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Malaria Morbidity in High and Seasonal Malaria
Transmission Area of Burkina Faso
Alphonse Oue
´draogo
1
, Alfred B. Tiono
1
, Amidou Diarra
1
, Souleymane Sanon
1
, Jean Baptiste Yaro
1
,
Esperance Ouedraogo
1
, Edith C. Bougouma
1
, Issiaka Soulama
1
, Adama Gansane
´
1
, Amathe Ouedraogo
1
,
Amadou T. Konate
1
, Issa Nebie
1
, Nora L. Watson
2
, Megan Sanza
2
, Tina J. T. Dube
2
, Sodiomon
Bienvenu Sirima
1,3
*
1Centre National de Recherche et de Formation sur le Paludisme, Ouagadougou, Burkina Faso, 2The EMMES Corporation, Rockville, Maryland, United States of America,
3Groupe d’action et de Recherche en Sante
´, Ouagadougou, Burkina Faso
Abstract
Background:
Malariometric parameters are often primary endpoints of efficacy trials of malaria vaccine candidates. This
study aims to describe the epidemiology of malaria prior to the conduct of a series of drug and vaccine trials in a rural area
of Burkina Faso.
Methods:
Malaria incidence was prospectively evaluated over one year follow-up among two cohorts of children aged 0–5
years living in the Sapone
´health district. The parents of 1089 children comprising a passive case detection cohort were
encouraged to seek care from the local health clinic at any time their child felt sick. Among this cohort, 555 children were
randomly selected for inclusion in an active surveillance sub-cohort evaluated for clinical malaria during twice weekly home
visits. Malaria prevalence was evaluated by cross-sectional survey during the low and high transmission seasons.
Results:
Number of episodes per child ranged from 0 to 6 per year. Cumulative incidence was 67.4% in the passive and
86.2% in the active cohort and was highest among children 0–1 years. Clinical malaria prevalence was 9.8% in the low and
13.0% in the high season (p.0.05). Median days to first malaria episode ranged from 187 (95% CI 180–193) among children
0–1 years to 228 (95% CI 212, 242) among children 4–5 years. The alternative parasite thresholds for the malaria case
definition that achieved optimal sensitivity and specificity (70–80%) were 3150 parasites/ml in the high and 1350 parasites/ml
in the low season.
Conclusion:
Clinical malaria burden was highest among the youngest age group children, who may represent the most
appropriate target population for malaria vaccine candidate development. The pyrogenic threshold of parasitaemia varied
markedly by season, suggesting a value for alternative parasitaemia levels in the malaria case defintion. Regional
epidemiology of malaria described, Sapone area field centers are positioned for future conduct of malaria vaccine trials.
Citation: Oue
´draogo A, Tiono AB, Diarra A, Sanon S, Yaro JB, et al. (2013) Malaria Morbidity in High and Seasonal Malaria Transmission Area of Burkina Faso. PLoS
ONE 8(1): e50036. doi:10.1371/journal.pone.0050036
Editor: Vasee Moorthy, World Health Organization, Switzerland
Received June 25, 2012; Accepted October 15, 2012; Published January 8, 2013
Copyright: ß2013 Oue
´draogo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Sources of funding NIAID/DMID (subcontract NuHHSN266200416C). The funders had no role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have read the journal’s policy and have the following conflicts: Authors Nora L. Watson, Megan Sanza, Tina J.T. Dube are
employed by commercial company The EMMES Corporation. There are no patents, products in development or marketed products to declare. This does not alter
the authors’ adherence to all the PLOS ONE policies on sharing data and materials.
* E-mail: s.sirima.cnlp@fasonet.bf
Introduction
Despite suggestion of a general decline in malaria burden in
sub-Saharan Africa, Burkina Faso has shown no evidence of
decrease in high year-round endemicity; health facility reports in
fact suggest an increase nationally from an estimated 1 million in
the year 2006 to 4 million in 2009 [1–4]. Malaria prevention in the
region is challenged by an underdeveloped health care system and
inadequate coverage of effective drugs and vector control tools.
Development of an effective malaria vaccine as an additional tool
in the arsenal against malaria is widely recognized an important
strategy for reduction in global malaria burden. Several malaria
vaccines candidates in various stages of development have shown
favourable safety and immunogenicity profiles [5].
Development and evaluation of malaria vaccine trials demands
knowledge of the regional epidemiology of malaria; in fact malaria
infection, clinical malaria, and mortality rates have represented
primary endpoints of phase 2b or 3 of malaria vaccine trials [6].
