Could malaria stage a comeback in the US, Europe and elsewhere because of climate change?
The mosquitoes that spread malaria are expanding their geographic distributions because of warming temperatures and altered rainfall patterns.
- 24 April 2024
- 9 min read
- by Linda Geddes
In August 2023, a person living in Maryland, US, close to Washington DC, contacted their doctor because they had been experiencing fever, body aches and feeling unwell for the past seven days. Hospital tests revealed that they were anaemic, with evidence of parasites living inside their red blood cells.
“Climate change is increasing the number of people around the world who are devastated by environmentally-linked crises – floods, droughts, food insecurity. This dramatically increases their vulnerability to debilitating diseases such as malaria. And these threats fall hardest on those populations that are already most vulnerable – in societies and around the world.”
– W. Scott Gordon, Head of Gavi’s Malaria Vaccine Programme
Because the patient had recently been bitten by a tick, doctors initially suspected that they might have a parasitic disease called babesiosis. However, further tests revealed the presence of Plasmodium falciparum – the deadliest type of malaria parasite – even though the patient had not recently travelled abroad.
First US case in 20 years
Malaria was once common in the US and many European countries, but the draining of wetlands where mosquitoes breed, plus the use of insecticides, window screens, antimalarial drugs and improved diagnostics, led to its gradual elimination. Most countries in these regions were declared malaria-free by the 1970s.
In the US, the Anophelesmosquitoes that carry malaria are still present and around 2,000 cases of malaria are detected in travellers returning from countries where the disease is endemic each year. However, because these cases are relatively rare and tend to be swiftly detected and treated, onwards transmission – through a mosquito biting an infected person, and then biting someone else – is rarer still.
Before last year, no such cases had been reported since 2003, and none had been reported in Maryland for more than 40 years. However, between May and October 2023, ten people were diagnosed with locally acquired malaria in Florida, Texas, Arkansas and Maryland – including the patient described above.
Is climate change to blame?
While it is too soon to know if such cases mark the start of a trend, scientists are increasingly predicting that malaria has the potential to rebound in countries or areas long declared malaria-free, including parts of the US, because of climate change.
"There is stuff happening that you wouldn’t necessarily say is consistent with the trend of malaria eradication, including in places where it has historically been eradicated, such as locally acquired malaria in, for example, Greece.”
– Chris Murray, professor of climate change and health, London School of Tropical Medicine at the MRC unit in the Gambia
"Climate change is increasing the number of people around the world who are devastated by environmentally-linked crises – floods, droughts, food insecurity. This dramatically increases their vulnerability to debilitating diseases such as malaria," said W. Scott Gordon, Head of Gavi's Malaria Vaccine Programme. "And these threats fall hardest on those populations that are already most vulnerable – in societies and around the world."
Malaria isn't the only disease that researchers and health professionals are worried about. Other vector-borne diseases (those transmitted by blood-sucking arthropods) such as dengue, Zika, West Nile virus and chikungunya – are also expected to expand their geographic distributions to more temperate regions further north or south because of warming temperatures and altered rainfall patterns.
"Vector-borne and zoonotic diseases [those transmitted from animals to humans] in particular, are climate-sensitive, because it [climate] impacts the life history, distribution and success of the hosts these things transmit from," said Chris Murray, a professor of climate change and health at the London School of Tropical Medicine at the MRC unit in the Gambia.
In its latest World Malaria Report, published in November 2023, the World Health Organization warned that climate change had the potential to undermine gains in the global fight against the disease, particularly in vulnerable regions where disrupted access to antimalarial drugs, vaccines and other preventive measures such as insecticide-treated bed-nets, e.g. as a result of conflict or natural disasters, further increases the risk of malaria outbreaks.
Longer summers and warmer winters could extend the breeding season for mosquitoes and enable more of them to survive and flourish, while stagnant pools of water left by receding floods or in plastic containers used to store water in temporary human settlements, make ideal breeding grounds. Warmer temperatures also speed up the growth cycle of malaria parasites, increasing their numbers.
Malaria transmission is changing
Already, there are indications that things are changing: "There is stuff happening that you wouldn't necessarily say is consistent with the trend of malaria eradication, including in places where it has historically been eradicated, such as locally acquired malaria in, for example, Greece," said Murray. "However, attributing individual events to climate change is difficult, because there are other factors that can contribute, such as changing [malaria] importation rates related to changing flows of people.
"The season of possible transmission in a lot of places is also increasing. Particularly in temperate areas, you've got a situation where local transmission is only possible within a given season – typically summer in Europe or the US. And what looks to be happening, at least from a theoretical perspective, is that the season is expanding."
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Some of the strongest evidence for a link between climate change and malaria transmission comes from long-term observations of cases in African highland areas on the fringes of endemic transmission, the WHO report said. These suggest that rising temperatures have led to the expansion of the disease in recent decades.
