The war on malaria: Mosquitoes gain ground as search for new weapons intensifies

Gates Foundation/flickr 


By Cheryl Katz
Environmental Health News

July 29, 2013

KARATU, Tanzania – Dr. Frank Artress is loath to get into an arms race with mosquitoes. “You hate to drag out all the heavy poisons,” he says, standing in front of the medical clinic he and his wife built in this rural town. But to fend off the voracious insects and their payload of malaria parasites, he knows there are few other choices.

Artress, a physician from California, frowns as he looks out over the tiny earthen houses straggled across the flank of the Ngorongoro Crater. Their screenless windows and doors, open to damp forest and red, puddle-pocked fields, are bullseyes for mosquitoes. Like many communities in sub-Saharan Africa, Karatu is reliant on house nets laced with insecticides called pyrethroids to keep malaria at bay.

Carlos Avila Gonzalez, San Francisco Chronicle

But a decade of blanketing Africa with pyrethroids has fueled resistance to this front-line chemical weapon. Now pyrethroid-immune mosquitoes are spreading quickly throughout the continent.

“At some level, to really control the mosquitoes,” Artress says, “they’re going to have to do more.”

What that “more” is, however, is uncertain. Because of a lack of research, no new chemicals for killing malaria-infected mosquitoes have emerged in more than 40 years.

Now pesticide companies and public health agencies are trying to develop low-toxic and inexpensive – yet powerful and long-lasting – new insecticides. Other researchers are working on novel approaches such as genetically modifying mosquitoes so they can’t harbor parasites.

It's likely to be years before new tools are widely available. In the meantime, health officials say, pyrethroid failure could trigger a malaria resurgence that kills hundreds of thousands of people worldwide.

To fill the void, some are turning to “green” methods, such as botanical oils or other plants that keep mosquitoes away. Others are practicing mosquito birth control by draining ditches where they breed and stocking ponds with larvae-eating fish or larvae-killing bacteria.

However, for a growing number of communities battling malaria, the controversial pesticide DDT, banned in most of the world, may become a more frequent weapon of choice.

A disease of poverty, environment and race

The need for new safe and sustainable malaria-fighting tools is resounding throughout the world’s neediest regions, where the disease sickens an estimated 219 million people and threatens more than 3 billion.

Mosquitoes that are invulnerable to one or more approved indoor insecticides are already active in two-thirds of malaria-ridden countries, according to the World Health Organization. And that figure is probably a “gross underestimate,” said Abraham Mnzava, coordinator of malaria vector control for the World Health Organization’s Global Malaria Programme.

Sub-Saharan Africa and India are hit hardest, but resistant mosquitoes have turned up as far away as Bolivia, Turkey and China. The problem is compounded by the recent emergence of malaria parasites that are immune to the leading medication, artemisinin.

US Centers for Disease Control 

A disease of poverty, environment and race, malaria kills an estimated 660,000 people – mainly children – every year.

Once prevalent worldwide, malaria was quashed in affluent nations decades ago, largely due to efficient insect control and housing improvements. But as of last year, 104 countries were still plagued by virulent species of parasitic Plasmodium protozoa that live inside female Anopheles mosquitoes and invade human bloodstreams when the insects feed. Half the world’s population remains at risk for malaria, according to the WHO.

Making matters worse, warming temperatures, coupled with deforestation, crop irrigation and other man-made environmental changes, are driving mosquitoes to new grounds across the globe. In sub-Saharan Africa, malaria is now reaching higher altitudes than ever before.

Karatu is one such place. When Artress and his wife moved there seven years ago, they were told “no mosquitoes, you’re above the malaria line,” he said. But “apparently with global warming, there are actually a lot of mosquitoes.”

For communities first encountering the disease, the results can be devastating.

“The appearance of malaria mosquitoes in the African highlands is real,” Mnzava said. “These areas are now prone to malaria epidemics with severe consequences to local populations due to their lack of immunity.”

Pyrethroids losing effectiveness

Developed in the 1970s, pyrethroids are the last new class of pesticides produced for public health uses in nearly half a century.

These synthetic compounds, based on a natural substance found in chrysanthemums, are considered "safe," according to the Environmental Protection Agency. Animal studies, however, have found that pyrethroids alter developing brains, and there have been virtually no studies of their potential effects on human health.

Cheryl Katz

“Given uncertainty about the existence of long-term health effects of exposure to pyrethroids, particularly under realistic scenarios, we should be cautious when promoting pyrethroid products as safe methods for pest control,” a group of environmental health scientists in Canada recently advised.

Nonetheless, pyrethroids are the backbone of all major malaria-control programs. Pyrethroid treatments, provided in a massive, decade-long campaign by U.S. and international aid organizations, have made significant inroads against the disease in recent years. The insecticide saved more than a million lives between 2000 and 2010, according to the WHO.

But reliance on pyrethroids, the only insecticide permitted for mosquito nets and the main ingredient in most indoor sprays, has allowed insects with a gene conferring immunity to flourish. That immunity now threatens to roll back those gains.

The surge of resistance to antimalarial weapons is analogous to today’s spate of “superbugs” that are invulnerable to antibiotics. Both were set off by unsustainable use of chemicals.

“Attacking malaria around the world and using a lot of drugs will generate drug-resistant parasites. And attacking mosquitoes around the world and using a lot of insecticides will generate insecticide-resistant mosquitoes,” said Sir Richard Feachem, former executive director of the Global Fund to Fight AIDS, Tuberculosis and Malaria and current director of the Global Health Group at the University of California, San Francisco.

“We always knew that – no fundamental surprise here – we always knew it would happen,” he said. “What we didn’t know was how quickly it would happen.”

The potential impact is huge. If resistance renders pyrethroids in mosquito nets and sprays useless, the toll could be 120,000 malaria deaths and as many as 26 million new cases a year, according to the WHO.

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