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Malaria Mosquitoes Evolving

The mosquito species most responsible for spreading malaria in Africa seems to be evolving into 2 separate species with different traits, researchers have found. The development may complicate efforts to control the disease.

Malaria is one of the most common infectious diseases in the tropical world and an enormous public health problem. It can bring fever, chills and flu-like illness. Left untreated, malaria can cause life-threatening complications. Each year, up to 3 million people die of the disease worldwide. The majority are young children in Sub-Saharan Africa.

Malaria is caused by a single-cell parasite called Plasmodium. Female mosquitoes can become infected after feeding on an infected human. They, in turn, can infect a new person when they feed again.

Anopheles gambiae is the mosquito that most commonly spreads human malaria in sub-Saharan Africa. New populations of the insect evolve to exploit changing habitats and seasonal conditions, but these populations can’t always be distinguished from each other by traditional means. Genetic studies have found that there are at least 2 physically indistinguishable forms of A. gambiae, dubbed M and S. The success of mosquito control efforts depends on understanding the characteristics of these different mosquito populations, such as their feeding behavior and susceptibility to insecticides.

Two teams of researchers set out to use genomics to better understand the characteristics of A. gambiae populations. A group led by researchers at Imperial College London studied the complete M and S genome sequences. Another group, led by scientists at the Broad Institute of MIT and Harvard, analyzed single nucleotide polymorphisms (SNPs)—single differences in DNA sequence—among different A. gambiae populations. The projects were partly supported by NIH's National Institute of Allergy and Infectious Diseases (NIAID) and National Human Genome Research Institute (NHGRI).

The findings appeared in back-to-back papers in the October 22, 2010, issue of Science. When the researchers compared the genomes of the A. gambiae M and S forms, they found them to be more different than expected. The results suggest that the 2 forms may be developing into 2 separate species. The SNP analysis revealed that different populations can quickly evolve to behave differently and thrive in different habitats.

These studies lay a foundation for further investigating these emerging species. Future studies can now explore how genetic changes affect the ability of different mosquito populations to compete in various habitats. The results will be used to refine existing malaria interventions and inform the development of new disease prevention strategies.

"From our new studies, we can see that mosquitoes are evolving more quickly than we thought and that unfortunately, strategies that might work against one strain of mosquito might not be effective against another,” says one of the lead authors, Dr. Mara Lawniczak of Imperial College London. "It's important to identify and monitor these hidden genetic changes in mosquitoes if we are to succeed in bringing malaria under control by targeting mosquitoes."

—by Harrison Wein, Ph.D.