de la malaria a une valeur prédictive positive très faible dans ce cadre urbain et sa spécificité faible conduit à l'application des soins inappropriés pour une large proportion des patients. Cela a un impact majeur sur les frais économiques des services de santé et des familles. Dans le Sahel, l'utilization systématique de diagnostic basé sur la microscopie et/ou des tests de diagnostic rapides devrait être considérée pour une prise en charge adéquate des cas de malaria et non-malaria.


Objetivo  Valorar la prevalencia de las tasas de malaria y los patrones estacionales entre los casos de malaria diagnosticada clínicamente a nivel de centros de atención primaria en un entorno urbano de Sahel.

Método  Se evaluaron todos los pacientes que consultaron dos centros privados y dos centros gubernamentales en un día por semana seleccionado al azar durante un período de 9 meses. A los pacientes con tratamiento presuntivo para malaria se les realizó un examen de sangre.

Resultados  De 1658 pacientes incluidos en la encuesta, el 47% fueron diagnosticados clínicamente y tratados como casos de malaria. La malaria fue diagnosticada más a menudo en centros privados. No se observó un patrón estacional claro para la malaria presuntiva. Un 30% de los casos de malaria diagnosticados clínicamente fueron positivos para Plasmodium (todos falciparum) por gota gruesa. Por lo tanto, los falsos positivos constituyeron más del 70% de los casos de malaria diagnosticada clínicamente. Las tasas de prevalencia fueron más altas al final y un poco después de la estación lluviosa (44 a 47%) mientras que la más baja fue durante la época seca (2%).

Conclusiones  El diagnóstico clínico de malaria tiene valores predictivos bajos en esta área urbana de baja endemicidad, y su baja especificidad conlleva a una atención inadecuada para una proporción grande de los pacientes. Esto tiene un gran impacto económico en los servicios sanitarios y en las familias. En Sahel, el uso sistemático de diagnóstico microscópico o de pruebas diagnósticas rápidas debería ser considerado para manejar apropiadamente tanto los casos de malaria como aquellos que no lo son.


Whilst more than 70% of malaria cases do not present initially to health facilities (Amexo et al. 2004), malaria is still the most important reason for consulting a health service in many countries of sub-Saharian Africa. This especially holds for private and public primary care facilities such as dispensaries, health centres and out-patient departments of hospitals where an important share of patients are diagnosed as malaria cases, based on a clinical examination only (Le Sueur 1996; Hamel et al. 2001; Caldas de Castro et al. 2004). For example, in Chad the annual statistical abstract indicates that 22% of all patients consulting a governmental or private health service are diagnosed for malaria, followed by acute respiratory infections (8%), diarrhoea (7%) and dysentery (3%) (MSP 2003).

In many places, malaria diagnosis is solely based on the medical history and clinical examination of patients, since there is no trained staff and equipment available to confirm the presence of Plasmodium parasites. The etiology of fever is thus often notified as malaria (Baudon et al. 1996; Gruenais 2003; Rogier 2004). Numerous studies have shown that a reliable diagnosis of malaria cannot be done on clinical grounds only (e.g. Genton et al. 1994), even if algorithm and scores are used (Chandramohan et al. 2002). There is therefore a significant risk for mistreatment, either in excess or lack (Olivar et al. 1991; Chandramohan et al. 2001; Nsimba et al. 2002; Amexo et al. 2004).

These observations are true for most of the malaria endemic settings. A particular complexity arises in areas of highly season transmission patterns such as in the Sahelian zones. In urban settings, open and/or clean water bodies, which are the most important precondition for the reproduction of most Anopheles species, do usually not exist much beyond the end of the rainy season. Consequently, the reproduction and density of malaria vectors, and hence of malaria, is limited to relatively short, well-defined time periods. While published evidence for a seasonal transmission of malaria in rural Sahelian settings does exist (e.g. Hamad et al. 2002; Koram et al. 2003), such information is still scarce for urban settings (Wang 2004).

