Effect of climate change on malarial mosquitoes may depend on daily temperature fluctuations

16 Feb 2009

Paul Chinnock

Source: Eureka Alert (see original article)

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Anopheles gambiae: adult female bloodfeeding on human skin.
WHO/TDR/Stammers.

Scientists studying the likely effects of global warming on malarial mosquitoes have focused on changes in average annual or monthly temperatures, but a leading entomologist says that the variations within each day could be the crucial factor.

It has been known for some time that temperature influences the speed at which malaria parasites develop in mosquitoes, but Professor Matthew Thomas of Penn State University, USA says that temperature’s effects are more complicated than previously thought and he points out that a day in the tropics may vary from, for example, 18 °C at night to 30°C in the day, even though the daily average may be 25°C: “Our research suggests this fluctuation matters because it alters the parasite incubation period in the mosquito, which is the most important factor in the spread of malaria.”

Adult female Anopheles mosquitoes can live for up to eight weeks but most die within two or three weeks. Malaria parasites must therefore complete their development before the last time a female feeds to infect humans. The warmer the ambient temperature, the faster the malaria parasite develops. If the incubation period takes longer than the life of the mosquito, the parasite will never infect a human. In some places, especially at higher elevations, malaria does not exist or is seasonal because, with cooler temperatures the mosquitoes die before the parasites are mature.

“Daily temperature fluctuation can increase or decrease malaria risk, depending on background conditions,” says Professor Thomas. During the first 12 hours of parasite development, temperature fluctuations can be fatal. Most mosquitoes bite to feed on blood in the evening or at night, after which temperatures remain low for 12 hours. Others feed much closer to morning. If the morning feeders face rapidly rising daytime temperatures within the next 12 hours, then the malaria parasite development can be stopped.

“If climate change increases the frequency of days when the temperature quickly exceeds the threshold temperature, then entire cohorts of mosquitoes could fail to develop the parasite,” says Thomas.

“We need higher resolution environmental and biological data to understand how climate change will affect the spread of the malaria parasite,” says Matthew Thomas. Unfortunately, the areas where we need to get more sensitive temperature readings are also sometimes the most difficult places to obtain data.”

Professor Thomas was speaking at the 2009 Annual Meeting of the American Association for the Advancement of Science.