Am. J. Trop. Med. Hyg., 82(2), 2010, pp. 176–184
doi:10.4269/ajtmh.2010.09-0373
Copyright © 2010 by The American Society of Tropical Medicine and Hygiene
Is Mosquito Larval Source Management Appropriate for Reducing Malaria in Areas
of Extensive Flooding in The Gambia? A Cross-over Intervention Trial
Silas Majambere,† Margaret Pinder,† Ulrike Fillinger, David Ameh, David J. Conway, Clare Green,
David Jeffries, Musa Jawara, Paul J. Milligan, Robert Hutchinson, and Steven W. Lindsay*
School of Biological and Biomedical Sciences, Durham University, Durham, United Kingdom; Medical Research Councils
Laboratories, Fajara, The Gambia; London School of Hygiene and Tropical Medicine, London, United Kingdom
Abstract. Larviciding to control malaria was assessed in rural areas with extensive seasonal flooding. Larval and adult
mosquitoes and malaria incidence were surveyed routinely in four 100-km2 areas either side of the Gambia River. Baseline
data were collected in 2005. Microbial larvicide was applied to all water bodies by hand application with water-dispersible
granular formulations and corn granules weekly from May to November in two areas in 2006 and in the other two areas
in 2007 in a cross-over design. The intervention was associated with a reduction in habitats with late stage anopheline
larvae and an 88% reduction in larval densities (P < 0.001). The effect of the intervention on mosquito densities was not
pronounced and was confounded by the distance of villages to the major breeding sites and year (P = 0.002). There was
no reduction in clinical malaria or anemia. Ground applications of non-residual larvicides with simple equipment are not
effective in riverine areas with extensive flooding, where many habitats are poorly demarcated, highly mobile, and inaccessible on foot.
is confined largely to the rainy season,18 which lasts from June
to October, we assumed this intervention would be effective
when targeted solely during this period. Before the large-scale
trial the commercial larvicide VectoBac® (Bacillus thuringiensis var. israelensis [Bti]; Valent BioSciences Corporation,
Libertyville, IL) was found to be highly effective at killing
mosquito larvae in rural Gambia.19 The application of larvicide was designed to be locally appropriate, and the microbial
larvicides were delivered by hand by field teams using simple
equipment.
This pilot trial was part of a series of studies to assess the
feasibility of LSM and its impact on malaria morbidity in different eco-epidemiological settings (rural towns,20 highland
valleys,13 desert fringe,14 urban areas16,21). The aim of this study
was to evaluate the impact of LSM using microbial larvicides
on vector populations and malaria incidence in the extensive
seasonally flooded areas of the lower reaches of the river in
rural Gambia.
INTRODUCTION
The realization that successful malaria control cannot rely
on a single tool and the need for evidence-based vector control
methods has prompted the World Health Organization to promote a global framework for integrated vector management.1,2
This framework is designed to rationalize vector control by
using evidence collected in the field to make decisions about
what combination of control measures to use to achieve maximum control of one or more vector-borne diseases. Control
measures such as the use of long-lasting insecticide-treated
nets (ITNs) and indoor residual spraying are effective tools
for malaria control and are currently the mainstay of vector
control in sub-Saharan Africa. However, the increasing resistance of malaria vectors to pyrethroids in Africa,3,4 the capacity of adult mosquitoes to avoid interventions,5 and the recent
call for malaria eradication6 has lead to a renewed interest in
the use of larval source management (LSM)7 for inclusion in
integrated vector management programs for malaria control
in Africa. Over the past five years, a series of small-scale studies have been undertaken to investigate the efficacy of LSM
in different biomes in sub-Saharan Africa. It was shown that
larval control works well where breeding sites are aggregated
in urban8,9 and rural areas.10–14
In contrast, we describe the results of an LSM trial with
microbial larvicides in a markedly different habitat in The
Gambia, an area of extensive wetland. The floodplain of lower
reaches of the Gambia River floods each rainy season to produce extensive areas of pooled sediment, which are ideal
breeding sites for mosquito larvae.15–17 Habitats most often
colonized are found in the first 1 km of the landward edges
of the floodplains in shallow water bodies. Because these sites
appeared to be readily accessible, we hypothesized that larviciding could decrease larval numbers sufficiently to reduce
malaria transmission. Moreover, because malaria transmission
METHODS
Study design. The study was carried out in four separate
areas (referred to as zones 1 to 4) two on the north banks
and two on the south banks of the Gambia River east of
Farafenni (universal transverse mercator zone 28 1500200mN,
435500mE; Figure 1). The area was flat open Sudan savannah
broadly consisting of farmlands, sparse woodland and the
extensive alluvial floodplains of the river. Villages were discrete
entities populated by mainly subsistence farmers predominantly
of the Wolof, Mandinka, and Fula ethnic groups. The four study
zones were approximately 12 × 8 km in area and divided into
three parallel 4 km-wide bands (subzones) perpendicular to
the river. Study villages were recruited from the central band
of each zone. We assumed that when larvicide was applied to
an entire study zone, the two 4-km bands, either side of the
central band, would be sufficiently wide to minimize mosquito
movement from untreated sites outside the study zone into the
central band, where the study villages were located.22,23
Baseline entomological data, but not clinical data, were collected during July–November 2005. In 2006 and 2007, entomological and clinical data collection started in May and ended in
* Address correspondence to Steven W. Lindsay, School of Biological and Biomedical Sciences, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, United Kingdom. E-mail:
s.w.lindsay@durham.ac.uk
† These authors have contributed equally to this work.