Therefore, this study assessed the frequency of parasitaemia among clinically diagnosed malaria cases in an urban Sahelian setting, in view of better understand the magnitude of false positive diagnoses, their seasonal patterns, as well as implications in terms of changes in diagnostic standards, such as moving from clinical presumptive diagnosis to microscopy-based diagnosis or the introduction of rapid diagnostic tests for the management of uncomplicated malaria in primary care settings.

Material and methods

Chad is classified among the poorest countries of the world with a Gross National Product per head of US$ 220 in 2002 (World Bank 2004). Its capital city N'Djaména is located in the Sahelian zone, near lake Chad, has a size of around 5190 H and had in 2002 an estimated population of around 1 million inhabitants. There is a typical wide offer in private and governmental health services including two national referral hospitals, two district hospitals and around 50 first contact facilities (‘centre de santé’) (Wyss et al. 2003).

Between June 2002 and March 2003, all patients admitted to two private and two governmental first contact facilities were included in the study. Study facilities were selected to represent typical first contact facilities in N'Djaména. The facilities serve each one a population of between 15 000 and 50 000 people, are spread over the whole city with two being localized in the peri-urban and two in the central areas, and were thought be used by the general population. All health staff involved was informed prior to data collection on the objective and the study methodology. On a randomly selected and alternating day of the week, the personnel conducting clinical examinations registered demographic and health related information on a separate sheet, which was based on the standard reporting registers notifying the diagnosis of patients. The information system of the Ministry of Health separates and lists diseases at district level along 26 health problems, among them uncomplicated and complicated malaria. Patients consulting for a follow-up visit were excluded from the study.

Patients diagnosed with presumptive malaria gave a blood sample and a standardized exit interview based on a questionnaire to a trained interviewer for assessing treatment costs. Oral informed consent was obtained from all patients before taking the blood by a clinical officer. Slides were then transferred at the end of the day to the national referral laboratory and stained using standard methods. One experienced laboratory technician examined the stained blood. The parasite density/μl was calculated by counting the number of parasites per 200 white blood cells (WBC), assuming a WBC count of 8000/μl (Shute 1988). As quality control, a 10% random sample of slides were reread at the national reference laboratory of the Swiss Tropical Institute.

Laboratory results were registered on an examination bulletin and a retro-information was provided to the patient. The clinical and laboratory data were entered using EpiInfo 6.04 software and analysed using SPSS 10.1. The parasite density was classified in three categories: less than 1000 parasites/μl, from 1000 to 10 000 parasites/μl, and more than 10 000 parasites/μl of blood.


Characteristics of the sample population

During the study period, 1568 patients consulted the curative departments of the primary care facilities selected (Table 1). Among them, 731 (47%) used a public provider and 837 (53%) a private one. Women consulted private services (44.1% of patients) less frequently than governmental services (51.3%) (χ2 = 7.86, P < 0.005). The distribution of the study population across months and type of provider indicates that during the rainy season, i.e. from June to October, more patients consulted health services than during the consecutive months. Adults from 15 to 44 years were the most important group in the sample (Figure 1). However, compared to their relative demographic weight in the population of N'Djaména, they were slightly underrepresented. As expected, children <5 years of age consulted a health facility more frequently than their respective demographic weight in the population.

Table 1.  Proportional distribution of the attendances of study population by months and type of provider
MonthGovernmental providers (n = 731)Private providers (n = 837) Total (n = 1568)
  1. Values are in percentages.

Figure 1.

Age distribution of the population of N'Djaména in comparison with the study population.

Frequency of presumptive malaria

Uncomplicated malaria was diagnosed as the main health problem for 729 patients. In other words, nearly one in two patients (46%) was judged and treated as an uncomplicated malaria case. This diagnosis was significantly more often established by private providers (57% of patients) than governmental providers (35% of patients) (χ2 = 75.06, P < 0.001). Children younger than 5 years were less often diagnosed as malaria cases (27% among patients less than 1-year old, 32% among patients 1 to 4 years old) than aged 5 years and older (57% among patients 5 to 14 years old, 53% among patients 15–44, and 52% among patients 45 years and older). No significant differences could be found between men and women. Conversely, severe malaria was significantly more often diagnosed in governmental services (2% of patients) than in private services (1%) (χ2 = 1.79, P < 0.001).

