Is the current decline in malaria burden in sub-Saharan Africa due to a decrease in vector population?

Abstract

Background: In sub-Saharan Africa (SSA), malaria caused by Plasmodium falciparum has historically been a major contributor to morbidity and mortality. Recent reports indicate a pronounced decline in infection and disease rates which are commonly ascribed to large-scale bed net programmes and improved case management. However, the decline has also occurred in areas with limited or no intervention. The present study assessed temporal changes in Anopheline populations in two highly malaria-endemic communities of NE Tanzania during the period 1998-2009.

Methods: Between 1998 and 2001 (1st period) and between 2003 and 2009 (2nd period), mosquitoes were collected weekly in 50 households using CDC light traps. Data on rainfall were obtained from the nearby climate station and were used to analyze the association between monthly rainfall and malaria mosquito populations.

Results: The average number of Anopheles gambiae and Anopheles funestus per trap decreased by 76.8% and 55.3%, respectively over the 1st period, and by 99.7% and 99.8% over the 2nd period. During the last year of sampling (2009), the use of 2368 traps produced a total of only 14 Anopheline mosquitoes. With the exception of the decline in An. gambiae during the 1st period, the results did not reveal any statistical association between mean trend in monthly rainfall and declining malaria vector populations.

Conclusion: A longitudinal decline in the density of malaria mosquito vectors was seen during both study periods despite the absence of organized vector control. Part of the decline could be associated with changes in the pattern of monthly rainfall, but other factors may also contribute to the dramatic downward trend. A similar decline in malaria vector densities could contribute to the decrease in levels of malaria infection reported from many parts of SSA.

Figure 1

The prevalence of in-door caught Anopheles gambiae and An. funestus and monthly rainfall pattern in Masaika village, 1998-2001 (a) and Kirare, 2004-2009 (b), Tanga region, Tanzania. The prevalence is shown as mean number of An. gambiae or An. funestus caught per CDC light trap per month. Note that this parameter has different scale on the y-axis in the two sub-figures. The monthly rainfall has been recorded at the Maji Depot Tanga Rainfall station (Ministry of Water and Irrigation, Tanzania) and is shown as a gray area plot.

Figure 2

Cumulative periodogram for Masaika (a) and Kirare (b) monthly rainfall series data (solid lines). Dashed lines indicate the upper and lower limits for the null hypothesis that the respective series is purely noise.