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An international collaboration comparing the DNA of four species of parasite that cause malaria has identified key parasite genes that could be used to create a vaccine preventing people with the disease from spreading it to others.

The researchers, who published their findings last week (7 January) in Science, compared one of the parasites responsible for human malaria with three species that cause the disease in rodents.

By studying the parasites' complete set of DNA — or genome — the researchers were able to look at the way the parasites' genes switch on and off at different stages of their life cycle.

During its lifespan, the malaria parasite changes form a number of times. The form of the parasite causing disease in humans is very different from the form transmitted by the bite of a mosquito or the form that mosquitoes pick up when they take their 'blood meal'.

The researchers identified a 'DNA signature' of genes that are important to the latter form of the parasite. They hope their finding could lead to 'transmission-blocking' vaccines, which would stimulate the human immune system to attack that form of the parasite.

If successful, the vaccine would stop infected people from spreading the disease, but would not protect people from parasites injected by the bite of a mosquito.

Vaccine researchers are now trying to develop vaccines comprising multiple subunits that would combine a vaccine that blocks transmission with one that protects health by working against the form of the parasite causing symptoms of malaria.

According the researchers, the parasite genes involved in interactions with its mosquito and mammal hosts are those that seem to be evolving most rapidly. Lead author of the Science paper Neil Hall, of the UK-based Wellcome Trust Sanger Institute, told SciDev.Net that this is because the parasite is always trying to evade the host's immune response and needs to change rapidly to avoid detection.

Thus, says Hall, "even without knowing the function of a gene, if you can tell it is evolving rapidly, it is likely to be involved in host interactions — which makes it a good vaccine candidate".

Parasite genes that are 'switched on' in the mammal host have evolved more rapidly than those active in the mosquito — the more complex the host, explains Hall, the more the parasite has needed to evolve.

Link to paper by Hall et al in Science

Reference: Science 307, 82 (2005)