World Mosquito Day commemorates Sir Ronald Ross’ 1897 discovery of the link between mosquitoes and malaria transmission. The event raises awareness about the threat of malaria and other diseases transmitted by the world’s deadliest animal.
On this occasion, we bring you 10 fascinating facts about malaria transmission.
Malaria is transmitted to humans by ~40 species of the mosquito genus Anopheles. Female mosquitoes take blood meals from humans to feed their babies, making the link between the human and mosquito hosts in the parasite lifecycle.1
In humans, malaria parasites grow and multiply first in the liver cells and then exponentially in the red blood cells (shown above in white). It is the blood stage of the parasite lifecycle that causes the symptoms of malaria in humans.
When certain forms of blood-stage parasites, known as gametocytes, are picked up by a mosquito during a blood meal, a new cycle of malaria parasite growth begins in the mosquito’s midgut that ends up in the salivary glands. After approximately 10-12 days, the gametocytes will have developed into new parasites, called sporozoites (pictured above, in green), which can be transmitted back to humans through the saliva of the mosquito as she takes another blood meal.
How can we stop the transmission of malaria? One important way is to stop mosquitoes biting people. A key intervention is the use of insecticide treated nets (ITNs). Across sub-Saharan Africa, the proportion of households with sufficient nets (i.e. one net for every two people) remains inadequate, at 40% in 20172. Another limitation is the preference of some species to bite outdoors and during the daytime, in particular those transmitting P. vivax malaria.
Indoor residual spraying (IRS), another key intervention, coats walls and other surfaces with a residual insecticide lasting for several months; when the female Anopheles pauses for a rest on a wall before or after a blood meal, for example, she is killed. Unfortunately, IRS protection has declined in recent years, from a peak of 5.8% in 2010 to 3% in 2017.3
Administering treatment to children to prevent malaria during the high transmission season (otherwise known as seasonal malaria chemoprevention) has been shown to be able to dramatically reduce disease incidence in the under 5s. Given the success of this intervention, MMV is supporting its scale-up in 15 countries.
Blocking the transmission of malaria is key for elimination of the disease at the local level, the country level and eradication globally. MMV and partners are researching new antimalarials that kill gametocytes or render them inactive and could thus potentially block malaria transmission.
To protect pregnant women living in areas of moderate-to-high malaria transmission, WHO recommends intermittent preventive treatment in pregnancy (IPTp) with sulfadoxine-pyrimethamine (SP). This is a time-tested drug combination whose safety profile is very well understood. Unfortunately, only 22% of pregnant women across 35 African countries receive the minimum of 3 or more doses as recommended by the WHO. MMV is working to improve coverage of the treatment in areas where it remains effective.4
An obstacle to blocking the transmission of malaria is resistance to insecticides. Since 2010, a total of 61 countries have reported resistance to at least one class of insecticide. Organizations such as IVCC are working hard to combat this growing issue.5
Blocking the transmission of the malaria parasite may be possible through genome editing. Organizations such as Target Malaria are evaluating several approaches to reduce the number of female Anopheles gambiae by creating genetically modified strains of the species.