176
LARVICIDING FOR MALARIA CONTROL IN THE GAMBIA
Figure 1. Study area in The Gambia. Larval control was applied
in each zone over the area enclosed by the two broken lines furthest
from the center of each zone and the study villages are enclosed by the
two broken lines nearest the center of each zone.
November. A cross-over design was used for the application of
larvicide. From June to November 2006, larvicide was applied
to all accessible aquatic habitats in zones 1 and 3 at weekly
intervals and zones 2 and 4 served as controls. From May to
November 2007, larvicide intervention was applied to zones 2
and 4 and zones 1 and 3 served as controls.
Eligibility criteria for study participants. A census of residents, including children 6 months to 10 years of age, was
carried out in 50 study villages during the dry season in 2006. In
addition to demographic data, information was also collected
on malaria risk factors from all consenting inhabitants,
including use of bed nets and ITNs, the presence of open or
closed eaves in sleeping rooms, and ethnicity. A total of 14,112
inhabitants were enumerated and children were selected
from random lists with the total in each village proportional
to village size. Informed consent was obtained from parents
or caregivers, and children were enrolled to enable a study of
approximately 500 children in each zone (Figure 2). In 2007,
participants who had reached 10 years of age were replaced
177
by infants 6–18 months of age and those who left the study
were replaced by additional children from the same village.
All children were provided with a study photo identity card to
verify their study number and village at clinical consultations
and during cross-sectional surveys.
Intervention. Detailed descriptions of the application of
microbial larvicides are provided elsewhere.16,20,24 Briefly,
water-dispersible granular formulations (WDG) and corn
granules (CG) of the commercial strain of Bti (VectoBac® strain
AM65-52; Valent BioSciences Corporation) were applied
weekly from the end of the dry season in May until the end
of the rainy season in November to all water bodies within an
intervention zone that could be reached by applicators on foot.
The WDG formulations were applied as liquid with knapsack
compression sprayers (15-liter capacity diaphragm knapsack
sprayers, Solo 475; Solo Kleinmotoren GmbH, Sindelfingen,
Germany) at 0.2 kg/hectare in areas with low vegetation
coverage. The CG was applied by hand from buckets held with a
strap around the waist or neck or motorized knapsack granuleblowers (13-liter capacity motorized sprayers; MD 150DX13; Maruyama, Tokyo, Japan) at 5.0 kg/hectare when aquatic
habitats were covered by vegetation and difficult to access.
Field applicators were recruited from communities within
each zone to make use of their local knowledge of the environment. They were supervised by one field supervisor in each
zone and trained for one month before larviciding. Applicators
worked five days a week from 7:00 am to 1:00 pm to avoid
the hottest time of the day. Teams of 3–4 applicators walked
abreast 8 meters apart. Each applicator covered a 180° swath
in front of him as he walked and applied larvicide from the
edge of a water body to the end of it or until progress was
impossible because of deep water.
Objective and outcome measures. To assess the impact of
LSM we used clinical and entomological outcome measures.
The primary clinical outcome was the incidence of clinical
malaria in study children defined as a history of fever within
the last 48 hours or axillary temperature ≥ 37.5°C plus the
presence of Plasmodium falciparum identified microscopically.
The primary entomological outcomes were adult densities in
houses, a proxy measure for indoor human biting rates, and
mosquito larval abundance, which served to evaluate the
effectiveness of the larvicide.
Figure 2. Trial profile, The Gambia.
178
MAJAMBERE, PINDER AND OTHERS
Larval vector abundance. Larval surveys were carried
out continuously by the zone supervisor. In 2005, during the
baseline period, all aquatic habitats in each zone were visited
and the presence or absence of anopheline and culicine larvae
recorded as described elsewhere.24 Each habitat was visited
monthly.
During the intervention years (2006 and 2007) random larval spot checks were implemented throughout the season to
estimate the proportion of habitats containing early and late
instar larvae to determine the effectiveness of larvicide application. Of the total number of habitats identified in each zone
during baseline (n = 1,076), 40 habitats were randomly (computer-generated) selected every day for each zone respectively
by the program manager (S.M.) and the habitat identification number, including global positioning system coordinates,
forwarded to the field supervisor for habitat inspection as
described above. Selection of sites was stratified according to
subzone and the timetable of larvicide application to ensure
that inspection of sites took place 1–2 days after the habitat
was treated with larvicide and that an equal number of sites
were visited weekly in all three subzones in each zone. In
addition, 10 sentinel habitats per zone were randomly selected
after the first round of complete habitat surveys in 2005 and
larval densities measured weekly in these.
