Climate change highlights risk of malaria resurgence

When health official Tatiana Marrufo visited a colleague in Mozambique’s Manica province this year, she was surprised by what he reported: there had been an increase in malaria cases reported by the local health system.

The inland highland region in the west of the country has traditionally had few cases of the parasite-borne disease. Its cooler and drier environment is a less hospitable habitat for the mosquitoes that transmit the parasite to humans in lower, hotter and wetter coastal parts of the country.

“He said they had not prioritised the province for indoor spraying with insecticides, because in the past we didn’t have [many] cases,” explains Marrufo, head of Mozambique’s Central Office of the National Observatory of Health — referring to the typical, but relatively costly, prevention technique. “But the climate is changing, and creating the conditions to have more mosquitoes.”

This increase highlights the risk of a continued spread of malaria — cause of the world’s biggest infectious disease burden, estimated by the World Health Organization (WHO) at nearly 250mn infections and more than 600,000 deaths in 2022.

Even as new tools emerge for prevention, diagnosis and treatment, Covid-19 has set back short-term progress, which had seen numbers on a downward trajectory, overall, since the turn of the century. Now, in the longer term, climate change looks set to confound, and compound, the problem of mosquito-borne infections, including malaria and dengue.

Marrufo highlights the effects of an escalation of extreme weather events, notably the cyclones that regularly strike Mozambique’s coast. The result is a surge in diseases such as cholera and in diarrhoeal infections linked to flooding and poor sanitation, and food insecurity for the mainly subsistence farmers.

The consequent displacement of populations exacerbates the health problems by spreading disease. It also puts fresh pressures on clean water and food supplies, hygiene and health systems — notably in parts of the country ravaged by civil war and poorly resourced ever since. “The past five years have been terrible for us with the number of cyclones and heavy rain,” Marrufo says.

Around the globe, increased greenhouse emissions, resultant rises in temperatures, and shifts in climate patterns are posing multiple new threats to human health, as extreme weather triggers death, disease and migration.

While some effects are clear — such as mortality and morbidity linked to air pollution, extreme heat, and physical harm — others are more ambiguous. In its latest annual report on malaria in 2023, the WHO was cautious, concluding that experts remain divided on how far climate change is increasing cases, with clear evidence still “sparse”.

The WHO cites a detailed study of children in sub-Saharan Africa, suggesting the odds are two-to-one that anthropogenic climate change — that driven by human activity — has led to an overall increase in the prevalence of malaria since the start of the 20th century. The report also highlights the indirect effects of climate, including a deterioration in storage of diagnostic tests and drugs, as well as further strains on health systems and wider resources.

But, irrespective of the effect of climate change, David Reddy, outgoing head of the Medicines for Malaria Venture, a non-profit partnership that develops new treatments, highlights the biological cunning of both the vector — the mosquito — and the parasite that it transmits. “It’s a fascinating disease and the situation really points to evolutionary warfare,” he observes.

The parasite continues to build resistance to drugs and diagnostic tests. Similarly, the mosquito has developed its own survival tactics, evolving resistance to the insecticides found in sprays and on bed nets, and shifting its habitats and feeding times — for example, by biting more in the daytime, which makes nighttime indoor preventive measures less effective.

Helen Jamet, deputy director for vector control at the Bill & Melinda Gates Foundation, also points to the displacement of the type of mosquito transmitting malaria, with the emergence — in south Asia and Djibouti, east Africa — of Anopheles stephensi, which breeds in water tanks and is spreading malaria into urban areas.

“Climate change presents new obstacles,” Jamet says. “There are a lot of complex links with transmission. Warmer temperatures and different rainfall patterns can expand its reach and extreme weather can fuel outbreaks. Droughts can also create ideal breeding spots when a body of water retracts.”

However, in spite of the setbacks, a new generation of prevention and treatment tools are gaining ground. The WHO has approved a rollout of bed nets treated with Pyrethroid-chlorfenapyr, as well as R21/Matrix-M, a second malaria vaccine to complement RTS, S. 

New drug combinations are being tested and a single-dose cure for the Plasmodium vivax strain of malaria is being distributed. Experimental use of genetically modified sterile mosquitoes is also under way to reduce numbers, and insects are being exposed to the Wolbachia bacteria to cut dengue infection.

Meanwhile, ahead of the rainy season in Mozambique, Marrufo is preparing the latest readout from an “early warning system” she has helped develop. It combines weather predictions with malaria reports to prioritise and target interventions in individual districts, such as insecticide spraying to more effectively tackle the parasite.

“We’re not reaching our health targets even without climate change,” she notes. “With it, the situation is even worse.”

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