The fight against malaria could eventually be transformed by releasing into
disease-ridden areas genetically modified mosquitoes that cannot transmit
Scientists in America have engineered a species of mosquito which is resistant
to the malaria infection. Its ability to block the infection suggests that
it could come to dominate mosquito populations if released into the wild.
The findings offer the strongest suggestion yet that engineering mosquitoes to
resist the parasite could help to control a disease that takes up to 2.7
million lives each year, chiefly in Africa. Malaria infects between 300
million and 500 million people each year. Only HIV/ Aids causes more deaths
from infectious disease.
Large numbers of GM mosquitoes would be released in areas where malaria is
common, where they would interbreed with wild ones. Over several
generations, resistance should spread through the mosquito population, so
that fewer insects carry malaria. However, this approach would prove
controversial with environmental groups, as it would involve supplanting a
naturally occurring species with a genetically engineered variant.
Critics have argued that it is difficult to be certain of the effects of
introducing new genes. Even the scientists involved accept that further
research is needed before any GM insects could be introduced into the wild.
Though the first GM mosquitoes were created seven years ago, they proved to be
less fit than their wild counterparts. This would mean that they would
quickly die out, and have no effect on malaria transmission.
But a new study, led by Mauro Marrelli, of Johns Hopkins University in
Maryland, reversed this position. The GM mosquitoes express a protein called
SM1 that blocks malaria infection, and a gene that makes their eyes glow red
or green, allowing them to be easily distinguished from wild insects.
The scientists found that while these modified mosquitoes have no advantage
when feeding on uninfected blood, they are much better adapted when blood
carries the malaria parasite. Infection with the Plasmodium organism
does not kill normal mosquitoes, but it does reduce breeding efficiency.
The GM mosquitoes did not suffer from this and over nine generations (several
months) they grew in number to make up 70 per cent of a laboratory
population, compared with 50 per cent at the outset.
“When fed on Plasmodium-infected blood, the transgenic
malaria-resistant mosquitoes had a significant fitness advantage over
wild-type,” the scientists wrote in Proceedings of the National
Academy of Sciences.
However, the species of both mosquito and malaria parasite used in the
experiment are not those that are most harmful to humans. The mosquito was
the Anopheles stephensi species, the main Asian vector, but the
Anopheles gambiae species is more likely to infect humans, particularly in
Africa where the malaria burden is worst.
The experimental parasite was Plasmodium berghei, which does not infect
humans but is considered a very good laboratory model for Plasmodium
falciparum, the most dangerous of the four strains that do.
A further problem is that only a very small proportion of wild mosquitoes are
exposed to malaria, and the transgenic insects did not have a competitive
advantage when the parasite was not present. This would slow the rate at
which they might have an impact on malaria transmission.
A different approach has been adopted by a British team, led by Andrea
Cristiani, of Imperial College, London. His team has developed a GM mosquito
in which the males have fluorescent testicles, allowing them to be easily
identified and sterilised. The goal is to introduce large numbers of sterile
males, which would mate with normal females, reducing the number of eggs
laid, and thus of malaria mosquitoes. As the mosquitoes are sterile, they
would not transmit transferred genes into wild populations.
The deadly bite
— The most severe form of malaria is caused by the Plasmodium
falciparum parasite, transmitted by the bite of the Anopheles
genus, particularly Anopheles gambiae. It is spread by pregnant
— Malaria killed people in the Fens until the 19th century
— It has been predicted that global warming may result in malaria
returning to Britain
— Malaria infects between 300 million and 500 million people a year,
mainly in Africa
— In sub-Saharan Africa, malaria affects mostly young children, with
almost 3,000 dying every day
— Symptoms include neck stiffness, convulsions, abnormal breathing and
fever of up to 40C (104F)
— Distribution of the tropical disease mainly affects developing
countries. About 90 per cent of cases are in Africa
— It costs £6.8 billion a year in Africa in lost GDP. Death and
disability lead to the loss of 45 million years of productive life each year
— Alexander the Great, Genghis Khan, Oliver Cromwell, Caravaggio and
David Livingstone are thought to have died of it
— Those who had it but recovered include Lord Nelson, Sir Arthur Conan
Doyle, Gandhi and Hemingway
Source: WHO / UN / Times database