At each site visit, purposive dipping was used to sample larvae (10 dips per site), which were categorized as early (first
and second instars) stages and late (third and fourth) stages.
Late instar anopheline larvae and all pupae were stored in
98% ethanol and taken to the laboratory for species identification by polymerase chain reaction (PCR).25
Adult vector abundance. Adult vector surveys were implemented in 39 villages (10 in zone 1, 11 in zone 2, 9 in zone 3,
and 9 in zone 4) at two-week intervals from July through
November in 2005 and for the duration of larviciding in the
intervention years. Each zone had 15 traps divided between
the villages with 1–3 sentinel houses per village proportional
to village size. Within randomly selected compounds, all
houses with open eaves, a thatched roof, no ceiling, and where
a single man slept were numbered and one was selected
randomly. Mosquitoes were sampled using miniature CDC
light traps (Model 512; John W. Hock Company, Gainesville,
FL) positioned one meter above the floor at the foot end of
the bed where a man slept under an untreated bed net. Traps
were set at 7:00 pm and collected at 7:00 am the following
morning. If the occupant moved house, the trap was moved to
the nearest similar house in the same village. If the occupant
did not spend the night in the selected room or the trap was
faulty, the data were excluded from the analysis.
Mosquitoes were identified to the level of species by microscopy and the numbers of Anopheles gambiae s.l. females
recorded. The presence of sporozoites was identified using an
enzyme-linked immunosorbent assay.26 In 2005 and 2006, a 1%
random sample of the An. gambiae s.l. females, stratified by
zone and sampling period was typed to the species by PCR.27
Malaria in children. Cross-sectional surveys were implemented before (May–June) and after (November–December)
the main transmission season in 2006 and 2007. At each
survey, children were questioned and examined for malaria
signs and symptoms, including axillary temperature, recent
clinical history, drug ingestion, anemia, and splenomegaly.
In children with an axillary temperature ≥ 37.5°C or recent
history of fever, a rapid diagnostic test (RDT; ICT Malaria
Pf Cassette Test; ICT Diagnostics, Cape Town, South Africa)
was conducted in the field and treatment was given if it was
positive. Anemia was measured in all children at each survey
(Hemocue AB, Ängelholm, Sweden) and thick blood smears
collected for subsequent determination of parasitemia. At
surveys and consultations, clinical conditions were treated
according to standard Gambian treatment guidelines. A short
history of each child’s health and mosquito prevention and
control measures at their home were recorded at each survey.
Travel history was collected at the end of season survey.
From June to December each year, passive case detection
was used to monitor clinical cases of malaria. Parents or caregivers were encouraged to consult study nurses if a study child
became ill. One study nurse was stationed in each of two centrally located study villages in each zone (total = 8), and they
collaborated closely with government village health workers
(VHWs) to cover their zones. At any consultation, children
were identified by their study cards, signs and symptoms were
recorded, a blood sample was tested for parasites by the RDT,
and a thick blood smear made if fever was present. If parasites
were detected by RDT, children were referred to the VHW
for treatment immediately. The thick blood film was stained
and read immediately. If the result was positive, but the result
of the RDT was negative, children were treated the next day.
All conditions were treated according to current standard
Gambian treatment guidelines. Moderate anemia (Hb < 8 g/dL)
was treated with iron sulfate, and severe cases (Hb < 5 g/dL)
were transported to a health facility. Uncomplicated malaria
was treated with chloroquine and pyrimethamine/sulfadoxine.
Formal and on-the-job training was provided to all VHWs by
the study doctor and nurses according to the Gambian VHW
training guidelines.
Blinding. Reading of blood films and ELISA results was
blinded. Entomological data collection was not blinded to
the assignment of mosquito larval control interventions in the
study areas. However, field applicators were blinded to the sites
selected for larval surveys. Residents were aware of ongoing
interventions. Light-trap collections of adult mosquitoes were
identified and counted by technicians blinded to the identity
of the village.
Protection of human subjects. Institutional and ethical
clearance was granted by the National Institutes for Health,
the Gambian Government/Medical Research Council Laboratories Joint Ethics Committee, and the Ethics Advisory
Committee of Durham University. Verbal consent for the
study was obtained from local leaders and the community at
large before collecting entomological baseline data. Before
implementation of larval control operations, the community
was again briefed on the nature of the intervention. Informed
consent was obtained from house occupiers for setting and
collecting the adult mosquito traps. Interviews and malaria
parasite screening were only started after the purpose of
the study had been clearly explained to the participants and
parents or guardians of children and an informed consent
form was read in an appropriate language and signed by the
parents/guardian and a witness. Assent was also sought and
obtained from older children. Approval to use microbial
larvicides was granted by the National Environment Agency
of The Gambia.
