Implications of bio-efficacy and persistence of insecticides when indoor residual spraying and long-lasting insecticide nets are combined for malaria prevention

Abstract

BackgroundBio-efficacy and residual activity of insecticides used for indoor residual spraying (IRS) and long-lasting insecticide nets (LLINs) were assessed against laboratory-reared and wild populations of the malaria vector, Anopheles arabiensis in south eastern Tanzania. Implications of the findings are examined in the context of potential synergies and redundancies where IRS and LLINs are combined.MethodsBioassays were conducted monthly for six months on three LLIN types (Olyset® PermaNet 2.0®,and Icon Life®) and three IRS treatments (2 g/m2 pirimiphos-methyl, 2 g/m2 DDT and 0.03 g/m2 lambda-cyhalothrin, sprayed on mud walls and palm ceilings of experimental huts). Tests used susceptible laboratory-reared An. arabiensis exposed in cones (nets and IRS) or wire balls (nets only). Susceptibility of wild populations was assessed using WHO diagnostic concentrations and PCR for knock-down resistance (kdr) genes.ResultsIRS treatments killed ≥ 85% of mosquitoes exposed on palm ceilings and ≥ 90% of those exposed on mud walls, but up to 50% of this toxicity decayed within 1–3 months, except for DDT. By 6th month, only 7.5%, 42.5% and 30.0% of mosquitoes died when exposed to ceilings sprayed with pirimiphos-methyl, DDT or lambda-cyhalothrin respectively, while 12.5%, 36.0% and 27.5% died after exposure to mud walls sprayed with the same insecticides. In wire-ball assays, mortality decreased from 98.1% in 1st month to 92.6% in 6th month in tests on PermaNet 2.0®, from 100% to 61.1% on Icon Life® and from 93.2% to 33.3% on Olyset® nets. In cone bioassays, mortality reduced from 92.8% in 1st month to 83.3% in 6th month on PermaNet 2.0®, from 96.9% to 43.80% on Icon Life® and from 85.6% to 14.6% on Olyset®. Wild An. arabiensis were 100% susceptible to DDT, 95.8% to deltamethrin, 90.2% to lambda cyhalothrin and 95.2% susceptible to permethrin. No kdr gene mutations were detected.ConclusionsIn bioassays where sufficient contact with treated surfaces is assured, LLINs and IRS kill high proportions of susceptible An. arabiensis mosquitoes, though these efficacies decay gradually for LLINs and rapidly for IRS. It is, therefore, important to always add intact nets in sprayed houses, guaranteeing protection even after the IRS decays, and to ensure accurate timing, quality control and regular re-spraying in IRS programmes. By contrast, adding IRS in houses with intact LLINs is unlikely to improve protection relative to LLINs alone, since there is no guarantee that unfed vectors would rest long enough on the sprayed surfaces, and because of the rapid IRS decay. However, there is need to clarify these effects using data from observations of free flying mosquitoes in huts. Physiological susceptibility of An. arabiensis in the area remains 100% against DDT, but is slightly reduced against pyrethroids, necessitating caution over possible spread of resistance. The loss of LLIN toxicity, particularly Olyset® nets suggests that protection offered by these nets against An. arabiensis may be primarily due to physical bite prevention rather ...

Data collection for risk assessments on animal health (Acronym: DACRAH) : Final Report

“…The European Food Safety Authority reserves its rights, view and position as regards the issues addressed and the conclusions reached in the present document, without prejudice to the rights of the authors. Mortality of mosquitoes was 97% in washed nets, and reduced to 84%, 54% and 7% after 6, 12 and 18 months (with respective average of times washed of 1.1, 1.9 and 3) deltamethrin (Okumu et al, 2012) Treated mosquito nets Concentrations of 62-65 mg/m 2 Mortality (24h mortality of Anopheles arabiensis) was above 90% after 30 days (94 and 97% for two products tested) but reduced below 90% after 4 months.…”

“…Shorter residual efficacy (42-56 days) was found in wall substrates made of cement, limestone, mud-daub, oil paint and white wash. Burnt bricks retained the residual efficacy up to 134 days while galvanized iron sheets, palm thatch and wood retained the recommended residual efficacy beyond 152 days lambdacyhalothrin (Okumu et al, 2012) Indoor spraying 0.03 g/m 2 sprayed on mud walls Mortality (24h mortality of Anopheles arabiensis) was 90% after 30 days but…”

“…deltamethrin (Okumu et al, 2012) Treated mosquito nets polyethylene net, impregnated during manufacture with synthetic deltamethrin at 0.2% w/w (approximately 65 mg of active ingredient/m2) 50% mortality (recorded over 24h) in mosquitoes (Anopheles arabiensis) exposed to nets 60 days after treatment. 2 ) was able to kill over 90% of mosquitoes (3 min exposure, 24h-mortality) for up to 30 DACRAH Disclaimer: The present document has been produced and adopted by the bodies identified above as author(s).…”

Landscape Epidemiology Modeling Using an Agent-Based Model and a Geographic Information System

“…These interventions are often applied in isolation and in combination, and their impacts have been investigated by numerous early and recent studies [38][39][40][41][42][43][44][45]. In addition to these time-tested, established tools, new and novel intervention tactics and strategies such as new drugs, vaccines, insecticides, improved surveillance methods, etc., are also being investigated [46].…”

Malaria: Control, Elimination, and Eradication

“…49 Both ITNs and LLIN are simple to use, easy to deliver to rural communities, and cost-effective when used in highly endemic malarious areas. 55 Pyrethroids are the only class of insecticide currently used for ITNs and it has a repellent effect, by preventing entry of mosquitoes into houses as well as disrupting blood feeding and premature exit of mosquitoes from the home. 49 The emergence and spread of insecticide resistance threatens the effectiveness of ITNs and indoor residual house spraying.…”