Dramatic decreases of malaria transmission intensities in Ifakara, south-eastern Tanzania since early 2000s

Abstract

BackgroundOngoing epidemiological transitions across Africa are particularly evident in fast-growing towns, such as Ifakara in the Kilombero valley, south-eastern Tanzania. This town and its environs (population ~ 70,000) historically experienced moderate to high malaria transmission, mediated mostly by Anopheles gambiae and Anopheles funestus. In early 2000s, malaria transmission [Plasmodium falciparum entomological inoculation rate (PfEIR)] was estimated at ~ 30 infectious bites/person/year (ib/p/yr). This study assessed the PfEIR after 15 years, during which there had been rapid urbanization and expanded use of insecticide-treated nets (ITNs).MethodsRandomly-selected 110 households were sampled across Ifakara town and four adjacent wards. Mosquitoes were trapped nightly or monthly (June.2015–May.2016) using CDC-light-traps indoors, Suna® traps outdoors and human landing catches (HLC) indoors and outdoors. All Anopheles mosquitoes were morphologically identified and analysed by ELISA for Plasmodium circumsporozoite proteins. Mosquito blood meals were identified using ELISA, and sub-samples of An. gambiae and An. funestus examined by PCR to distinguish morphologically-similar siblings. Insecticide resistance was assessed using WHO-susceptibility assays, and some Anopheles were dissected to examine ovariole tracheoles for parity.ResultsAfter 3572 trap-nights, one Plasmodium-infected Anopheles was found (an An. funestus caught outdoors in Katindiuka-ward by HLC), resulting in overall PfEIR of 0.102 ib/p/yr. Nearly 80% of malaria vectors were from Katindiuka and Mlabani wards. Anopheles gambiae densities were higher outdoors (64%) than indoors (36%), but no such difference was observed for An. funestus. All An. funestus and 75% of An. gambiae dissected were parous. Anopheles gambiae complex consisted entirely of Anopheles arabiensis, while An. funestus included 84.2% An. funestus s.s., 4.5% Anopheles rivulorum, 1.4% Anopheles leesoni and 9.9% with unamplified-DNA. Anopheles gambiae were susceptible to bendiocarb and malathion, but resistant to pyrethroids, DDT and pirimiphos-methyl. Most houses had brick walls and/or iron roofs (> 90%), and 52% had screened windows.ConclusionMalaria transmission in Ifakara has decreased by > 99% since early-2000s, reaching levels nearly undetectable with current entomological methods. These declines are likely associated with ITNs use, urbanization and improved housing. Remaining risk is now mostly in peri-urban wards, but concerted efforts could further decrease local transmission. Parasitological surveys are required to assess actual prevalence, incidence and importation rates.

Opinions of key stakeholders on alternative interventions for malaria control and elimination in Tanzania

“…In developing countries, simple modifications like screening windows and doors and closing eave spaces have resulted in some cases, in a 50% decline in entomological inoculation rates [38]. In Tanzania for example, housing improvement was linked to significant historical declines of malaria in Dar es Salaam [39], and was likely a major factor in more than 99% decline in malaria in Ifakara town, the main town in the area of our study [33] Larval source management Larval source management (LSM) refers to environmental manipulations to target mosquito larval habitats [13]. LSM can include the use of larvicides as well as environmental management methods [13,14,40].…”

“…The group of policy-makers included senior officials from government ministries Finda et al Malar J (2020) 19:164 in Dodoma, Tanzania's administrative capital, all of them with direct or indirect influence on malaria control Lastly, community members were comprised of local community leaders drawn from 10 rural and urban wards in Ulanga and Kilombero districts in the Kilombero valley, in south-eastern Tanzania. Residents in the area are mostly subsistence farmers, pastoralists or small business owners [33,34]. Malaria prevalence in these districts is highly heterogeneous, ranging from < 1% in the urban and peri-urban sites to > 40% in some of the villages (Swai et al, unpublished).…”

“…Malaria prevalence in these districts is highly heterogeneous, ranging from < 1% in the urban and peri-urban sites to > 40% in some of the villages (Swai et al, unpublished). Transmission intensities are also highly diverse, varying from less than 1 to ~ 20 infectious bites per person per year [33,34].…”

Anopheles funestus mosquitoes from rural south-eastern Tanzania may have stronger resistance to pyrethroids than the other malaria vector, Anopheles arabiensis

“…In the villages of south-eastern Tanzania, where Anopheles funestus have been reported to implicate most of the ongoing malaria transmission [28][29][30], signs of resistance to most public health pesticides have been observed. An.…”

Impact of ENSO 2016–17 on regional climate and malaria vector dynamics in Tanzania

“…The Kilombero Valley in southern Tanzania has experienced historically high malaria transmission with 226 infective bites per person per year in 2012 (Lwetoijera et al 2014). Since then, it has seen a reduction in infective bites to 15.9 ib/p/yr in 2015 (Kaindoa et al 2017b, Finda et al 2018 due to control measures (LLINs and IRS). The availability of high quality historical data on vector ecology and transmission, and a high coverage with LLINs make this valley a good model for quantifying impacts of extreme climate events in areas of Africa were transmission has also been declining.…”