Active and passive surveillance methods have more recently been
proposed for the evaluation of malaria vaccine efficacy. Consensus
guidelines [7] recognize the value of active case detection (ACD) in
minimizing the influence of health care-seeking behaviour and
allowing inference of age-associated and temporal patterns of
disease. Limited to those seeking medical care, passive case
detection (PCD) methods are likely to underestimate population
PLOS ONE | www.plosone.org 1 January 2013 | Volume 8 | Issue 1 | e50036
burden of malaria but may provide information of greater
relevance to a health services perspective [8].
Current malaria epidemiology data in endemic regions are
needed to evaluate the public health significance of alternative
interventions and to monitor effects of malaria control activities
over time. We sought to describe the epidemiology of malaria
prior to the conduct of a series of drug and vaccine trials by
combing active and passive case detection of malaria episodes
among children under five years living in a rural area of Burkina
Faso.
Patients, Materials and Methods
Study area and population
Enrollment was initiated in January 2007 in the Sapone´ health
district in the Baze`ga province where the Centre National de
Recherche et de Formation sur le Paludisme (CNRFP) has
maintained a continuous demographic surveillance system (DSS)
since 2005. The Sapone DSS is located in the centre of Burkina
Faso, 40 km at the south from the capital, Ouagadougou in the
Sudan-Sahelian eco-climatic zone (isohyets 600–800). The DSS
covers an area of 1700 Km2 within a plateau dissected by the
Nazinon river (White Volta river). Mean temperature recorded
during 2007 was 29uC and total rainfall for the same period was
713 mm. The climate is characterized by a rainy season from June
to September and dry season from October to May. The total
resident population is estimated as 100,000 inhabitants and infant
and children mortality is 105.3 per 1000 live births. Children
below five years represent 19% of the population and total fertility
rate is an estimated 45%. The predominating ethnic groups in the
area are Mossi and Fulani. Most of the population depends on
subsistence farming of millet and domestic animals (poultry, cattle
etc.). Houses are typically made of mud walls and grass or
corrugated iron roofs [9]. One reference district hospital (CMA de
Sapone´ with 32 beds) and 14 peripheral health facilities constitute
the government health network in the district.
Study randomization was performed at two levels to achieve
target sample sizes of PCD and ACD cohorts: i) two of the 14
peripheral health centres catchment areas composed of 13 villages
were randomly selected, ii) 9 villages of the 13 villages were
randomized to conduct passive case detection and 4 to conduct
active case detection of malaria over one year follow-up. All 13
villages were situated within <7.5 km of the peripheral health
centres, facilitating access to each PCD surveillance system.
Malaria transmission in the region is markedly seasonal and
intense during the rainy season. The entomological inoculation
rate is estimated at 0.3 and 44.4 infective bites/person/month
during the dry and rainy seasons, respectively [10]. The main
malaria vectors are Anopheles gambiae,An. Arabiensis and An funestus. P
falciparum accounts for a 90% of malaria infections. Oral
chloroquine was still in use in the communities during the study
period despite parasite resistance of 57% (Sirima and al.
unpublished data). The first line treatment for uncomplicated
malaria in Burkina is now CoartemHor Artesunate Amodiaquine.
In the study region these medications are distributed primarily by
peripheral health centres; there is no private clinic or pharmacy
alternative although prohibited or sub-standard antimalarials are
sold from the local market. In 2007, bed net coverage was less than
5% of the DSS population, reflecting the absence of large scale
bed net distribution in the region.
Study design and subjects
Malaria surveillance over one year follow-up consisted of
passive case detection among all patients attending the outpatient
clinic; and active case detection by twice weekly home visits and
two cross-sectional surveys among selected villages. The first cross
sectional survey occurred during low malaria transmission season
and the second during high transmission season. Overall and age-
specific incidence of clinical malaria were compared among the
passive and active cohorts.
All children aged under five years and living for at least three
months in the study area were eligible for inclusion in either cohort
at study initiation. Exclusion criteria were presence of major
congenital defect, chronic disease or severe anaemia (haemoglobin
value less than 6 g/dl); these conditions were considered to
increase participant risks associated with blood draws, and
potentially require absence from the study area for receipt of
specialized care. An age-stratified sample was generated from the
updated census of Sapone´ demographic surveillance system area.
Five age-specific age strata (0–11, 12–23, 24–35, 36–47 and 48–59
months) included 245, 201, 225, 219 and 199 children respectively
in the PCD, and 131, 103, 113, 115 and 93 children in the ACD
cohort. Children under one year were recruited quarterly to
maintain balance among cohorts. Signed or thumb-printed (in the
presence of an impartial witness) written informed consent from
child’s parent/legal guardian was required prior to inclusion in the
study. Each child was provided with an identity card and the
caregiver was given instructions how to access health centre in the
event of illness.
