Continued decline of malaria in The Gambia with implications for elimination

Abstract

Background: A substantial decline in malaria was reported to have occurred over several years until 2007 in the western part of The Gambia, encouraging consideration of future elimination in this previously highly endemic region. Scale up of interventions has since increased with support from the Global Fund and other donors.

Methodology/principal findings: We continued to examine laboratory records at four health facilities previously studied and investigated six additional facilities for a 7 year period, adding data from 243,707 slide examinations, to determine trends throughout the country until the end of 2009. We actively detected infections in a community cohort of 800 children living in rural villages throughout the 2008 malaria season, and assayed serological changes in another rural population between 2006 and 2009. Proportions of malaria positive slides declined significantly at all of the 10 health facilities between 2003 (annual mean across all sites, 38.7%) and 2009 (annual mean, 7.9%). Statistical modelling of trends confirmed significant seasonality and decline over time at each facility. Slide positivity was lowest in 2009 at all sites, except two where lowest levels were observed in 2006. Mapping households of cases presenting at the latter sites in 2007-2009 indicated that these were not restricted to a few residual foci. Only 2.8% (22/800) of a rural cohort of children had a malaria episode in the 2008 season, and there was substantial serological decline between 2006 and 2009 in a separate rural area.

Conclusions: Malaria has continued to decline in The Gambia, as indicated by a downward trend in slide positivity at health facilities, and unprecedented low incidence and seroprevalence in community surveys. We recommend intensification of control interventions for several years to further reduce incidence, prior to considering an elimination programme.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Location of health facilities and populations surveyed, rainfall patterns and Global Fund support for malaria control from 2003 to 2009 in The Gambia.

(a) Sites of 10 health facilities at which laboratory slide data were analysed (circular symbols indicate 4 sites previously studied until the end of 2007, square symbols indicate 6 sites not previously studied); sites of the Farafenni village cluster and Brefet village are marked with triangular symbols. (b) Monthly rainfall for two sites at opposite ends of the country (Banjul at the coast in the west and Basse in the east) over the 7 year period. (c) Changes in first-line antimalarial therapy and Global Fund support for malaria control over the period. First-line therapy: CQ, Chloroquine; SP, Sulphadoxine-Pyrimethamine; AL, Artemether-Lumefantrine. The Global Fund has supported Insecticide treated bednet (ITN) provision for children 5 years of age and pregnant women, and intermittent preventative treatment with SP during pregnancy (IPTp) under Round 3 from 2004 onwards (Western Region) and Round 6 from 2007 onwards (countrywide), as well as the purchase of AL (Coartem) to allow free treatment with an effective artemisinin-based combination therapy from 2008 onwards. Coverage of interventions countrywide by the midpoint year (2006) reached 50% for ITN use by children under 5 years of age and 30% for receipt of at least 2 doses of IPTp .

Figure 2. Malaria trends during 2003–2009 at 6 newly-surveyed health facilities in The Gambia.

Monthly numbers (left panel) and annual proportions (right panel) of malaria positive slides from January 2003 to December 2009 at (a) Fajikunda Health Centre (b) Serekunda Health Centre, (c) Soma Health Centre, (d) Basse Hospital, (e) Bansang Hospital, and (f) Bwiam Hospital.

Figure 3. Malaria trends during 2003–2009 at 4 health facilities in The Gambia that had been previously surveyed until 2007.

Monthly numbers (left panel) and annual proportions (right panel) of malaria positive slides from January 2003 to December 2009 at (a) Fajara MRC outpatient clinic, (b) Brikama Health Centre, (c) Farafenni AFPRC Hospital, and (d) Keneba MRC clinic.

Figure 4. Spatial distribution of cases at two health facilities in the coastal area.

Map of household locations of 285 malaria cases sampled from those presenting to the Fajara MRC clinic and Brikama Health Centre in three conseculive malaria seasons 2007 to 2009. Cases presenting to Fajara are represented as circular symbols, and those presenting to Brikama are represented with square symbols.The locations of these two health facilities are shown (green crosses), along with the most densely populated areas (shaded in yellow).

Figure 5. Serological surveys of malaria in Brefet village at the end of annual malaria seasons, December 2006 (n = 211) and December 2009 (n = 98).

Proportions with detectable serum IgG to the MSP-119 antigen are plotted by age group (numbers in 2006 and 2009 respectively sampled from each age group: 0–4 years, n = 35 and 17; 5–9 years, n = 31 and 19; 10–14 years, n = 31 and 17; 15–24 years, n = 19 and 13; 25–39 years, n = 46 and 17; >39 years, 49 and 15).