Statistical analysis. All data were collected on forms, checked
for completeness, double entered into Access databases,
verified and checked for consistency. The incidence of clinical
179
0.8
75.1
19.7
0
0.8
73.9
19.4
0
0
13.0
37.7
41.4
0
12.8
37.0
40.6
27.1
0
68.5
0.4
* CI = confidence interval.
83.1
0.4
16.3
0
82.5
0.4
16.1
0
27.0
0
68.1
0.4
88.8 (86.1–91.5)
90.0 (87.4–92.6)
83.9 (80.7–87.1)
82.3 (79.0–85.6)
11.7 (8.9–14.4)
21.3 (17.7–25.0)
17.9 (14.6–21.3)
12.2 (9.3–15.1)
70.4 (66.3–74.6)
34.3 (30.2–38.5)
71.2 (67.0–75.4)
38.3 (34.1–42.5)
81.4 (77.9– 84.9)
37.2 (32.8–41.7)
27.6 (23.6–31.5)
6.1 (4.0–8.2)
2007
Yes
527
41.6
4.9
28.5 (24.4–32.6)
88.9 (86.1–91.8)
2006
No
510
42.2
5.2
39.2 (34.9–43.4)
78.9 (75.3–82.5)
2007
No
503
50.9
4.8
43.4 (38.8–48.0)
91.9 (89.3– 94.4)
Zone 3
2006
Yes
525
51.1
4.7
46.3 (42.0–50.6)
75.8 (72.1– 79.5)
2007
Yes
523
49.3
4.9
68.4 (64.1–72.6)
97.7 (96.3– 99.0)
Zone 2
2006
No
508
49.2
4.9
74.9 (71.1–78.7)
69.7 (65.6–73.8)
2007
No
492
52.6
5.1
46.9 (42.3–51.5)
43.4 (38.8–48.0)
2006
Yes
496
50.6
5.2
57.4 (53.0–61.8)
32.7 (28.5–36.8)
Zone 1
Characteristic
Participants, demographics, and follow-up. Approximately
500 children were surveyed in each zone at the start of the
transmission season in 2006 and 2007 (Figure 2). Most children
were surveyed again at the end of each season; 84.9% in 2006
and 90.8% in 2007. Of those not surveyed most had traveled
for a religious holiday for a few weeks. Most participants were
enrolled in the study both years (Figure 2).
The age and sex of children were similar in each zone and
between intervention arms, although there were slightly more
boys in zone 4 (Table 1). There were 16% more clinical consultations in 2007 than in 2006 (787 compared with 678), which
indicated that the lower incidence of malaria in 2007 (Table 2)
was not caused by fewer consultations that year. At consultation, 99% (1,458 of 1,466) of the patients had a recorded temperature of ≥ 37.5°C or reported history of fever, 34% (488
of 1,458) of these patients were slide positive, and 72% (352
of 488) had a fever ≥ 37.5°C. Slides results were available for
> 98% of clinical and survey visits; 2% (85 of 4,443) missing in
2006 and 1% (44 of 4,828) in 2007.
Malaria risk factors. Data on risk factors were available for
> 94% of children for both years of the study. Most risk factors
varied in a similar manner between the zones each year;
these included the distance of homes from the floodplain, the
Year
Larviciding
No. children
Females (%)
Age, years
% Houses with open eaves (95% CI)
% Children using bed nets (95% CI)
% Children using treated bed nets
(95% CI)
% Villages < 1 km from floodplain
(95% CI)
Ethnicity
% Wollof
% Mandinka
% Fula
% Serrehule
RESULTS
Table 1
Characteristics of children enrolled by study zones, The Gambia*
malaria was calculated from the number of study children
who consulted with malaria/100 child-years of exposure. The
time of exposure for each child was the duration of passive
surveillance corrected for absences of over one week and
by subtracting 28 days if a child received anti-malaria drugs.
Time of exposure was censored at the first attack in children
with clinical malaria. The potential effect of mosquito larval
control was examined by calculating the incidence of clinical
malaria, prevalence of parasitemia and splenomegaly, and
mean ± SD Hb levels (g/dL) for each survey, zone, and year.
Hemoglobin levels in intervention and control groups each
year were compared at the end of each season by using a t-test.
The incidence of malaria allowing for time of exposure was
analyzed as a cross-over study, with a multilevel generalized
linear model (GLLAMM, Stata version 9.1; Stata Corporation,
College Station, TX). The clustering effects of village and
subject were included as random effects (the intra cluster
correlation for zone was minimal once subject and village
were included in the model).