Extreme rainfall and flooding in Pakistan during 2022 also resulted in a fivefold increase in malaria cases compared with 2021, the report noted, and such extreme weather events are predicted to become more commonplace due to climate change.
Some regions could see a fall in cases
However, it is not all bad news. Some areas where malaria is currently endemic could experience a decrease in malaria because of changing temperatures.
Prof Jane Carlton, Director of the Malaria Research Institute at the Johns Hopkins Bloomberg School of Public Health, said: "An interesting fact about malaria is that the parasite itself can only develop in a mosquito over quite a tight temperature range: if it is too cold, the parasite won't grow, and if it is too hot, the parasite won't grow. This means that if it gets too hot in a particular area, the parasite will die, and you may get fewer malaria cases.
“An interesting fact about malaria is that the parasite itself can only develop in a mosquito over quite a tight temperature range: if it is too cold, the parasite won’t grow, and if it is too hot, the parasite won’t grow. This means that if it gets too hot in a particular area, the parasite will die, and you may get fewer malaria cases."
– Prof Jane Carlton, Director of the Malaria Research Institute at the Johns Hopkins Bloomberg School of Public Health
"However, it also means that in more temperate regions, including highland areas of countries where malaria is already endemic, there could be increases in cases because the range that the mosquitoes can live in, and that the parasite can develop in, will change. So, we might see a geographical change in the distribution of malaria, which is concerning."
Whether such factors contributed to the flurry of locally acquired malaria cases in the US last year is impossible to say for sure. Seven of these cases occurred in Sarasota County, Florida, south of Tampa; one occurred in Texas, just north of the border with Mexico; and another occurred near Little Rock, Arkansas. Unlike the Maryland case, these ones involved Plasmodium vivax, the dominant malaria parasite outside sub-Saharan Africa. All of the patients recovered, and there is no evidence to suggest that any of the cases were related, the US Centers for Disease Control and Prevention has said.
"If there are more [locally acquired] cases this year, then that will certainly be something to be more aware of," said Carlton. "But I think what these cases have really triggered is a realisation that we need to do more studies to better understand the local mosquito populations."
Italian studies
Such studies are already taking place elsewhere. For instance, in Italy, where malaria was endemic until the 1970s, a cluster of seven non-imported, and seemingly unrelated, cases of malaria occurred in 2017, including one case of P. falciparum malaria in which a four-year-old girl died. A later investigation suggested she might have caught it during a hospital stay, as the malaria strain was the same as one infecting another family at the hospital who had caught it during a recent trip to Burkina Faso. Possibly, the reuse of a needle or some other human error resulted in the girl becoming infected, investigators said.
“Humans are a critical element in the transmission cycle of malaria and many other vector-borne diseases. If we can ensure better care for our most vulnerable populations – such as making sure that they are vaccinated, properly nourished and have access to prevention such as bed-nets – we may be able to limit some of the most dramatic impacts from the changing climate.”
– W. Scott Gordon, Head of Gavi’s Malaria Vaccine Programme
Increased surveillance in the wake of these cases has detected mosquito species capable of carrying malaria parasites in several regions of Italy, including rice fields in northern regions, while a study published in Parasites and Vectors earlier this month found Anopheles sacharovi mosquitoes – historically, one of the two key malaria vectors in Italy, and which were thought to have disappeared from the country more than 50 years ago – in the southern Apulia region.
Although the density of these mosquitoes is unlikely to pose a health threat at the current time, "to prevent the risk of reintroduction of the disease, the need to strengthen the surveillance of [Anopheles mosquitoes] throughout the South should be considered," the study authors said.
Even though the risk of catching malaria in temperate regions remains very low at the current time, "it is important that we have systems in place that we can detect unusual changes in the distribution or the abundance of [disease-carrying] species," said Murray.
Also needed is greater public awareness of the signs and symptoms of malaria and other vector-borne diseases in countries where they are not currently widespread. "It is important to start to inform populations in the US about how to protect themselves during the summer months, such as by wearing long-sleeved shirts, and just being more aware of if they are being bitten," Carlton said.
Focus on the most vulnerable countries
Yet it is important not to lose sight of the huge gains that have been made against malaria in recent decades, and to recognise that the bulk of transmission still occurs in low-income countries. Continuing to focus prevention and surveillance efforts on these countries is crucial, as health disparities only magnify global and regional socio-economic disparities, leaving populations even more vulnerable to the impact of climate-related disasters and other crises.
T he biggest threat to the progress that has been made against malaria is the disruption of public health systems, whether because of civil strife, poverty or climate-related disaster. "That's when you start to get an increase in malaria cases," Carlton said.
Our global climate is changing. The best way to protect ourselves against malaria is to shore up those health care systems to ensure that everyone stays safe. "Humans are a critical element in the transmission cycle of malaria and many other vector-borne diseases. If we can ensure better care for our most vulnerable populations – such as making sure that they are vaccinated, properly nourished and have access to prevention such as bed-nets – we may be able to limit some of the most dramatic impacts from the changing climate," said Gordon.
