People often ask why malaria persists despite decades of control efforts. Malaria is not just a problem of mosquitoes and parasites, but it’s a disease shaped by people, the environment, and even climate. Understanding where and why Plasmodium thrives helps explain why malaria remains a global challenge and why elimination is so difficult.

Where the parasite thrives

Plasmodium parasites rely on two essential components for survival, namely humans to infect and Anopheles mosquitoes to carry them between hosts. Because of this dependence, malaria is most prevalent in tropical and subtropical regions, where Anopheles mosquitoes thrive throughout the year and temperatures allow the parasite to develop efficiently within the mosquito. Other environmental factors, such as stable mosquito populations, abundant standing water for breeding, and high humidity, further create ideal conditions for ongoing transmission. Seasonal rains, irrigation, and human activities that create water collections can also influence mosquito abundance, making certain areas more prone to malaria outbreaks. Together, these ecological and climatic factors shape where and when malaria can persist, highlighting why control efforts must consider both the parasite and its environment.

Asymptomatic carriers, parasite density

Not everyone infected with malaria develops symptoms. Some people carry low levels of parasites without feeling sick. These people are known as asymptomatic carriers, and they are of great concern for malaria control programmes. Even though these people appear healthy, their blood can still infect mosquitoes, allowing the parasite to circulate silently within communities. This hidden reservoir makes malaria elimination particularly challenging, because relying only on testing and treating symptomatic individuals leaves many infections undetected. Asymptomatic carriers can maintain transmission over long periods, sustaining outbreaks and undermining control efforts unless targeted through active surveillance and community-wide interventions.

The number of parasites in the blood, or parasitaemia, influences both disease severity and transmission potential. High parasitaemia often causes severe illness and is easier to detect with diagnostic tests. Low parasitaemia may produce mild or no symptoms but can still allow mosquitoes to pick up gametocytes and continue transmission. Detecting these low-level infections requires sensitive diagnostic tools, highlighting the importance of surveillance even among healthy-looking individuals.

Environmental and climatic influences

Malaria transmission is highly sensitive to environmental and climate factors, including temperature, rainfall, and humidity.

Temperature influences the speed at which Plasmodium parasites develop inside Anopheles mosquitoes. Warmer temperatures can shorten the parasite’s development time, increasing the likelihood of transmission. Rainfall and water availability create breeding sites for mosquitoes, while humidity affects their survival and lifespan, determining how long they can transmit the parasite. Seasonal variations, such as wet and dry periods, can therefore cause fluctuations in malaria cases.

Long-term changes in climate, including rising temperatures and altered rainfall patterns, can expand malaria into previously unaffected regions, change transmission dynamics, and trigger sudden outbreaks in areas where the disease was once rare. Understanding these environmental influences is crucial for predicting malaria risk and planning effective control strategies.

A need for parasite surveillance

Stopping malaria requires more than just treating symptomatic cases; it also demands identifying hidden infections, tracking parasite populations, and monitoring mosquito habitats. Without careful surveillance, asymptomatic carriers and environmental changes can allow malaria to persist or re-emerge, even in areas with strong control programs. Controlling Plasmodium in these hidden reservoirs is essential to interrupt transmission. Targeted strategies include active case detection using sensitive diagnostic tools to find asymptomatic carriers, followed by treatment with blood-stage antimalarials to eliminate the parasites. Environmental management, such as removing mosquito breeding sites and improving community sanitation, further reduces opportunities for parasites to move from humans to mosquitoes. By addressing both hidden infections and the environmental conditions that support transmission, we can significantly weaken the parasite’s foothold in endemic areas and move closer to malaria elimination.

The next and final article will explore how antimalarial medicines fight Plasmodium, why drug resistance is growing, and what strategies are being developed to outsmart this adaptable parasite.

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