Key Characteristics of Residual Malaria Transmission in Two Districts in South-Eastern Tanzania—Implications for Improved Control

Abstract

After 2 decades of using insecticide-treated nets (ITNs) and improved case management, malaria burden in the historically-holoendemic Kilombero valley in Tanzania has significantly declined. We review key characteristics of the residual transmission and recommend options for improvement. Transmission has declined by >10-fold since 2000 but remains heterogeneous over small distances. Following the crash of Anopheles gambiae, which coincided with ITN scale-up around 2005-2012, Anopheles funestus now dominates malaria transmission. While most infections still occur indoors, substantial biting happens outdoors and before bed-time. There is widespread resistance to pyrethroids and carbamates; An. funestus being particularly strongly-resistant. In short and medium-term, these challenges could be addressed using high-quality indoor residual spraying with nonpyrethroids, or ITNs incorporating synergists. Supplementary tools, eg, spatial-repellents may expand protection outdoors. However, sustainable control requires resilience-building approaches, particularly improved housing and larval-source management to suppress mosquitoes, stronger health systems guaranteeing case-detection and treatment, greater community-engagement and expanded health education.

Persistently high proportions of Plasmodium-infected Anopheles funestus mosquitoes in two villages in the Kilombero valley, south-eastern Tanzania

“…arabiensis [6,21,39]. malaria transmission has declined significantly in the Kilombero valley since 2000, mostly due to the wide coverage of ITNs and effective case management [20,22].…”

“…Since then, several other studies have confirmed the dominance of An. funestus in this area [20]. In addition, reports of higher entomological inoculation rates by An.…”

“…In rural south-eastern Tanzanian villages, where ITNs have been widely used for more than 20 years, malaria transmission intensities have greatly reduced, in some cases from highs of >300 infectious bites per person per year (ib/p/y) in early 2000s to new averages below 30 ib/p/y [20]. These gains have been mostly attributed to widespread use of ITNs and effective case management [20,21].…”

“…In rural south-eastern Tanzanian villages, where ITNs have been widely used for more than 20 years, malaria transmission intensities have greatly reduced, in some cases from highs of >300 infectious bites per person per year (ib/p/y) in early 2000s to new averages below 30 ib/p/y [20]. These gains have been mostly attributed to widespread use of ITNs and effective case management [20,21]. Today, malaria transmission remains highly varied over small distances [20,22], and recent parasite surveys have demonstrated ranges of <1% prevalence in urban centers of Ifakara town (<150m above sea level) to highs of >40% in some higher altitude villages (∼300m above sea level) just 30km away (Swai et al ., unpublished).…”

“…These gains have been mostly attributed to widespread use of ITNs and effective case management [20,21]. Today, malaria transmission remains highly varied over small distances [20,22], and recent parasite surveys have demonstrated ranges of <1% prevalence in urban centers of Ifakara town (<150m above sea level) to highs of >40% in some higher altitude villages (∼300m above sea level) just 30km away (Swai et al ., unpublished).…”