Marta Maia: Vector control to fight malaria
Malaria is mostly controlled through the use of bed nets and insecticides, but progress has stalled and we need new vector control interventions. Mosquitoes can be affected by endectocides carried in the blood of hosts. Clinical trials will determine whether ivermectin administered to human or cattle can be used to impact malaria transmission.
My name is Marta Maia, I am a medical entomologist based in Kenya, in Kilifi, a coastal town. I work with insects that transmit disease, in particular malaria, so I am a specialist in malaria mosquitoes.
Malaria is mostly controlled through the use of bed nets and indoor residual spraying, where people spay insecticides on the walls of houses. Bed nets control mosquitoes because they are first of all a barrier to the person who is sleeping, and also they are impregnated with insecticide. This has been the main method for controlling malaria transmission for the past decades, and it has resulted in a dramatic reduction in malaria transmission. However in the past two years, we have not seen a further reduction.
I’m working on a new method for controlling malaria mosquitoes, with endectocides. Ivermectin is an endectocide; that means it is a drug that host takes, in this case human takes this drug, and then the mosquitoes if they bite you, the blood that they take has a small concentration of this drug, which kills them. Some call this a systemic insecticide; basically, if you treat all the hosts in a given population, not just humans it can also be the cattle, the mosquitoes will be cornered and this will impact the population by reducing the density of mosquitoes, and their survival and their fitness in the environment, which would then lead to an impact on malaria transmission.
Ivermectin is a drug that has been known for many decades. It’s been used to control other diseases, neglected tropical diseases like onchocerciasis and filariasis; and now we consider repurposing the drug for malaria control, because it’s been discovered that ivermectin in slow concentration in human blood results in mosquito mortality and reduction of mosquito survival. Even if you don’t see an immediate impact on the survival of mosquitoes, it will hinder them from reaching the age when they are able to transmit the disease.
In order for mosquitoes to transmit malaria, they have to reach 10 to 14 days of age, because mosquitoes are not born with malaria. They have to take a blood meal from an infected person, and then the parasite needs a specific number of days depending on the temperature to develop in the mosquito. If you could impact the survival of the mosquito, even by just two or three days, you may have a dramatic impact on malaria transmission.
Ivermectin is a drug known to impact parasites and helminths, soil transmitted helminths, but we also found out that it has an impact on insects. This is something that is well known in the veterinary field, and it’s not been adopted in human medicine. But actually, it has an application for malaria control because it will kill the mosquito. The repurposing of ivermectin for malaria control has been a very good breakthrough in the past three years. And now, there are trials that are going to investigate the epidemiological impact of this intervention in the field, in Africa.
Research to find new ways to control mosquitoes is really important at the moment, because the existing interventions, bed nets and IRS (indoor residual spraying), are no longer as effective as they used to be. Mosquitoes have adapted; they are biting earlier in the evening. They are also biting other hosts: they’re finding refuge in other hosts. So finding new methods to control mosquitoes is really important. This would not only reduce mortality and mobility but also stop the hindrance that malaria has been for economic growth in developing countries. We really need to invest more in finding new vector control interventions.
We are going to conduct two clinical trials in sub-Saharan Africa to find out if mass drug administration of ivermectin to humans and cattle may impact malaria transmission. The trials are funded by Unitaid, and if we prove that there is an epidemiological impact on malaria transmission, this could lead to evidence to advice policy makers on new interventions that they may implement to eliminate the disease.
This interview was recorded in September 2019