During the study period, rainfalls were recorded in N'Djaména from June to October 2002 (Figure 2). With regard to seasonal patterns in the frequency of presumptive malaria, no marked changes or tendencies at the level of governmental providers were notified and presumptive malaria rates varied from 29% (June 2002) to 39% (September 2002) of all diagnoses. At the level of private providers, there were more marked differences, but no consistent tendency. Presumptive malaria was most often identified at the end of the rainy season in October (67% of all patients) and December (70% of patients).

Figure 2.

Seasonality of presumptive malaria in first contact level health facilities of governmental and private providers. Source rainfall data June 2002 to March 2003. Direction of Water Resources and Meteorology, Ministry of Water and Environment, Chad.

Clinical vs. microscopy-based diagnosis

For 712 of the 729 presumptive malaria patients, blood films and microscopy examinations were performed. A 30% of these (211 slides) were positive for Plasmodium, whereas 70% (501 slides) showed no malaria parasites at all. All those found positive were infected with Plasmodium falciparum, and 42% had a high parasitic density (≥10 000 parasites/μl), 24% a density between 1000 and 10 000 parasites/μl, and 34% a low parasitic density (≤1000 parasites/μl).

At governmental providers, the positive predictive value of the clinical diagnosis was not significantly higher than at private providers (33%vs. 28%; χ2 = 1.90, P = 0,16). In the age-category of those 5 to 14 years old, the frequency of malaria as confirmed by microscopy-based examination was highest, whereas in age categories of those <1 year and ≥45 years, only about 1 in 10 presumptive malaria cases could be confirmed by microscopy (Table 2).

Table 2.  Frequency of presumptive malaria and microscopy-confirmed diagnoses by age category and type of provider
Age category (years)Governmental providersPrivate providersTotal
Presumptive malaria (n)Positive parasitaemia (%)Presumptive malaria (n)Positive parasitaemia (%)Presumptive malaria (n)Positive parasitaemia (%)

Table 3 shows the frequency of presumptive malaria confirmed by microscopy according to the level of training of the treating personnel. At both levels, governmental and private providers, clinical officers performed significantly better than physicians (governmental providers: χ2 = 87.15, p < 0.001; private providers: χ2 = 7.91, P < 0.001). At the level of governmental facilities, physicians diagnosed 79% of patients as malaria cases, whereas only 19% had documented parasitaemia by microscopy.

Table 3.  Frequency of presumptive malaria and microscopy-confirmed diagnoses by level of training of health professionals
Skill levelGovernmental providersPrivate providersTotal
nPresumptive malaria (n)Positive parasitaemia (%)nPresumptive malaria (n)Positive parasitaemia (%)nPresumptive malaria (n)Positive parasitaemia (%)
Clinical officers with 2–3 years medical training59425%44%64261%29%123643%33%

Seasonal patterns of prevalence of presumptive malaria cases confirmed by micrscopy indicate that there is a steady increase of the percentage of patients in the period from June to December, corresponding to the rainy season and the period shortly afterwards (Figure 3). In the following months the percentage of false positive malaria cases increases drastically and, in the month of March, only 2% of the patients diagnosed as malaria clinically had detectable parasites in the blood.

Figure 3.

Frequency of microscopy-confirmed presumptive malaria cases by month.


