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Epidemiology Bill Brieger | 08 Feb 2009 09:24 am

Darwin at 200, as malaria evolves

Charles Darwin’s 200th birthday is this Thursday, 12 February 2009, and Darwin published his masterwork, On the Origin of Species, 150 years ago and died in 1882. This was just two years after Alphonse Laveran identified the parasite that caused malaria.  Since then it has been possible to learn how the wheels of evolution roll along for humans, mosquitoes and the plasmodia species that cause malaria. These lessons aid in the continuing battle to save human life.

Within humans, evolution of “the protective effects of sickle cell trait (HbAS) against severe malaria and the resulting survival advantage are well known.” Malaria Journal reports that “protection against mild malaria episodes” among those with the trait may also result in improved child nutrition and reduced stunting. A reduced survival rate of SS fetal genotype may be associated with placental malaria. The LA Times also describes the human side of the malaria evolution triangle:

Among the genes whose purpose is understood, the biggest category is devoted to fighting infectious diseases. For instance, the researchers found more than a dozen new genetic variants involved in fighting malaria to be spreading rapidly among Africans.  Scientists had previously identified several mutations that offered protection against the disease. Most were shared by people of African descent, because the scourge is most widespread on that continent. But malaria afflicts people throughout the tropics and subtropics, and additional mutations to combat the disease arose in Thailand and New Guinea, Hawks said. One of the newly discovered mutations helps defend against a form of the disease in which malaria parasites congregate in blood cells in the placenta, causing a high rate of miscarriage.

Issues like drug resistance by malaria parasites provide focus to the discussion on continuing evolution of the malaria parasites.  Durand and colleagues explain that, “Drug development programs exhibit a high attrition rate and parasite resistance to hemotherapeutic drugs exacerbates the problem. Strategies that limit the development of resistance and minimize host side-effects are therefore of major importance.” They report on efforts to use an understanding of evolution to design better malaria drugs.

The mosquitoes themselves evolve to adapt to changing environments, human behavior and pesticides. Muller and co-researchers encourage continued study on how “Insects exposed to pesticides undergo strong natural selection and have developed various adaptive mechanisms to survive.”

Resistance to pyrethroid insecticides in the malaria vector Anopheles gambiae is receiving increasing attention because it threatens the sustainability of malaria vector control programs in sub-Saharan Africa. An understanding of the molecular mechanisms conferring pyrethroid resistance gives insight into the processes of evolution of adaptive traits and facilitates the development of simple monitoring tools and novel strategies to restore the efficacy of insecticides.

With continued changes in human behavior such as in agriculture and urbanization, as well as human impact on the environment, malaria will continue to evolve. Darwin has given a guide to understanding the complex evolution of malaria. Two hundred years on, we must continue to fund research that advances this understanding and uses it to keep ahead of the disease.

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