[health-vn] Evolution-Proof Insecticides May Stall Malaria Forever

[Vern Weitzel]

http://www.terradaily.com/reports/Evolution_Proof_Insecticides_May_Stall_Malaria_Forever_999.html


EPIDEMICS
Evolution-Proof Insecticides May Stall Malaria Forever

"We are working on a fungal pesticide that kills mosquitoes late in life," said 
Thomas. "We could spray it onto walls or onto treated materials such as bed 
nets, from where the mosquito would get infected by the fungal spores."
by Staff Writers
University Park PA (SPX) Apr 08, 2009
Killing just the older mosquitoes would be a more sustainable way of controlling 
malaria, according to entomologists who add that the approach may lead to 
evolution-proof insecticides that never become obsolete.
Each year malaria - spread through mosquito bites - kills about a million 
people, but many of the chemicals used to kill the insects become ineffective. 
Repeated exposure to an insecticide breeds a new generation of mosquitoes that 
are resistant to that particular insecticide.

"Insecticides sprayed on house walls or bed nets are some of the most successful 
ways of controlling malaria," said Andrew Read, professor of biology and 
entomology, Penn State. "But they work by killing the insects or denying them 
the human blood they turn into eggs. This imposes an enormous selection in favor 
of insecticide-resistant mosquitoes."

Read and his colleagues Matthew Thomas, professor of entomology, Penn State, and 
Penelope Lynch, doctoral student, Open University, UK, argue that insecticides - 
chemical or biological - that kill only older mosquitoes are a more sustainable 
way to fight the deadly disease.

"If we killed only older mosquitoes we could control malaria and solve the 
problem of resistant mosquitoes," said Read. "This could be done by changing the 
way we use existing insecticides, even by simply diluting them," he added.

Aging mosquitoes are easier to kill with insecticides like DDT but new 
generation pesticides could do it too. Read and his colleagues are working with 
a biopesticide that kills older mosquitoes.

"It is one of the great ironies of malaria," explained Read, whose team's 
findings appear in PLoS Biology.

"Most mosquitoes do not live long enough to transmit the disease. To stop 
malaria, we only need to kill the old mosquitoes."

Since most mosquitoes die before they become dangerous, late-acting insecticides 
will not have much impact on breeding, so there is much less pressure for the 
mosquitoes to evolve resistance, explained Read, who is also associated with the 
Penn State Center for Infectious Disease Dynamics.

"This means that late-life insecticides will be useful for much, much longer - 
maybe forever - than conventional insecticides," he added.

"Insects usually have to pay a price for resistance, and if only a few older 
mosquitoes gain the benefits, evolutionary economics can stop resistance from 
ever spreading."

"We are working on a fungal pesticide that kills mosquitoes late in life," said 
Thomas. "We could spray it onto walls or onto treated materials such as bed 
nets, from where the mosquito would get infected by the fungal spores."

The fungi take 10 to 12 days to kill the insects. This achieves the benefit of 
killing the old, dangerous mosquitoes, while dramatically reducing the selection 
for the evolution of resistance, Thomas explained.

To study the impact of late-acting insecticides on mosquito populations, the 
researchers constructed a mathematical model of malaria transmission using 
factors such as the egg laying cycle of the mosquito and the development of 
parasites within the insect.

Once malaria parasites infect a mosquito, they need at least 10 to 14 days - or 
two to six cycles of egg production - to mature and migrate to the insect's 
salivary glands. From there they can pass into humans when a mosquito bites.

Analyses of the model using data on mosquito lifespan and malaria development 
from hotspots in Africa and Papua New Guinea reveal that insecticides killing 
only mosquitoes that have completed at least four cycles of egg production 
reduce the number of infectious bites by about 95 percent.

Critically, the researchers also found that resistance to late-acting 
insecticides spreads much more slowly among mosquitoes, compared to conventional 
insecticides, and that in many cases, it never spreads at all.

Read says the development of biological or chemical insecticides that are more 
effective against older, malaria-infected mosquitoes could save the millions 
dollars that will have to be spent to endlessly find new insecticides to replace 
ones that have become ineffective.

"Insecticides that kill indiscriminately impose maximal selection for mosquitoes 
that render those insecticides useless. Late-life acting insecticides would 
avoid that fate," Read added. "Done right, a one-off investment could create a 
single insecticide that would solve the problem of mosquito resistance forever."

S