Malaria is a serious parasitic disease. In 2022, there were an estimated 249 million cases and 608,000 deaths worldwide, with most occurring in the WHO African Region. Understanding how malaria spreads is important for public health efforts to control it.
Malaria Transmission and Contagious Period
Malaria is not directly contagious from person to person through casual contact. Instead, it spreads through the bite of an infected female Anopheles mosquito. The parasite, Plasmodium, has a complex life cycle that involves both humans and mosquitoes. When an infected mosquito bites a human, it injects sporozoites, which travel to the liver and multiply.
After multiplying in the liver, the parasites, called merozoites, are released into the bloodstream and infect red blood cells, where they reproduce asexually, causing malaria symptoms. A small percentage of these merozoites develop into sexual forms called gametocytes, which circulate in the human bloodstream. These gametocytes are the only forms of the parasite that can infect a mosquito.
When an Anopheles mosquito bites an infected person, it ingests these gametocytes. Inside the mosquito’s gut, the gametocytes undergo sexual reproduction, forming new parasites that migrate to the mosquito’s salivary glands, ready to infect another human. A person remains “contagious” to mosquitoes as long as these gametocytes are present in their blood. For Plasmodium falciparum, gametocytes can persist in the bloodstream for several weeks, even after the asexual parasites causing illness have been cleared by treatment. P. falciparum gametocytes can be detectable for up to two months.
Factors Influencing Contagion
The duration of malaria transmission to mosquitoes is influenced by several factors, including the Plasmodium species. Plasmodium falciparum gametocytes take about 8-12 days to mature and circulate in the blood for approximately 3-4 weeks if untreated, though they can persist longer. In contrast, Plasmodium vivax gametocytes develop more quickly, maturing in about 48 hours, and are cleared from the bloodstream within three days post-maturation.
Treatment with antimalarial drugs reduces the period of contagiousness. While many antimalarial drugs effectively target the asexual forms of the parasite that cause illness, their effect on gametocytes can vary. Some treatments, particularly artemisinin-based combination therapies (ACTs), can reduce gametocyte levels, but mature gametocytes may still persist for some time after asexual parasites are cleared. For instance, even after ACT treatment, P. falciparum gametocytes can be detected in some patients for up to 28 days.
Immune status also influences contagiousness. People living in malaria-endemic areas often develop partial immunity, which can lead to lower asexual parasite densities and lower gametocyte levels, reducing their contribution to transmission. However, even asymptomatic individuals with low levels of parasites can still harbor gametocytes and contribute to the infectious reservoir. Susceptible Anopheles mosquitoes must also be present for transmission.