Passive surveillance. In January 2007 the PCD cohort
enrolled a total of 1090 children from 9 villages in the catchment
areas of two health facilities. In the two community clinics
providing routine care and in the Sapone´ District Hospital
emergency unit, local health staff received training in study
procedures and were provided with study case report forms to
record clinical information of children enrolled in either cohort. At
each attendance to a health facility (community clinics or Sapone´
District Hospital), study children received a brief physical
examination and parents reported previous health events. In the
event of reported history of fever within 24 hours or measured
axillary temperature $37.5uC, a finger-prick blood sample was
collected for preparation of duplicate thick blood smears. One
slide was read immediately (within two hours) in the Sapone´
District Hospital and each transferred to the CNRFP lab for
quality assurance. Rapid diagnosis tests (RDT) were performed in
the two community clinics of the study area where an operating
lab was not present. Results were used to guide diagnosis and
prompt adequate management of malaria cases. Antimalarial
medication was provided as needed at no cost to study
participants. This system operated 24 hours per day each day of
the year to continuously ascertain malaria episodes among
presenting study children. Free drug treatment was made available
as indicated for malaria and non-malaria illnesses.
Active surveillance. The active surveillance cohort consisted
of 555 children from four randomly selected villages who were
visited in the home twice per week for detection of clinical malaria
episodes over one year follow up. The target sample size was
achieved after recruitment of children living in the first four of
thirteen villages randomly selected for potential study inclusion.
Trained fieldworkers visited the children in their homes twice a
week on the same week day and before 11:00 AM in order to
minimize the bias due to diurnal variation of the body temperature
[11]. A morbidity questionnaire was administered for evaluation of
fever history and medication use. Respondents to the question-
naire were the children’s parents/guardians who were aged $18
years and were at home at the time of the visit. A brief physical
examination was performed and axillary temperature recorded. In
the event of repeated fever in the past 24 hours or axillary
Malaria Morbidity in Burkina Faso
PLOS ONE | www.plosone.org 2 January 2013 | Volume 8 | Issue 1 | e50036
temperature $37.5uC, a finger prick blood sample was collected
for preparation of thick blood smears for microscopic examination.
Feverish children received a malaria rapid diagnostic test for
prompt diagnosis and treatment. All symptomatic children were
referred to the nearest local heath staff or to the Sapone´ District
Hospital to receive adequate treatment and follow-up as clinically
appropriate at no cost to the participant. Between the two
scheduled field-worker visits, parents were encouraged to report to
the nearest community clinic or hospital at any time whenever
their children showed signs of sickness. Children were provided
with study identification cards for presentation during clinical or
hospital attendance.
Diagnostic procedures were repeated during two seasonal cross
sectional surveys, which contributed to analyses of prevalence of
malariometric indicators and malaria attributable fraction of
fevers. The first cross sectional survey occurred during the dry
season and the second one during the wet season of the same year.
Laboratory methods
Thick and thin blood films were prepared, air-dried, fixed,
Giemsa-stained, and examined using a light microscope fitted with
1006oil immersion lens. Number of parasites and leucocytes were
counted to reach 200 leucocytes for positive slides. Slides were
declared negative only after 100 high power fields has been read.
Parasite numbers were converted to a count/mL. All slides were
read twice and slides with discrepant result were then read a third
time. Haemoglobin was measured at each survey using a portable
Hemocue.
Study Definitions
Fever was defined as axillary temperature $37.5uC and or
reported fever within the previous 24 hours. A clinical malaria
episode was defined as an axillary temperature of $37.5uCor
history of fever in last 24 hours, and positive asexual parasitemia
(P. falciparum trophozoite count greater than 0). Analyses of clinical
malaria incidence were repeated using the alternative parasitemia
thresholds of .0, $2500, and $5000.
In analyses of clinical malaria incidence, children were censored
for 28 days after recording an episode, to ensure that the infection
causing the episode was only recorded once.
Statistical analysis
Overall and age-specific fever and malaria incidence rates over
one year follow-up were calculated as the number of events
malaria cases divided by the total person time at risk. Cumulative
incidences were estimated by the Kaplan-Meier method. Children
were censored for each day he or she recorded a fever or for 28
days after recording an episode, to ensure that the infection
causing the episode was only recorded once. In analyses of both
fever and clinical malaria incidence, children were censored after
365 days of follow-up, or at the date of death/out migration
(N = 25). Those who did not experience death/migration but were
followed for less than one year were censored at the last recorded
passive case detection visit (Febuary 29
th
2008). The low
transmission season was defined as the date of first enrollment
(January 23
rd
2007) through May 31
st
2007 and November 1
st
2007 through date of last observation (February 29
th
2008). The
high transmission season was defined as June 1
st
2007 through Oct
31
st
2007.
