Changing malaria epidemiology in four urban settings in sub-Saharan Africa

Background
An estimated 200 million persons in sub-Saharan Africa (SSA) live currently in urban centres in
malaria endemic areas. The epidemiology and control of urban malaria poses a number of specific
challenges in comparison to rural areas, most notably the heterogeneous spatial distribution of
transmission and the low state of immunity in the population. Interestingly, much less is currently
known about malaria in urban settings than in rural areas. As a result there is an essential need for
more information on disease burden, distribution and control strategies. In this multi-country study
we undertook to study systematically key malariological features in four large SSA cities: Abidjan
(Côte d’Ivoire), Ouagadougou (Burkina Faso), Cotonou (Benin) and Dar es Salaam (United
Republic of Tanzania).
Objectives
The general objective of this series of case studies was to further our understanding of malaria
transmission and epidemiology in the urban environment in SSA, in view of developing and
implementing effective control measures.
Study methodology
The basic study design of RUMA in each site included six components: 1) An extensive literature
review and contacts with national malaria experts, 2) The collection of routine health statistics,
disaggregated by sex, age and residence, 3) The mapping of health facilities and the identification
of the main breeding sites on the basis of existing maps, 4) School parasitaemia surveys (200 school
children aged 5-10 years in 3-4 schools, 5) Health facility-based fever surveys (200 fever cases and
200 non-fever controls in 3-4 facilities) and 6) A systematic description of the health care delivery
system. For components 4 and 5 we categorized each city into 3-4 areas (centre, intermediate,
periphery and rural areas), and randomly chose one clinic and one nearby school from each area.
All work was completed within six to ten weeks on-site.
The main emphasis was put on describing the burden of malaria (components 1, 2, 4), transmission
patterns (components 1, 2, 3, 4) and the diagnosis of malaria in urban settings (component 5, 6).
Finally, key risk factors for infection were explored (components 4 and 5).
Key results
Abidjan: The field work was carried out in August-September 2002 during the rainy season.
According to national statistics, approximately 240,000 malaria cases were reported by health
facilities in Abidjan in 2001 (40.2% of all consultations). The peak malaria incidence was in
July-September. In the health facilities of the Yopougon commune, the malaria infection rates in
presenting fever cases were 22.1% (under 1 year-old), 42.8% (1-5 years-old), 42.0% (6-15 years-old)
and 26.8% (over 15 years-old), while those in the control group were 13.0%, 26.7%, 21.8% and
14.6%. Malaria prevalence in health facilities was homogenous in the different areas of Yopougon.
The malaria-attributable fractions (MAFs) among presenting fever cases were 0.12, 0.22, 0.27 and
0.13 for the age groups listed above, suggesting that malaria played only a low to moderate role in
fever episodes during the rainy season.
Among all patients, 10.1% used a mosquito net (treated or not) the night before the survey and this
was protective (OR=0.52, 95% CI 0.29-0.97). Travel to rural areas within the last three months was
frequent (31% of all respondents) and associated with a malaria infection (OR=1.75, 95% CI
1.25-2.45). The health facility and breeding site mapping, as well as the school surveys could not be
carried out because of political troubles.
Ouagadougou: The field work was carried out in November-December 2002 at the start of cold and
dry season. Seasonal variations in reported clinical malaria cases were marked. The highest
incidence rate was reported from July to September and incidence rates went down in
October-December until a low point during the dry season, from January to March. In 2001, there
were 203,466 simple malaria cases (29.3-41.4% of consultations) and 19 deaths reported among
596,365 consultations in all public health facilities. A further 20,071 complicated malaria cases
were reported.