The impact of larviciding on the presence of late stage larvae in water bodies was analyzed as a cross-over study by using
GLLAMM with clustering by water body and zone included
as random effects. Odd ratios were adjusted for the year of
intervention. The density of female anopheline in traps was
highly over-dispersed, and we used a generalized estimating
equation with a negative binomial distribution to examine the
effect of larviciding on this. Comparisons were adjusted for
month, baseline densities in 2005, distance of villages to the
edge of alluvial floodplains, and clustering by trap and village.
Binary logistic regression was used to examine the presence
and absence of An. gambiae with sporozoites by zone over the
two intervention years. Seasonal entomological inoculation
rates were calculated by multiplying the mean density of mosquitoes collected in light traps from July to November in each
zone by the proportion positive for P. falciparum sporozoites
and by 153, the number of days in the season.
Zone 4
LARVICIDING FOR MALARIA CONTROL IN THE GAMBIA
180
MAJAMBERE, PINDER AND OTHERS
Table 2
Impact of the intervention on malarial indices in children 6 months to 10 years of age, The Gambia*
Zone
Variable
1
2
Incidence of malaria cases/100 child-years (95% CI)
2006
70.9 (58.8–85.6)
30.3 (23.1–39.7)
2007
7.2 (4.3–11.9)
17.0 (12.4−23.5)
Prevalence of Plasmodium falciparum infection (no. parasitemic/total)
Start 2006
38.4% (158/411)
16.8% (85/505)
End 2006
41.0% (163/398)
12.2% (54/443)
Start 2007
17.0% (82/482)
3.3% (17/514)
End 2007
20.7% (95/458)
8.2% (39/474)
Mean hemogloblin level, g/dL (SD)
Start 2006
10.4 (1.7)
10.4 (1.9)
End 2006
10.2 (1.8)
10.5 (1.9)
Start 2007
10.4 (1.6)
10.2 (1.7)
End 2007
10.4 (1.6)
10.6 (1.9)
Prevalence of splenomegaly (Hacket’s score > 0)
Start 2006
11.6% (57/493)
3.9% (20/507)
End 2006
12.0% (47/393)
5.9% (26/442)
Start 2007
4.5% (22/491)
1.9% (10/524)
End 2007
7.7% (35/456)
2.6% (12/471)
Prevalence of gametocytemia
Start 2006
4.2% (17/411)
1.4% (7/505)
End 2006
5.5% (22/398)
2.5% (11/443)
Start 2007
3.1% (15/482)
0.8% (4/514)
End 2007
6.3% (29/458)
4.2% (20/474)
3
44.1 (35.2–55.2)
27.2 (20.9–35.4)
16.0% (84/524)
12.8% (57/447)
1.0% (5/502)
10.4% (47/452)
10.8 (1.7)
10.7 (1.7)
10.5 (1.5)
10.4 (1.6)
4
29.1 (22.1–38.4)
24.7 (18.8–32.3)
9.5% (48/508)
10.5% (45/430)
2.3% (12/513)
22.3% (105/472)
10. 7 (1.6)
10.7 (1.7)
10.4 (1.7)
10.0 (1.6)
All zones
42.9 (38.2–48.1)
19.0 (16.3–22.2)
19.3% (375/1,948)
18.6% (319/1,718)
5.7% (116/2,011)
15.4% (286/1,856)
10.6 (1.8)
10.5 (1. 8)
10.3 (1.7)
10.3 (1.7)
4.2% (22/524)
6.5% (29/447)
2.2% (11/503)
2.6% (12/455)
3.9% (20/510)
5.8% (25/434)
1.1% (6/527)
3.8% (18/471)
5.9% (119/2,034)
7.4% (127/1,716)
2.4% (49/2,045)
4.2% (77/1,853)
1.9% (10/524)
3.1% (14/447)
0.2% (1/502)
2.7% (12/452)
0.4% (2/508)
4.7% (20/430)
0.8% (4/513)
6.4% (30/472)
1.9% (36/1948)
3.9% (67/1718)
1.2% (24/2011)
4.9% (91/1856)
* CI = confidence interval. Incidence of malaria was estimated by passive case detection during the main malaria transmission seasons. Prevalence of P. falciparum infection and anemia was estimated at the start and end of each transmission season. Start or end denote surveys done in June or November/December, i.e., at the start and end of the malaria transmission season each year.
percentage of subjects living in houses with closed eaves, bed
net use, and ethnicity (Table 1). Comparison of risk factors by
intervention for each year shows that only bed net and ITN use
varied with the intervention in both years; ITN use increased
from a range of 6.1–38.3% in 2006 to 37.2–81.4% in 2007.
Larvicide application. Sixty-four men applied 4,933 kg of
Bti WDG and 2,712 kg of Bti CG to zones 1 and 3 in 2006, and
6,705 kg of Bti WDG and 7,553 kg of Bti CG to zones 2 and 4
in 2007.
