[afro-nets] Mosquito isn't a Happy Host for Malaria, Tests Indicate

[Claudio Schuftan <claudio@hcmc.netnam.vn>]

Mosquito isn't a Happy Host for Malaria, Tests Indicate
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http://www.nytimes.com/2006/04/28/science/28malaria.html?_r=1&oref=slogin

Many mosquitoes seem to kill naturally the malaria parasites
they ingest, and it may be possible to exploit that genetic
trait to fight malaria, according to a study being published to-
day.

Researchers have long dreamed of inserting an antimalaria gene
into mosquitoes, but this study suggests for the first time that
this may be unnecessary because "most mosquitoes are malaria-
resistant and the susceptible ones are the oddballs," said Dr.
Kenneth D. Vernick, a microbiologist at the University of Minne-
sota and the lead author.

The study, appearing in the journal Science, is a "major step
forward" in understanding mosquito genetics, said Dr. Allan
Schapira, a coordinator of the World Health Organization's ma-
laria program. Dr. Dyann F. Wirth, chief of the malaria program
at the Harvard School of Public Health, called it "a nice piece
of work."

The discovery changes the terrain in the war on malaria, which
kills more than a million people a year, most of them children
and pregnant women. On this shifting battlefield, mutating para-
sites and mosquitoes eventually outwit each ingenious new drug
or pesticide created to destroy them.

Natural resistance in mosquitoes to the malaria parasite, plas-
modium falciparum, is good news for researchers because it is
theoretically easier to bolster an existing gene than to implant
one from another species. Also, the study found that the resis-
tance centers on a small section of one chromosome, rather than
on many diverse sites, making gene manipulation easier.

However, even if a better mosquito could be created in a lab,
the idea of releasing manipulated bugs into the wild to hunt hu-
man blood would be fraught with political perils. After lobbying
by environmental groups, some African countries now refuse food
aid containing genetically modified corn and are skeptical of
genetically modified seeds that may confer drought resistance.

As an alternative, Dr. Vernick suggested that a soil fungus that
devoured insects, whose mosquito-killing powers were described
by British scientists last year, could be used to hunt down the
most malaria-susceptible bugs in any swarm and knock them out of
the gene pool.

Dr. Schapira called the idea "interesting," but he cautioned
that years of testing would be needed to see if it was practical
and safe.

The fungus, Beauveria bassiana, is harmless to humans and ap-
proved for use on aphids. It grows in insects that land on sur-
faces where it has been sprayed.

It has long been known that fewer than 10 percent of any swarm
of mosquitoes in the wild will transmit malaria. The conven-
tional wisdom has been that this is just chance: a female must
first bite a human unlucky enough to be infected already, then
it takes about 14 days for the parasites to develop in her gut
and migrate to her salivary glands, from which they exit into
her next victim. Mosquitoes have short lives, and a female is
usually infectious for only her last few days.

This study makes it clear that genetics play a part, too, and
that mosquitoes are not just passive squirt guns for malaria
parasites.

Plasmodium parasites do hurt mosquitoes, Dr. Vernick and Dr.
Wirth said. They damage salivary tissue, make the mosquitoes fly
less vigorously and lay fewer eggs and, to gain a toehold in the
insect, may depress its immune system.

"The mosquito doesn't want to be infected, so it has responded
with this very powerful mechanism," Dr. Vernick said, referring
to what he called the "resistance island" on the mosquito ge-
nome.

By a very different route, the fungus also weakens mosquitoes;
they fly badly and bite less, and many die within 14 days. For
unknown reasons, it weakens plasmodium-carrying mosquitoes more
than it does others, Dr. Vernick said, so if a strain of the
fungus just strong enough to kill off old, weak, malarial mos-
quitoes could be developed, it could "tip the balance," he said.
It would suppress the malaria-susceptible mosquitoes without
creating mutation pressure on all mosquitoes to evolve a fungus-
resistant form, as DDT created pressure to evolve pesticide-
resistant forms.

For the study, scientists from the University of Minnesota, the
University of Bamako in Mali, the Fred Hutchinson Cancer Re-
search Center in Seattle and Princeton University collected wild
anopheles gambiae mosquitoes, the species most likely to spread
malaria in Africa, in villages in Mali.

They let each produce a generation of offspring, then let them
suck malaria-infected blood (drawn from a villager, but fed
through a plastic membrane). A week later, they dissected them
to see where the parasites grew. They were surprised to find
that 22 percent had no parasites at all, and that many others
had low numbers. Then they compared mosquito genomes and nar-
rowed the search with a gene they named APL1. When they disabled
it, they found that parasites grew well.