Malaria is a life-threatening disease that is spread to humans through the bite of infected mosquitoes. In 2022, it affected more than 249 million people around the globe — including Australia’s closest neighbour, Papua New Guinea (PNG). New research from the Australian Institute of Tropical Health and Medicine (AITHM) indicates that people could be better protected from mosquito bites by using a method that dates back to World War II, but with a 21st century twist.
Malaria is primarily found in countries with a tropical climate, where people generally rely on the protection of mosquito nets that are treated with an insecticide. However, this leaves people vulnerable to bites when outside the nets.
PhD candidate Ms Rebecca Vinit from the PNG Institute of Medical Research, with her advisor Associate Professor Stephan Karl from the Australian Institute of Tropical Health and Medicine (AITHM), is examining if indoor residual spraying (IRS) significantly reduces the number of malaria cases in PNG.
IRS involves spraying walls and other surfaces of the house with an insecticide. The insecticide will kill mosquitos that land on these treated surfaces, and typically lasts for several months.
“IRS is not new to PNG; it was first used back in the 1930s and during World War II,” Ms Vinit said. “After it became known that Australian soldiers were killed faster by malaria than in combat, indoor residual spraying proved to be very efficient in reducing the number of infections during the 1960s. But after PNG became independent in 1975, the program was abandoned.”
Some insecticides that were used then were harmful to not just mosquitoes but also other animals. This is why in her research, Ms Vinit is using products that only affect mosquitos. “We call it ‘residual spraying’ because it involves a small but potent quantity,” she said. “It's enough to knock out a mosquito without having a negative effect on other animals or humans.”
There are several mosquito species that can transmit malaria in PNG, and they may respond differently to insecticides, but the actual mosquito population in a place depends on a variety of biological factors, such as how close to the coast a village is located.
Four villages along the North Coast of Madang Province in PNG participated in Ms Vinit’s study: two located on the coast and two about five kilometres inland.
“In 2021, we initially sprayed two villages to assess the impact of indoor residual spraying. This was followed by monthly surveys before and after, as we wanted to find out how long the insecticide was effective,” Ms Vinit said.
“In phase two, we sprayed all four villages and conducted two assessments to measure the effectiveness of the insecticide at three months and six months,” she said. All in all, Ms Vinit’s team sprayed around 330 homes in phase one.
“We have a hypothesis that indoor residual spraying is having a positive impact in our two inland villages, but in the coastal areas, where mosquitoes are more of a nuisance, it may be less effective,” Ms Vinit said. However, the analysis is still in progress, and Ms Vinit says it’s too early to provide any recommendations.
After Ms Vinit’s research project is finished in 2026, indoor residual spraying may become a part of PNG’s national malaria control program to complement the current program of insecticide treated bednets. “Indoor residual spraying is an expensive intervention for the government,” Ms Vinit said. “This is why I’m looking for an effective and efficient way to implement this only in places where indoor spraying can actually make a difference.”