Malaria attributable fraction of fevers, and sensitivity and
specificity of alternative parasite thresholds for the malaria case
definition were calculated according to methods described by
Dicko et al [12] Briefly, the odds ratio for the risk of fever
associated with increasing parasite density was estimated sepa-
rately for each transmission season, using data from the first and
second cross-sectional surveys. The model was of the form: Log(p
i
/
(i2p
i
)) = b
o
+b
1
x
i
t
, where p
i
is the fitted probability that individual i
is a fever case rather than a control, and x
i
t
is the parasite density
to the power of t. Several models were fit using alternative t
values, and a final twas selected as the value that provided the
model of best fit. Attributable fraction, sensitivity and specificity
were then defined as detailed by Smith et al [13].
Attributable fraction estimates have been found to vary
depending on whether fever cases are defined using objective
(temperature $37.5uC) or both objective and subjective fever [14].
To compare estimates obtained by each method, we repeated the
analyses using the following alternative definitions of fever.
Method 1: A fever case was defined as axillary temperature
$37.5 or reported history of fever in past 24 hours; a control was
defined as no axillary temperature $37.5 or reported history of
fever in past 24 hours. This analysis included 522 and 487
children who had fever and parasitemia data at the first and
second cross-sectional surveys, respectively. Method 2: A fever
case was defined as axillary temperature $37.5; a control was
defined as no axillary temperature $37.5 or reported history of
fever in past 24 hours. Of the children who had fever and
parasitemia data at each survey, this analysis excluded those who
did not meet either the definition of a fever case or control, as they
had no axillary temperature $37.5 but had reported history of
fever in past 24 hours. This exclusion yielded a final sample size of
494 and 447 children at the first and cross-sectional surveys,
respectively. While reduction in sample size may be seen as a
disadvantage of relying exclusively on objective fever data, this
approach offers the potentially important advantage of reducing
misclassification of fevers due to errors in reporting.
Sample sizes expected to yield robust estimates of malaria
incidence were calculated using Epi Info version 6 assuming
weekly incidence densities estimated in previous years from
regional DSS and targeted surveillance within one of the study
site villages. Access was used for double data entry and query
resolution and SAS 9.2 for data analysis.
The spatial analysis map of community clinic accessibility was
produced using Arc View GIS 3.2.
Quality assurance
Validity and completeness of data collection procedures were
reviewed weekly by a team of physician researchers. In the ACD
cohort, each week a sample of fifteen (average 50 children)
randomly selected households were revisited by a field supervisor.
The supervisor interviewed the parents/guardian to verify
reporting of fever or collection of a blood sample.
All slides were read twice independently; if the ratio of densities
from the first two readings was .1.5 or ,0.67 or if ,30 parasites
were counted with a difference in the number of parasites .10,
the slide was evaluated a third time. The definitive result was the
geometric mean of the parasite density. In the event of discrepancy
the slide was evaluated a third time and the final result defined as
the majority reading for positivity.
Data queries were generated weekly to identify and resolve
missing or erroneous values.
Ethics Statement
The study protocol and the informed consent form were
approved by the Burkina Faso ministry of health ethic committee.
The study was conducted in compliance with principles set out by
the International Conference on Harmonization Good Clinical
Practices, the Declaration of Helsinki and the regulatory
requirements of Burkina Faso. Individual written informed
Malaria Morbidity in Burkina Faso
PLOS ONE | www.plosone.org 3 January 2013 | Volume 8 | Issue 1 | e50036
consent was obtained from all children’s parents or legal
representatives, in the presence of an independent witness for
illiterate parents/legal representative. The conduct of the study
was monitored by the sponsor.
Results
Population characteristics
Of 1644 children enrolled, analyses include 1089 (66.3%) in
PCD and 555 (33.7%) in ACD. Eight (8) children died and 17
(1%) withdrew during the follow up period. Enrolment within each
cohort was approximately equally distributed among gender and
age groups (Table 1).
Median number of visits by child
ACD children where present during 55279 of 57720 (95.8%)
home visits attempted. Losses to follow up were due to parents
moving away from the DSS area. Median number of visits per
child was 100 (range 1–104). ACD children presented to the
community clinic without reference 3389 times; median number of
visits per child was 6 (range 1–20).
PCD children contributed 5742 visits; median number of visits
per child was 5 (range 1–14).
Incidence of fever
ACD scheduled & unscheduled visits identified 2668 cases of
fever; 79% (n = 2129) of these were detected through unscheduled
visits. 2382 cases of fever were detected by passive surveillance
visits.
