Climate change affects mosquito behavior, making it difficult to eradicate malaria in South Africa

Climate change affects mosquito behavior, making it difficult to eradicate malaria in South Africa

The global climate is changing rapidly. This has a range of public health implications. Credit: CDC/James Gathany

Changes in climatic factors – such as higher temperatures and increased precipitation – affect the growth, behavior and distribution patterns of insects such as mosquitoes. These changes have serious implications for the effective control of insect-borne diseases such as malaria.

Worryingly, temperatures across southern Africa are expected to increase by at least 0.8°C by 2035.

Malaria is currently prevalent in three provinces of South Africa: Limpopo, Mpumalanga and KwaZulu-Natal. Limpopo reported 62% of local cases, while KwaZulu-Natal reported only 6%.

Over the past 50 years, annual temperatures in South Africa have increased faster than the global average. The increases were most acute in Limpopo, where temperatures rose at a rate of 0.12°C every decade. Small annual shifts have big effects.

Higher temperatures increase the risk of malaria. This is because malaria mosquitoes and parasites are much happier at temperatures between 17°C and 35°C.

Warmer weather means that vector mosquitoes are able to evolve faster, invade new locations, and spread vector-borne diseases.

In addition, increased rainfall is likely to increase the number of mosquito vector breeding sites. Like the mosquitoes that transmit malaria, vector mosquitoes breed in stagnant, temporary bodies of water. Research in Limpopo has shown that heavy rains in the spring are usually associated with a higher number of malaria cases during the summer.

The impact of climate change on mosquitoes is very clear. But its effect on malaria transmission remains unclear. Some theoretical mathematical modeling studies predict a slight increase in the numbers of malaria cases due to climate change. But other models suggest that climate change will not have an effect on malaria. More data is needed to know the correct model. This is because the effect is difficult to test in the laboratory.

It has not been confirmed whether climate change will pose another challenge to achieving malaria elimination. Our research group is currently trying to address this knowledge gap.

What we know

The relationship between climate change and malaria is complex. But four things are clear: As the Earth warms, the malaria vector will evolve faster, allowing it to reproduce faster, bite more frequently, and expand into previously unsuitable habitats.

This means that mosquito larvae will develop into adults faster. The faster the bites, the faster the disease can be transmitted. If she bites repeatedly, she will spread more diseases.

The development of the malaria parasite within the mosquito is highly temperature dependent. At temperatures below 17°C, and above 35°C, the life cycle of the parasite inside the mosquito cannot be completed. This stops the transmission of malaria.

The transformation of mosquitoes from a larva to a free adult bird usually occurs at temperatures between 22°C and 34°C. Interestingly, research has shown that mosquitoes can change their behavior to spend most of their time resting in cooler places. This way they can survive when ambient temperatures rise. The mosquito’s behavior could help this parasite survive in temperatures that would otherwise stop its growth.

Characteristic changes are observed in the seasons, largely due to climate change. Southern Africa experiences more frequent days of intense heat and less cold days.

Therefore, winters are getting warmer, allowing mosquitoes to reproduce and transmit malaria in greater numbers during the winter months. The summer months are also getting hotter. In some cases, summer can be too hot for mosquitoes and parasites to thrive, preventing malaria transmission. Climate change could cause a shift in the malaria transmission season from the summer months to the cooler fall and winter months.

Precipitation also plays a major role in the transmission of malaria. In general, the incidence of malaria decreases during El Niño years (hotter but drier) and increases in La Niña years (cooler but wetter). This is particularly true in countries such as South Africa, where the adaptable malaria vector, Anopheles arabiensis, is the predominant vector. South Africa is currently going through a La Niña cycle so the next malaria season (October to February) is likely to be significant, given the conditions most favorable for malaria transmission and the relaxation of all COVID-related restrictions on movement.

The situation in South Africa

Our research group based at the National Institute of Infectious Diseases and the University of the Witwatersrand Institute of Malaria Research participated in identifying the mosquitoes behind the malaria epidemic in 2000. This outbreak coincided with severe flooding in southern Mozambique. The massive increase in available breeding sites allowed an insect-resistant mosquito from Mozambique, Anopheles funestus, to re-invade KwaZulu-Natal, resulting in an increased incidence of malaria.

Since this outbreak, our group has conducted extensive surveillance in endemic districts in South Africa. We have also been involved in research to understand the impact of climate change on malaria transmission in Southern Africa.

Research from our vector laboratories has shown that global warming would reduce the effectiveness of insecticides used in IRS. Additionally, pesticide-resistant mosquitoes appear to be more adapted to survive in warmer conditions than insecticide-sensitive mosquitoes.

Modeling experiments indicate that moisture levels will also affect the transmission of malaria in southern Africa. But this needs to be confirmed in laboratory conditions using live mosquitoes.

what should be done?

Clearly, the relationship between climate change and malaria is complex. More work needs to be done to understand this relationship so that effective control measures can be put in place. Crucially, malaria hotspots should be targeted for surveillance in order to understand the role of microclimate in malaria transmission. A microclimate is a set of local climatic conditions that may differ from the climate in general.

At present, there is no evidence of the breadth of areas at risk for malaria in Southern Africa. However, regardless of the climate or whether you’ve traveled this summer, it’s even more important to think about malaria when it comes to unexplained fevers. Learn the symptoms of malaria, how to reduce your risk of contracting it, and what to do if you suspect you have malaria.

Introduction of the conversation

This article has been republished from The Conversation under a Creative Commons license. Read the original article.Conversation This article has been republished from The Conversation under a Creative Commons license. Read the original article.Conversation

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