Identifying residual foci of Plasmodium falciparum infections for malaria elimination: the urban context of Khartoum, Sudan

Abstract

Background: Identifying the location and size of residual foci of infections is critical where malaria elimination is the primary goal. Here the spatial heterogeneity of Plasmodium falciparum infections within the urban extent of Khartoum state in Sudan is investigated using data from cross-sectional surveys undertaken from 1999 to 2008 to inform the Khartoum Malaria Free Initiative (KMFI).

Methods: From 1999-2008 the KMFI undertook cross-sectional surveys of 256 clusters across 203 random samples of residential blocks in the urban Khartoum state in September of each year. Within sampled blocks, at least five persons, including at least one child under the age of five years, were selected from each household. Blood smears were collected from the sampled individuals to examine the presence of P. falciparum parasites. Residential blocks were mapped. Data were analysed for spatial clustering using the Bernoulli model and the significance of clusters were tested using the Kulldorff scan statistic.

Results: A total of 128,510 malaria slide examinations were undertaken during the study period. In 1999, overall prevalence was 2.5%, rising to 3.2% in 2000 and consistently staying below 1% in subsequent years. From 2006, over 90% of all surveyed clusters reported no infections. Spatial clustering of infections was present in each year but not statistically significant in the years 2001, 2002, 2004 and 2008. Spatial clusters of high infection were often located at the junction of the Blue and White Niles.

Conclusion: Persisting foci of malaria infection in Khartoum are likely to distort wide area assessments and disproportionately affect future transmission within the city limits. Improved investments in surveillance that combines both passive and active case detection linked to a geographic information system and a more detailed analysis of the location and stability of foci should be undertaken to facilitate and track malaria elimination in the state of Khartoum.

Figure 1. Map of Khartoum state showing the distribution of survey locations.

A total of 256 P. falciparum prevalence surveys were undertaken in 203 locations (some survey locations were repeated in subsequent years) from 1999–2008. Inset is the state map of the Sudan showing the location of Khartoum state.

Figure 2. Graph of P. falciparum prevalence (solid line) and the total annual rainfall (bars) in Khartoum state by year of survey from 1999–2008.

Figure 3. Location of primary spatial clusters of P. falciparum prevalence.

Primary spatial clusters in Khartoum state in each year from 1999 to 2008 are shown as pink circles of varying radius or red zeros (when a cluster radius is indeterminate). The primary spatial clusters in 2001, 2002, 2004 and 2008 were all statistically not significant (Kulldorff scan statistic of >0.05). Years with statistically significant spatial clusters are shaded grey.