Partially-sprayed houses show promising results in IRS efficacy


Indoor residual spraying (IRS) is established as an effective method of malaria vector control. However, the cost of treating homes is seen as one of the barriers. Finding a way to reduce the cost of spraying, without reducing product efficacy, could go a long way to achieving malaria eradication.

A study was conducted in northern Ghana in May 2019 to explore the effectiveness of spraying only part of a house. Specifically, researchers were looking at the effectiveness of pirimiphos-methyl, the active ingredient in Actellic® 300CS insecticide, on Anopheles gambiae s.l. mosquitoes.

To compare efficacy, researchers looked at the following scenarios:

  • Spraying a hut or house completely — walls and ceiling
  • Partially spraying a hut or house — the upper part of the walls and ceiling (i.e. only 2/3 of the house)
  • Not spraying at all

The results were promising: researchers found that areas that were partially sprayed only saw a slightly higher mosquito biting rates (1.36x) than those that were fully sprayed, while unsprayed control sites still showed nearly 7.8x and 6x more bites respectively, compared to fully sprayed and partially sprayed.

  • Fully sprayed sites: 28 ib/p/y*
  • Partially sprayed sites: 38 ib/p/y
  • Unsprayed control site: 217 ib/p/y

Taking this a step further, the team looked at transmission models based on this data and determined that the efficacy of partial IRS would be about the same after six months as full IRS. Additionally, spray teams could see a 39.2% cost savings from the cost of treatment and a 36.2% overall cost savings in treatment and operational costs using a partial spray method.

To learn more about vector control and Actellic 300CS, visit

*ib/p/y: infectious bites/person/year


Partial indoor residual spraying with pirimiphos‑methyl as an effective and cost‑saving measure for the control of Anopheles gambiae s.l. in northern Ghana, Sylvester Coleman, et al., September 2021

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