In both ACD and PCD cohorts fever was more frequent among
infants and markedly lower among children aged 4–5 years
(Table 2). Fever incidence also varied substantially by malaria
transmission season: episodes per child-year-at-risk (95% CI) were
1.6 (1.5–1.7) (PCD) and 2.3 (1.9–2.7)(ACD) during the low season,
versus 3.2 (3.0–3.4) (PCD) and 7.0 (6.7–7.4) (ACD) in the high
season.
Incidence of clinical malaria episodes
Incidence of clinical malaria defined by alternative parasitemia
thresholds was nearly two fold higher in the ACD relative to the
PCD cohort over one year follow up. P. falciparum was identified
among 1071 (40%) of 2668 fevers in the ACD, and among 1208
(51%) of 2382 fevers in the PCD cohort. Clinical malaria
incidence was higher in younger age groups among both the
ACD and PCD cohorts (Table 3). Number of episodes per child
ranged from 0 to 6.
Incidence of clinical malaria defined by the alternative
parasitemia threshold of 5000 parasites/mL (episodes per child-
year-at-risk) (95% CI) was 1.6 (1.5–1.7) in the ACD and 0.8 (CI
0.8–0.9) in the PCD cohort (Table4).
Clinical malaria incidence was higher in younger children
within each season and among alternative parasite thresholds for
malaria case definition. Children aged $4 years experienced
fewest malaria episodes with incidence rates ranging between 0.8
and 2.2 episodes per child-year-at-risk. This pattern was apparent
by each method of incidence assessment (Table 3).
Among all age groups clinical malaria incidence was highest
following the peak rainfall (Figure 1a & 1b).
Malariometric parameters according to the season
Two cross-sectional surveys among children in the ACD cohort
evaluated prevalence of malariometric indices within the low and
high transmission seasons (Table 5).
Clinical malaria prevalence was 9.8% in the low versus 13.0%
in the high season (p = 0.12); prevalence of asexual P. falciparum
infection was 57.5% in the low versus 49.3% in the high season
(p = 0.01). Geometric mean parasite density (parasites/mL) (95%
CI) was lower during the low relative to high season: 1554
(1307.5–1848.9) versus 2673.2 (2077.1–3440.4) (p,0.01)
(Figure 2a & 2b).
Time to first malaria episode at enrollment
Median days to first malaria episode ranged from 187 (95% CI
180–193) among children 0–1 years to 228 (95% CI 212, 242)
among children 4–5 years in the ACD cohort. Cumulative
incidence among these age groups ranged from 93%–71%
(Figure 3).
Malaria attributable fraction of fevers
Malaria attributable fraction of fever was 41.9% in the high
season and 34.5% in the low season, where fever was defined as
reported fever in the past 24 hours and measured temperature
$37.5uC. The alternative parasite thresholds for the malaria case
definition that achieved optimal sensitivity and specificity (70–
80%) were 3150 parasites/ml in the high and 1350 parasites/mlin
the low season (Figure 4a & 4b).
The objective definition of fever (axillary temperature $37.5uC)
yielded a malaria attributable fraction of fever of 59.6% in the
high and 17.2% in the low transmission season. The thresholds
that achieved sensitivity and specificity were 2930 parasites/mlin
the high and 2780 parasites/ml in the low season (Figure 5a & 5b).
Discussion
In this cohort of children under five years living in Burkina
Faso, cumulative incidence by active case detection over one year
follow-up exceeded 90% in the youngest children. Sixty-five
percent of malaria episodes were detected during the rainy season.
These data characterize the Sapone´ district as a stable malaria
transmission area with marked seasonality [10,15]. Despite
introduction of effective medications and vector controls to the
region, malaria burden in the region remains significant and
consistent with other sub-Saharan African countries [16–19].
Incidence
Plasmodium falciparum is the most common vector of malaria in
the Balonghin region. Clinical malaria incidence was highest
among children under three years and showed graded decline
through five years, a pattern consistent with previous reports
[18,20]. Continued exposure to malaria among infants and
Table 1. Characteristics of children at recruitment.
Cohort C Cohort B
Gender M 289 (52.1%) 551 (50.6%)
F 266 (47.9%) 538 (49.4%)
Age group (years)
[0–1] 131 (24%) 245 (23%)
[.1–2] 103 (18%) 201 (18%)
[.2–3] 113 (20%) 225 (21%)
[.3–4] 115 (21%) 219 (20%)
[.4–5] 93 (17%) 199 (18%)
N 555 (100%) 1089 (100%)
doi:10.1371/journal.pone.0050036.t001
Malaria Morbidity in Burkina Faso
PLOS ONE | www.plosone.org 4 January 2013 | Volume 8 | Issue 1 | e50036
children living in this and other endemic regions may contribute to
development of partial immunity in early life [20].
