After a break of two weeks, here is my latest Mind and Matter column in
the Wall Street Journal:
April 25 is World Malaria Day, designed to draw attention to the
planet’s biggest infectious killer. The news is generally good.
Never has malaria, which is carried by the Anopheles mosquito, been
in more rapid retreat. Deaths are down by a third in Africa over
the past decade alone, and malaria has vanished from much of the
world, including the U.S.
As so often happens in the battle against disease, however,
evolution aids the enemy. The selection pressure on pathogens to
develop resistance to new drugs is huge. In recent weeks, the
emergence on the Thai-Myanmar border of malaria strains resistant
to artemisin, a plant-derived drug, have led to pessimistic
headlines and reminders of the setback caused by resistance to the
drug chloroquine, which began in the 1950s.
For this reason, prevention generally works better than cure in
eradicating infectious diseases: Vaccination beat smallpox, clean
water beats cholera, less crowded living beats tuberculosis and
protection from mosquitoes beats malaria. Good prevention keeps bad
evolution from getting started. Yet two can play at evolution. The
newest weapon in the fight against mosquitoes ingeniously turns the
evolutionary tables on the pests.
The mosquito Aedes aegypti lives almost
exclusively in human settlements, breeding in small pools inside
discarded objects like car tires and coconut husks. Dengue fever,
which it carries, now infects at least 50 million people a year and
rising. Both Aedes and dengue have evolved to exploit dense urban
settlements.
Yet this also makes the mosquito vulnerable to a new control
technique developed by a former Oxford University scientist named
Luke Alphey. He genetically modified mosquitoes so that they would
produce no viable progeny unless supplied with a dietary
supplement. His idea was to release the modified males (only the
females bite humans) and let them mate with wild females, whose
offspring would then die. He can also make females genetically
flightless or doomed to die young.
A similar technique eradicated the screw worm (a blowfly maggot
that infects livestock) from North and Central America. Gamma rays
made male flies sterile but didn’t affect their ability to mate,
and since females only mate once, this led to an epidemic of
infertility in wild flies. The beauty of this technique is that it
generates the opposite of diminishing returns: The rarer the pest
gets, the more likely it is that the released males will mate with
the few remaining females.
But gamma rays damage mosquitoes too much, so a subtler,
gene-based form of sterilization was needed. Enter Dr. Alphey and
his company Oxitec, which last year announced the results of a
trial of his technique in the Cayman Islands, showing that released
male Aedes mosquitoes did indeed succeed in mating with wild
females. Further tests are planned for Key West, Fla., and other
areas before moving into larger cities.
Predictably, perhaps, the genetic modifications have led to
objections from some Western pressure groups, showing their now
customary tendency to elevate theoretical principles above the
battle against human suffering. Yet the great advantage of Dr.
Alphey’s approach, in contrast to the fogging of dengue-affected
areas with insecticide, is that it is pest-specific. No other
insect is hurt.
It will work best for a mosquito like Aedes that breeds in
modest numbers and only in urban refuse. Common Culex mosquitoes,
which also breed in ditches, sewage systems and rural habitats, may
be too widespread to be controlled this way. But there is another
fastidious mosquito genus that prefers small pools close to human
habitation and that could also be vulnerable to a campaign of
genetic sterilization: Anopheles, the carrier of malaria. With such
a technique, the eventual eradication of human malaria from the
planet is far from being an impossible dream.