Mosquito abundance. Each year there were five months
of rain from June to October, with peaks in rainfall pattern
between July and September (Figure 3). Total annual rainfall
decreased slightly each year from 858.3 mm in 2005 to 807.9 mm
in 2006 and 751.4 mm in 2007. The proportion of sampled
habitats colonized with late instar Anopheles larvae at baseline
was 31% (439 of 1,408). A similar proportion was found in
2006 in the untreated zones (40%, 515 of 1,288), whereas only
1% (12 of 1,380) of sites were colonized with Anopheles in
zones where larviciding took place. However, in 2007 only
Figure 3. Rainfall during the study period, The Gambia.
12% of sites (165 of 1,389) were colonized in untreated zones
compared with 4% (55 of 1,439) in zones where larviciding
took place. The overall crude relative risk of sites being
colonized in the presence of larviciding was thus 0.12. Taking
into account clustering by zone and site, we observed that
the intervention significantly reduced the likelihood of water
bodies being colonized in both years, but was more effective
in 2006 (odds ratio [OR] = 0.01, 95% confidence interval [CI] =
0.01–0.02, P < 0.001) than 2007 (OR = 0.27, 95% CI = 0.18–
0.41; P < 0.001). Similar results were found in the sentinel sites
where the proportion of sites with late anopheline larvae in
the absence of larviciding was 51% (415 of 817) in 2005, 48%
(198 of 410) in 2006, and 22% (107 of 487) in 2007. In the
presence of larviciding, the proportion was 2% (9 of 489) in
2006 and 9% (42 of 488) in 2007. In these sentinel sites, when
we took clustering by zone and site into account, similar
reductions in colonization by larvae were found in 2006 (OR
< 0.01, 95% CI = 0.003–0.020, P < 0.001) and 2007 (OR = 0.44,
95% CI = 0.22–0.83, P < 0.01). The mean density of anopheline
larvae per dip per sentinel habitat was significantly reduced in
all zones during larviciding (P < 0.001; Figure 4).
On each sampling round, 60 CDC light traps were set in the
selected 39 villages and traps were sampled on 97.5% (2,053
of 2,100) occasions during the study. A subsample of 626 An.
gambiae s.l. females caught in houses in 2005 and 2006 was identified to species level by PCR; 13% (n = 82) of these samples
did not amplify and the rest consisted of 54% An. gambiae s.s.,
27% An. melas, and 19% An. arabiensis. There was a clear seasonal trend in adult vector densities with peaks in August or
September (Figure 4). The density of adult vectors collected
varied between zones with the lowest levels in zone 1 and highest in zone 3, except in the year of the intervention (Figure 4
and Table 3). Adult vector densities were higher in the baseline year than subsequent years, even in the absence of the
LARVICIDING FOR MALARIA CONTROL IN THE GAMBIA
181
Figure 4. Seasonal abundance of larval and adult Anopheles gambiae during the study, The Gambia. Gray bars represent periods of larvicide
application.
intervention (median = 10, interquartile range [IQR] = 2–32
compared with median = 7, IQR = 1–25, P = 0.011). Adult vector density was lower in zones 1 and 3 during larviciding but
there was little change in zones 2 and 4 (Figure 4 and Table 3).
Vector densities varied not only with larviciding and year, but
also with distance of villages from the floodplains (Figure 5).
In villages further from the floodplain, larviciding was associated with a reduction in vector densities in 2006, but not in
2007. Conversely, in villages closest to the river, larviciding was
associated with reduced vector density in 2007, but not 2006.
This three-way interaction between the intervention, time, and
location relative to the major breeding sites was highly significant when modeled (P = 0.002), making an estimation of the
effect of larviciding alone on vector density an unreasonable
simplification.
Sporozoite rates (Table 3) were generally lower during larviciding. However, binary logistic regression, when adjusted
for year and zone, showed no significant association between
larviciding and the presence of infective mosquitoes (OR =
0.65, 95% CI = 0.36–1.20, P = 0.17). Seasonal entomological
inoculation rate varied from below the level of detection to
19.5 (Table 3).
Plasmodium spp. infection. There was considerably more
malaria in 2006 than 2007 (Table 2). Incidence of clinical
episodes of malaria in 2006 was twice that in 2007. This
finding may have resulted from higher rainfall in 2006, and
182
MAJAMBERE, PINDER AND OTHERS
Table 3
Mean female Anopheles gambiae s.l. density and the sporozoite rate per zone, June–November, The Gambia*
Zone
Parameter
1
2
Median female An. gambiae s.l./trap/night (IQR)
2005
3 (0–7)
19 (4–44)
2006
1 (0–3)
13 (6–26)
2007
2 (0–5)
13 (4–26)
Sporozoite rate, no. with Plasmodium falciparum sporozoites/total
2005
1.09% (13/1,191)
0.19% (16/8,332)
2006
0% (0/469)
0% (0/4,105)
2007
0.37% (2/546)
0.08% (3/3,493)
Overall
0.68% (15/2,206)
0.12% (19/15,930)
Seasonal EIR
2005
8.80
8.29
2006
0
0
2007
2.24
2.32
3
4
24 (6–78)
12 (4–31)
34 (10–69)
11 (3–26)
3 (1–11)
9 (2–26)
0.11% (16/15,136)
0.08% (7/9,315)
0.16% (25/15,796)
0.12% (48/40,247)
0.23% (7/3,008)
0.24% (4/1,633)
0.14% (6/4,154)
0.19% (17/8,795)
16.55
5.82
17.00
6.13
3.13
3.91
* IQR = interquartile range; EIR = entomological inoculation rate.