Despite high all year-round endemicity, clinical malaria was
markedly more common during the rainy months of June-
November, a pattern previously reported in Burkina Faso
[15,21,22]. Age and seasonal patterns of malaria morbidity were
consistent among alternative parasite thresholds for the malaria
case definition.
Active and passive surveillance
Age and seasonal trends also persisted among passive (PCD) and
active case detection (ACD) cohorts; however, fever and clinical
malaria incidence detected by ACD were approximately twice that
by PCD.
These data suggest that as many as half of malaria episodes
identified by active follow-up may be undetected by passive
surveillance in this and comparable settings. This expected
advantage of ACD highlights the common occurrence of
unattended symptomatic malaria. Variation among cohorts may
also in part reflect study design limitations. Passive surveillance
outcomes may be underestimated due to alternative care seeking
from non-study clinics or traditional healers or use of anti-
malarials from the local market. Moreover, parents of ACD
children may have been more likely to seek medical care for
symptomatic illness as encouraged by field workers during regular
Table 2. Age-specific incidence of fever over one year follow-up.
Age group (years) Number of children Number of visits Number of fevers
Total p-yrs* at
risk
Incidence rate (95% CI)
(per p-yr at risk)
ACTIVE SURVEILLANCE
[0–1] 131 11117 789 112.4 7.0 (6.5, 7.5)
[.1–2] 103 10244 588 101.3 5.8 (5.3, 6.3)
[.2–3] 113 11733 558 111.4 5.0 (4.6, 5.4)
[.3–4] 115 12388 456 113.1 4.0 (3.7, 4.4)
[.4–5] 93 9797 277 92.2 3.0 (2.7, 3.4)
Total 555 55279 2668 530.5 5.0 (4.8, 5.2)
PASSIVE SURVEILLANCE
[0–1] 245 1738 813 216.5 3.8 (3.5, 4.0)
[.1–2] 201 1287 569 198.2 2.9 (2.6, 3.1)
[.2–3] 225 1121 474 220.1 2.2 (2.0, 2.4)
[.3–4] 219 863 291 217.4 1.3 (1.2, 1.5)
[.4–5] 199 733 235 195.5 1.2 (1.1, 1.4)
Total 1089 5742 2382 1047.7 2.3 (2.2, 2.4)
doi:10.1371/journal.pone.0050036.t002
Table 3. Age-specific incidence of clinical malaria episodes parasitemia threshold for malaria case definition: .0 parasites/ml
during high and low seasons.
Age group (years)
Number of
children
Number of
visits
Number of
episodes
Total p-yrs*
at risk
Incidence rate (95% CI)
(per p-yr at risk)
Cumulative
incidence
PASSIVE SURVEILLANCE
[.0–1] 245 1738 340 192.4 1.8 (1.6, 2.0) 83.6
[.1–2] 201 1287 291 176.9 1.6 (1.5, 1.8) 69.1
[.2–3] 225 1121 267 200.8 1.3 (1.2, 1.5) 67.9
[.3–4] 219 863 169 205.2 0.8 (0.7, 1.0) 41.6
[.4–5] 199 733 141 185.1 0.8 (0.6, 0.9) 55.9
Total 1089 5742 1208 960.4 1.3 (1.2, 1.3) 67.4
ACTIVE SURVEILLANCE
[0–1] 131 11117 243 95.5 2.5 (2.2, 2.9) 93.0
[.1–2] 103 10244 230 85.1 2.7 (2.4, 3.1) 89.6
[.2–3] 113 11733 260 92.7 2.8 (2.5, 3.2) 94.0
[.3–4] 115 12388 215 97.5 2.2 (1.9, 2.5) 80.7
[.4–5] 93 9797 123 83.3 1.5 (1.2, 1.8) 71.4
Total 555 55279 1071 454.1 2.4 (2.2, 2.5) 86.2
doi:10.1371/journal.pone.0050036.t003
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PLOS ONE | www.plosone.org 5 January 2013 | Volume 8 | Issue 1 | e50036
home visits; by contrast, parents of PCD children received this
instruction only at enrolment. It is also possible that time at risk for
malaria, particularly among ACD children, is in part overesti-
mated where chemoprophylaxis due to antimalarial treatment
may have extended beyond the 28 day censoring window imposed
after each observed episode. Refinement of censoring windows
according to treatment duration information not available in this
study may be expected to minimize associated inflation of time at
risk, yielding higher than current incidence estimates.