This study is one of the first to present information on the prevalence and seasonality of urban malaria in Sahelian towns, where malaria transmission is highly seasonal. Our results revealed that malaria is the most frequent reason for using a governmental and private primary care facility, and that presumptive malaria is diagnosed as the main health problem for 46% of patients. In Chad, seasonal variation of attendances for presumptive malaria is low. The frequent diagnosis of presumptive malaria contrasts with laboratory findings indicating that only 30% of those patients have detectable parasites by microscopy. In other words, at least 70% of presumptive malaria cases are misdiagnosed. This estimate is conservative, since some of the parasite positive patients may have fever due to other causes (Chandramohan et al. 2002).

This study also showed that there are marked seasonal differences along the rainy season, and very low microscopy-confirmed malaria prevalence rates in the months following the end of the raining period. During the driest period, hardly any patients show a positive parasitaemia. In operational terms, this implies that in urban Sahelian settings no malaria treatment should be given unless the clinical diagnosis is confirmed by a laboratory-based examination such as microscopy or dipstick tests.

When the policy is to give antimalarials to all febrile cases or all presumptive malaria cases, misdiagnosis has implications for non-malaria cases in terms of appropriate care and well-being. Indeed the cause of fever is not appropriately tackled, and delay in introducing appropriate treatment, especially antibiotics for bacterial infections, may lead to severe complications or even deaths (Reyburn et al. 2004). Burden and economic costs on health services and households can be serious, including due to an accelerated spread of drug resistance. In N'Djaména, patients paid considerable amounts of money to treat their presumptive malaria. In the context of this study, we estimated, through exit interviews, median expenditures among false positive malaria cases to be in the order of € 1.5 for those consulting a governmental provider and € 9 for those consulting a private service. With the current move to introduce artemisinin-based combination therapy that will be more expensive as the current practices, high levels of misdiagnoses have substantial implications. It results in a high burden on the poorest and most vulnerable population groups, who are less likely to endure unreasonably extended periods of sickness, with the related high utilization of health services and foregone opportunities for income generation.

Rapid and unprecedented urbanization in sub-Saharan Africa have profound implications for the epidemiology and control of malaria, and control measures in urban settings should therefore – besides placing emphasis on prompt diagnosis and treatment – also strengthen preventive actions through mother and child health programs, the promotion of insecticide-treated bed nets for the rapidly growing numbers of the urban poor, and environmental management and larviciding of vector productive sites (Caldas de Castro et al. 2004; Keiser et al. 2004). This study further underlines the crucial importance of improving the quality of prompt diagnosis and treatment among those who do consult a health service. It is interesting to note that the study revealed – for both governmental and private providers – that the quality of performance was not proportional to the level of training of the personnel reflected by physicians not performing as good as clinical officers. Reasons of this difference in performance need to be investigated, and ways to improve their skills identified. Approaches to improving quality of care should be based on continuous education measures, regular quality assessments, and careful monitoring activities that can then be translated into practice.

In light of the substantial overestimation of the number of true malaria cases observed in this study (in the range beyond 60%), and considering the abundant literature that points up the poor performance of medical staff to diagnose malaria on clinical grounds only, it is very unlikely that training of clinical skills will be the only approach to improve the present situation. The systematic inclusion of laboratory-based diagnosis should be promoted in the health facilities of urban settings showing low and/or highly seasonal endemicity. At this stage and as the equipment and the necessary technical skills for microscopy-based examination may be difficult to scale up in the short run, rapid diagnostics tests based on the detection of Plasmodium antigens may be the most efficient approach to appropriately manage malaria as well as non-malaria cases.


The Special Programme for Research and Training in Tropical Diseases (TDR) and WHO/AFRO provided financing for this study. Additional support for the write-up of this paper was provided by the Swiss National Science Foundation (SNSF) and the Swiss Agency for Development and Cooperation through the Individual Project 4 ‘Health and Well-being’ of the ‘NCCR North-South: Research Partnerships for Mitigating Syndromes of Global Change’. We are very grateful to the health workers, the interviewers and the patients who kindly agreed to participate in the study. We would like to thank Dr Daugla Doumagoum Moto for advice and helpful comments during the design and analysis of the study. Written research and ethical clearance for this study was obtained from the Ministry of Health, Chad.