particularly the heavy rains early in the season, compared with
2007 or changes in ITN coverage (Figure 3 and Table 1). There
was also a large variation between zones, with zone 1 having
approximately twice the malaria incidence found in the other
zones in 2006. This finding may have been a consistent pattern
because the prevalence of parasitemia in zone 1, at the start of
2006, which reflected the intensity of malaria transmission in
2005, was also double that of its neighbors. These differences
in incidence were also reflected by the differences in the rates
of enlarged spleens, with rates in zone 1 approximately twice
those in the other three zones and lower rates in 2007 than
2006 (Table 2). Gametocyte rates were higher in zone 1 in the
dry season surveys, and as expected these rates increased in the
wet season survey but only slight differences were apparent
between the zones in the wet season surveys, and there was
no apparent association with the intervention (Table 2). Mean
Hb levels remained fairly constant throughout the study and
between zones (Table 2). Overall the malaria indices seen in
each zone appear largely unaffected by the intervention.
Analysis of the effect of larviciding on malaria incidence,
taking into account the cross-over trial design and adjusting
for individual time of exposure and clustering by subject and
Figure 5. Impact of larval control on female Anopheles gambiae
s.l. densities stratified by intervention year and distance of the villages
to the nearest riverine floodplain, The Gambia.
village, indicated an increase in malaria incidence associated
with larviciding in 2006, but not 2007 (2006: OR = 2.89, 95%
CI = 1.79–4.68, P < 0.001; 2007: OR = 1.25, 95% CI = 0.74–2.09,
P = 0.404). Sex, age, bed net use, ITN use, sleeping in a room
with open eaves, distance of villages from the floodplains, and
ethnicity did not significantly impact on the OR for a model
including year. Year of study was the only variable with a significant effect (P = 0.031). There was also no significant impact
on anemia (P > 0.05; Table 3).
DISCUSSION
Larval control with microbial larvicides is effective in areas
with relatively few well-defined habitats13,14,20,21 but there has
been no detailed evaluation of this method in areas with
extensive habitats. This is the first large-scale assessment of
the impact of larviciding in a rural ecosystem that is dominated by large areas of flooding.
Although our data indicate that larviciding with microbials
reduced the proportion of water bodies containing Anopheles
larvae and the mean densities of late stage larvae per habitat
by an order of magnitude, the reduction in exposure to transmission was unsatisfactory and did not lead to any reduction
in clinical malaria, parasite prevalence, or anemia. The impact
of larviciding on adult vectors in this study was limited. This
finding is in marked contrast to studies in urban Tanzania and
lowland and highland Kenya where LSM reduced exposure
to transmission by 65–93%13,20,21 and was associated with a
68–72% reduction in new parasite infections.13,21
The study does not show any consistent change in malaria
associated with larval control. This result is heavily influenced
by the exceptionally variable level of malaria found in zone 1;
being extremely high during the intervention and extremely
low in the non-intervention year. The reasons for this variability are not understood because there is relatively little change
in net use in this zone during the study. This decrease may
reflect a trend in this area because similarly decreasing levels
were found in the community to the east of zone 128 and also in
selected hospitals in The Gambia.29 This finding illustrates the
general study design issue of finding large clusters that have
similar malaria ecologies.
Nonetheless we have demonstrated that larviciding reduced
the aquatic stages of anophelines, but only limited success in
reducing adult numbers in three of the four study zones. The
LARVICIDING FOR MALARIA CONTROL IN THE GAMBIA
question arises: why were we not able to achieve a significant reduction in transmission? There are a number of possible explanations. First, it is possible that mosquitoes might
have invaded the intervention zones from surrounding areas
outside the untreated areas. Earlier studies in The Gambia
indicate that although most An. gambiae s.l. fly no more than
2 km, a small proportion may fly much further.23 This longdistance flight may be a consequence of the local ecology of the
study area, which is flat, where persons live in small and discrete communities and where breeding sites are often far from
the villages. Thus, vectors flying from the abundant breeding
sites in the floodplain find it difficult to locate a human blood
meal, particularly because those persons living closer to the
floodplains are more likely to sleep under bed nets.30 Second,
not all water bodies in the alluvial floodplains were treated
with larvicide because deep water, especially during times of
high tides, made it impossible to reach some parts of the wetlands close to the river. Based on earlier published work,31 we
assumed that this might not affect the intervention because
these sites were more than 4 km from the study villages and
we expected most adult mosquitoes to emerge from the landward edge of the floodplain. Nevertheless, more recent work
has shown that low densities of larvae can be found over the
entire floodplain area, even close to the river.24,32 Away from
the landward edge, raised areas within the large flooded areas
can create edges suitable for colonization by mosquitoes compromising the success of targeted interventions such as the
current one. Third, in our study area the flooded areas close
to the Gambia River are subject to daily tidal movements33
that might disperse mosquito larvae away from sites regularly
treated with larvicide and dilute the larvicides in areas treated
at low tide. Furthermore, mosquito eggs can survive on damp
soil for several days34 and once these sites are flooded with
water, the eggs hatch and larvae develop successfully to adults.
