Estimation of malaria risks based on oversimplified temperature data

25 August 2010

When making malaria predictions you must not look at the average monthly temperature but rather at variations around the average temperature. These have proven to be far more important for the development and survival of malaria parasites and malaria mosquitoes. Rubicon researcher Krijn Paaijmans published this research finding in the journal PNAS. This new insight is important for determining the risk of malaria during climate change, but also for the control of the malaria mosquitoes with fungi, for example.

It is estimated that some one million people die from malaria each year, mainly in Africa. Estimating the risk of malaria is important for the effective control of this disease. The risk depends, for example, on five conditions: how many mosquitoes there are; how often they bite; how long they live; how quickly the parasite develops in the mosquito; and how many infected mosquitoes can make a person ill. These five risk factors were found to have different values at constant temperatures than at variable temperatures.

The use of constant temperatures in the laboratory situation to estimate the risk of malaria gives a distorted picture. The majority of malaria mosquitoes live in areas where the temperature can easily vary by 12°C or more per day. Fluctuations around a cold average temperature can accelerate processes such as development, whereas fluctuations around a warm average temperature delay the development.

Fungi

Paaijmans' research result is important for the development of a new form of malaria control, namely killing mosquitoes with fungi. Mosquitoes, but also other parasites, are once again becoming resistant to new chemical agents on the market. Fungi are a good alternative. Controlling mosquitoes with fungi is not only biologically responsible but can also increase the sensitivity of a mosquito for existing insecticides.

Initially, Paaijmans wanted to study the effect of a mosquito-killing fungus under realistic conditions and therefore not as usual at constant temperatures. Yet because it can take a fungus several days to several weeks to kill a mosquito, it was important to know how long the malaria parasite in the mosquito takes to complete its entire development. The fungus had to be capable of killing a mosquito before the malaria parasite made it life threatening. Paaijmans discovered that the development rate of the malaria parasite was strongly dependent on temperature variations.

A mosquito picks up parasites that multiply in a sort of bag on the mosquito’s stomach. Once they are fully developed, the bag bursts open and some of the many parasites ultimately end up in the salivary glands. When the mosquito bites they are 'spat' into the human victim. The development time in the bag varies from one to several weeks, dependent on the temperature.

Climate change

A widely held view is that a warmer climate will lead to more malaria, although scientists do not yet agree about this. The effect of temperature variations on the transmission of malaria discovered by Paaijmans has made this discussion even more complex still. After all, climate change can also exert a considerable influence on temperature variations.

In cold areas, such as the highlands in Africa, processes such as the growth of mosquitoes and malaria will now proceed more quickly than was assumed, and relatively speaking will speed up less if it becomes warmer. In warm areas, these processes can now proceed more slowly than we thought, and will become even slower still if the temperature increases. In a previous PNAS article, Paaijmans had already demonstrated that the risk of malaria is overestimated in warmer areas and underestimated in colder areas.

Paaijmans' article in PNAS (Proceedings of the National Academy of Sciences) was published on Tuesday 24 August 2010. He is working at the Pennsylvania State University, in the United States.

Rubicon

Paaijmans' research was funded by the Netherlands Organisation for Scientific Research (NWO). Paaijmans received a Rubicon grant from NWO. With Rubicon, Dutch scientists who have recently gained their PhDs can gain research experience abroad.

About NWO

The Netherlands Organisation for Scientific Research (NWO) is the principal Dutch science funding body and its mission is to facilitate excellent scientific research in the Netherlands by means of national competition. Each year NWO spends more than 700 million euro on grants for top researchers, on innovative instruments and equipment, and on institutes where top research is performed. NWO funds the research of more than 5300 talented researchers at universities and institutes. Independent experts select proposals by means of a peer review system. NWO facilitates the transfer of knowledge to society.

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last modified on 10 September 2010