The malaria infection rates in presenting fever cases were 12.1% (under 1 year-old), 25.9% (1-5
years-old), 37.1% (6-15 years-old) and 18.0% (over 15 years-old), while those in the control group
were 14.3%, 14.4%, 34.5% and 19.8%. The MAFs among presenting fever cases were 0.00, 0.13,
0.04 and 0.00 for the age groups cited above, suggesting that malaria played only a small role in
fever episodes at the start of cold and dry season. The school parasitaemia prevalence was rather
high (overall: 48.3%) and there was heterogeneity between the 3 surveyed schools (31.6%, 37.6%,
73.1%). The mapping of Anopheles sp. breeding sites correlated with this gradient of endemicity
between the urban centre and the periphery of Ouagadougou. We found a link between malaria
infections and urban agriculture activities and the availability of water supply. In total 42.0% of
patients used a mosquito net the night before the survey and this was protective (OR=0.74, 95% CI
0.54-1.00). Travelling to a rural area (8.7% of all respondents) did not increase the infection risk
(OR=1.14, 95% CI 0.70-1.90).
Cotonou: The field work was carried out in February-March 2003. In 2002, there were 100,257
reported simple malaria cases and 12,195 complicated malaria cases reported for 289,342
consultations in the public health facilities of Cotonou. Between 1996 and 2002, on average 34% of
total consultations were attributed to simple malaria and 1-4.2% to complicated malaria cases.
There was no clear seasonal pattern.
The malaria infection rates in presenting fever cases were 0% (under 1 year-old), 6.8% (1-5
years-old), 0% (6-15 years-old) and 0.9% (over 15 years-old), while those in the control group were
1.4%, 2.8%, 1.3% and 2.0%. The MAFs among presenting fever cases were 0.04 in the 1-5
years-old and 0 in the over 15 years-old. MAFs could not be calculated for the other two age
groups. Hence, malaria played only a small role in fever episodes at the end of the rainy season. In
the school parasitaemia surveys, a malaria infection was found in 5.2 % of all samples. The
prevalence of parasitaemia in the centre, intermediate and periphery areas was 2.6%, 9.0% and
2.5%, respectively. In total 69.2% of patients used a mosquito net the night before the survey
(OR=0.61, not significant). Traveling to a rural area (5.8% of all respondents) did not increase the
infection risk since none of those who had traveled had parasitaemia. No mapping of health
facilities and breeding sites could be carried out.
Dar es Salaam: The field work was carried out in June-August 2003. An estimated 1.1 million
annual malaria cases were reported in 2000 from a total of 2,200,000 outpatient visits in the health
facilities (49% of all consultations). A clear seasonal pattern of clinical malaria was recorded, with
high rates from March to June and a low point in July-August.
The malaria infection rates in presenting fever cases were 2.0 % (under 1 year-old), 7.0% (1-5
years-old), 7.2 (6-15 years-old) and 4.2 % (over 15 years-old), while those in the control group
were 3.4%, 4.5%, 3.6% and 1.9%. The MAFs were very low in all age groups: 0.00, 0.03, 0.04 and
0.02 for the age categories shown above. School surveys in Dar es Salaam during a prolonged dry
season in 2003 showed that the prevalence of malaria parasites was low: 0.8%, 1.4%, 2.7% and
3.7% in the centre, intermediate, periphery and rural areas, respectively. Anopheles sp. breeding
sites were fairly well distributed within the city. We found a remarkably high coverage rate of
mosquito nets in the households (91.8% users) and this seemed to be protective (OR=0.60, 95% CI
0.27-0.93). An increased malaria infection rate was seen in the 11.8% of children who traveled to
rural areas within last 3 month (OR=3.62, 95% CI 1.48-8.88).
Conclusion
RUMA was successfully implemented in 4 selected urban areas within a period of six to ten weeks
per site. The financial cost for conducting a RUMA in these four sites ranged from 8,500-13,000
USD. All components were feasible (with the exception of breeding site mapping which clearly
exceeded what can be done in such a short time period) and highly informative. The RUMA
allowed to describe transmission patterns in the four cities and highlighted the enormous level of
over-treatment with antimalarials. The collected information should prove of high value as a basis
for further investigations and for planning effective control interventions.