A similar situation might occur in The Gambia when eggs laid
at low tide on damp soil remain viable and hatch at high tide
when these sites are flooded. In such cases, a successful intervention would either require larviciding at shorter intervals
or the application of a more residual larvicide, which remains
viable even if it falls dry periodically, such as the insect growth
regulator pyriproxyfen.35 Fourth, even though considerable
effort was made to supervise the application of larvicides, we
cannot exclude the possibility that field applicators may have
missed aquatic habitats.
In this study, a low technology approach was used to apply
the larvicide because it was considered most appropriate
for community involvement in resource-poor countries in
Africa.20,24 However, in areas with extensive flooding, such as
river floodplains and major areas of irrigated rice, significant
impact might only be achieved with aerial application because
large areas can be treated rapidly at full coverage. Clearly,
it is imperative that appropriate methods are developed for
the large-scale application of larvicides in areas of extensive
flooding in Africa.
Notably, LSM can improve as the field teams gain more
experience, as has been demonstrated for the Urban Malaria
Control Program in Dar es Salaam, Tanzania.16 In a large and
difficult study area such as ours with locally recruited field
teams that have had relatively little experience with LSM, a
strategy implementing the intervention in the same zones over
consecutive years might have been more successful. The same
applies if the study would have been implemented 200 km
183
upriver where water bodies are less influenced directly by the
river, are no longer tidal and often much smaller.
The recent successes that have been achieved with LSM in
rural and urban settings in east Africa13,21 are clearly associated
with differences in the transmission setting and habitat characteristics compared with the extensive floodplains of the Gambia
River. Significant control of vectors can be achieved by hand
application of larvicide where malaria transmission is focal and
when water bodies are defined and accessible, where the water
is stagnant and the flight range of the adult vector is not more
than several kilometers. In such settings LSM has a significant
added benefit to personal protection measures such as ITNs.36
This trial was part of a series of studies to assess the impact
of LSM on malaria morbidity in different eco-epidemiological
settings. Despite a major effort, we were not able to reduce
malaria in this ecosystem dominated by riverine floodplains.
Because LSM using simple, low-cost technology is not an intervention that works everywhere, careful consideration needs to
be given to the habitat characteristics responsible for the proliferation of malaria vectors. Ground-based manual application
of larvicides is not an appropriate tool for areas with extensive flooding (e.g., large floodplains or large-scale rice cultivation where habitats are not defined and largely inaccessible on
foot and/or where the water is tidal). It is therefore crucial to
develop a decision support system for national malaria control
programs to guide where and when it is appropriate to consider
LSM in an integrated vector management approach.
Received July 1, 2009. Accepted for publication September 9, 2009.
Acknowledgments: We thank the study children, their parents, village
health workers and the Gambian Department of State for Health for
their continuous support for this work. We are grateful to the National
Malaria Control Program of The Gambia for their active collaboration with the larviciding operations. We acknowledge Paul Emerson,
Amy Ratcliffe, and Miguel A. San Joaquin Polo for their help at the
early stages of the study. We also thank the Medical Research Council
and Durham University associates of the LCP trial and other support
staff, in particular Sabine Schindler and Momodou Jasseh.
Financial support: This study was supported by the National Institutes
of Health (grant 1 UO1 AI058250-01).
Authors’ addresses: Silas Majambere, Margaret Pinder, Clare Green,
Robert Hutchinson, and Steve Lindsay, School of Biological and
Biomedical Sciences, Science Laboratories, South Road, Durham DH1
3LE, UK, E-mails: smajambere@ihi.or.tz, mpinder@mrc.gm, clare
.green@ucl.ac.uk, R.A.Hutchinson@insects.org, and Steve.Lindsay@
lshtm.ac.uk. David Ameh, David Conway, and David Jeffries, Medical
Research Councils Laboratories, Fajara, The Gambia, E-mails: dameh@
mrc.gm, dconway@mrc.gm, and djeffries@mrc.gm. Ulrike Fillinger
and Paul Milligan, London School of Hygiene and Tropical Medicine,
Keppel Street, London WC1E 7HT, UK, E-mails: ulrike.fillinger@
lshtm.ac.uk and paul.milligan@lshtm.ac.uk.
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