Malariometric indicators by cross sectional survey
Prevalence of clinical malaria in the ACD cohort ranged from
10–13%, and prevalence of P.f. infection from 49–58% among
transmission seasons. Age was not consistently associated with
prevalence of clinical malaria, P.f. infection or splenomegaly.
Prevalences of P.f. infection, gametocyte carriage, and splenomeg-
aly were slightly higher in the low relative to high transmission
season. These data may reflect protection provided by longitudinal
active surveillance, in which frequent contacts initiated following
the low season survey prompted early diagnosis and treatment and
potentially chemoprophylaxis effect particularly through the
subsequent high season. Prevalence of malariometric indices
may be considered underestimated as a result of this potentially
artificially healthy ACD cohort.
Time to the first episode
Time to first malaria episode from the low season survey was
lowest among ages 0–2 years, consistent with previously described
vulnerability associated with the loss of protection by maternal
immunoglobulin following the first six months of life. Cumulative
incidences of malaria were also highest among this age group,
potentially reflecting a critical period for induction of partial
immunity associated with repeat malaria exposure in early life.
Children 3–5 years were more likely to escape infection during the
high transmission season, suggesting development of immune
protection prior to our observation.
Fraction attributable
Estimated proportion of fever attributable to malaria parasitae-
mia varied substantially by malaria case definition and were nearly
two fold greater in the high relative to low transmission season.
These data yielded optimal parastemia thresholds for malaria case
definition ranging from approximately 3000 parasites/ml in the
high and 1350 parasites/ml in the low season. Marked dependence
of these characterizations on season and case definition have
previously been described among children living in an endemic
areas of Mali and Ghana [12,23]. Variation in estimates according
to fever definition was more recently characterized among cohorts
living in low to moderate transmission areas of rural Kenya Olotu
Table 4. Incidence density of malaria episode according to season and different level of parsitemia.
Stratum
Number of
observations
Number of children
having at least one
episode (
.
0
parasites/ml)
Incidence rate
per year at risk
.
0
parasite/mL
Incidence rate
per year at risk
.
2500 parasite/mL
Incidence rate per
year at risk
.
5000
parasite/mL
Active cohort 55279 480 2.4 (2.2, 2.5) 1.8 (1.7, 1.9) 1.6 (1.5, 1.7)
Passive cohort 5742 702 1.3 (1.2, 1.3) 0.9 (0.9, 1.0) 0.8 (0.8, 0.9)
Active cohort high season 22013 424 3.9 (3.6, 4.2) 3.2 (2.9, 3.4) 2.9 (2.7, 3.2)
Passive cohort high season 1824 585 2.2 (2.1, 2.4) 1.8 (1.6, 1.9) 1.6 (1.5, 1.7)
Active cohort low season 33266 304 1.4 (1.3, 1.6) 0.9 (0.8, 1.0) 0.8 (0.7, 0.9)
Passive cohort low season 3918 323 0.7 (0.6, 0.7) 0.4 (0.4, 0.5) 0.3 (0.3, 0.4)
doi:10.1371/journal.pone.0050036.t004
Figure 1. Age-specific malaria incidence according to the season. a. Seasonal variation in age-specific clinical malaria incidence evaluated by
active surveillance over one year. b. Seasonal variation in age-specific clinical malaria incidence evaluated by passive surveillance over one year.
doi:10.1371/journal.pone.0050036.g001
Malaria Morbidity in Burkina Faso
PLOS ONE | www.plosone.org 6 January 2013 | Volume 8 | Issue 1 | e50036
[14]. Among the current ACD cohort, nearly 80% of cases with
objective fever and 65% of the cases with subjective or objective
fever identified during the low season were in fact not malaria.
Moreover, during the high transmission season, subjective or
objective fever was associated with a consistently lower malaria
attributable fraction of fever than measured temperature only.
These data suggest an advantage of objective fever in defining
malaria outcomes due to low specificity of subjective fever.
Consideration of optimal parasitemia thresholds for malaria case
definitions, particularly during the high season, may also avoid
overestimation of malaria burden due to low specificity of low
parasitemia levels.
Practicians in this region may be advised to investigate
alternative etiology of fevers given parasitemia ,1350 in the dry
or ,3150 ml in the rainy season, the optimal thresholds derived
from the favored objective fever definition.
Conclusion
In this high and seasonal transmission region of Burkina Faso,
burden of malaria was highest among the youngest children, who
may be considered the most appropriate target for malaria vaccine
candidates. Active case detection identified nearly twice the
incidence of malaria episodes relative to the passive system,
highlighting the common occurrence of potentially clinically
important unattended illnesses and resulting advantage of
enhanced surveillance methods in estimation of disease burden.
The pyrogenic threshold for malaria case definition differed
markedly by season, suggesting that alternative etiologies of fever
Table 5. Malariometric parameters Prevalence by Age Group in active surveillance cohort according to the season.
Age group
(years) Clinical malaria
P. falciparum
infection Gametocyte carriage Splenomegaly Hemoglobin mean (SD)
Low Season
[0–1] 6 (5.7%) 24 (22.9%) 15 (14.3%) 4 (3.8%) 9.58 (2.13)
[.1–2] 10 (9.7%) 53 (51.5%) 30 (29.1%) 15 (14.6%) 9.06 (1.64)
[.2–3] 12 (10.6%) 74 (66.7%) 40 (36.0%) 18 (15.9%) 9.51 (1.31)
[.3–4] 15 (13.2%) 78 (70.3%) 36 (32.4%) 32 (28.1%) 9.88 (1.53)
[.4–5] 9 (9.7%) 71 (77.2%) 33 (35.9%) 19 (20.7%) 10.58 (1.16)
Total (N = 528) 52 (9.8%) 300 (57.5%) 154 (29.5%) 88 (16.7%) 9.71 (1.66)
High Season
[0–1] 15 (16.1%) 35 (37.6%) 6 (6.5%) 4 (4.3%) 9.59 (1.18)
[.1–2] 15 (16.5%) 49 (53.8%) 23 (25.3%) 6 (6.6%) 10.13 (1.30)
[.2–3] 17 (16.3%) 49 (47.1%) 12 (11.5%) 12 (11.7%) 10.73 (1.09)
[.3–4] 8 (7.1%) 61 (54.5%) 29 (25.9%) 14 (12.5%) 10.93 (1.19)
[.4–5] 8 (9.4%) 45 (52.9%) 12 (14.1%) 4 (4.7%) 11.11 (1.12)
Total (N = 485) 63 (13.0%) 239 (49.3%) 82 (16.9%) 40 (8.3%) 10.53 (1.29)
doi:10.1371/journal.pone.0050036.t005
Figure 2. Prevalence of parasitemia and geometric mean parasite densities according to the season. a. Age-specific prevalence of
parasitemia and geometric mean parasite densities in the high transmission season. Line indicates parasitemia prevalence. Bars indicate geometric
parasite density and 95% confidence interval. b. Age-specific prevalence of parasitemia and geometric mean parasite densities in the low
transmission season. Line indicates parasitemia prevalence. Bars indicate geometric parasite density and 95% confidence interval.
doi:10.1371/journal.pone.0050036.g002
Malaria Morbidity in Burkina Faso
PLOS ONE | www.plosone.org 7 January 2013 | Volume 8 | Issue 1 | e50036
should be considered in the presence of low parasitemia levels
particularly during the low transmission season. Characterization
of malaria epidemiology in the Balonghin region reinforces the
urgency of improved access to currently available medication and
vector control tools, while setting the stage for conduct of early
phase vaccine trials in local field sites.
Figure 3. Time to first malaria episode among children 0–5 years evaluated by active surveillance from the low through high
transmission season.
doi:10.1371/journal.pone.0050036.g003
Figure 4. Malaria case definition using objective and subjective fever according to the season. a. Sensitivity and specificity of alternative parasite
threshold for malaria case definition in the high transmission season using objective (.= 37.5 degrees C) and subjective fever. b. Sensitivity and specificity of
alternative parasite threshold for malaria case definition in the low transmission season using objective (.=37.5 degrees C) and subjective fever.
doi:10.1371/journal.pone.0050036.g004
Malaria Morbidity in Burkina Faso
PLOS ONE | www.plosone.org 8 January 2013 | Volume 8 | Issue 1 | e50036
Acknowledgments
We express our gratitude to study participants for their kind cooperation
and support, CNRFP staff, Prof Koram A. Kwadwo from NMIMR,
Ghana, Mr. Walter Jones and Dr. Steven Rosenthal from NIAID/DMID.
Author Contributions
Conceived and designed the experiments: Alphonse Oue´draogo ABT
ATK IN SBS. Performed the experiments: Alphonse Oue´draogo ABT
ATK IN SBS AD SS JBY EO ECB IS AG Amathe Ouedraogo. Analyzed
the data: NW MS TJTD. Contributed reagents/materials/analysis tools:
IN AD SS ECB IS AG Amathe Ouedraogo. Wrote the paper: Alphonse
Oue´draogo ABT ATK IN SBS NW MS TJTD.
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Malaria Morbidity in Burkina Faso
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