promising bunch of killers
In the heyday of the war on malaria, widespread DDT-spraying campaigns were used to stifle the pesky bloodsuckers. But DDT played havoc with the environment -- it killed birds and beneficial insects as well as anopheles mosquitoes. And the mosquitoes eventually gained resistance to DDT, and then to newer insecticides.
And that's led to a new attitude among people whose job is to make a mosquito's life nastier, more brutish, and particularly, shorter. The idea of "using a magic bullet against mosquitoes is history," says James Becnel of the mosquito and fly research unit of the U.S. Agricultural Research Service. "The idea of eradicating mosquitoes is not realistic." But Becnel does think a more realistic goal is achievable: "the reduction and management of the population below the threshold that would cause disease" through a combination of:
One biological control that might come to fruition in the next few years is a protozoan parasite that Becnel is helping develop. This so-called microsporidia was discovered in Thailand; it produces spores in adult female Aedes aegypti mosquitoes, the little devils that carry yellow fever and dengue fever.
In the progeny (offspring) of the infected adults, the organism lives in a fat body that stores energy for the mosquito. The spores don't kill the female, but they do infect her eggs.
Can we get to the part where the mosquitoes die?
As these larvae mature into adults, they carry the infection, thus continuing the cycle of infection and dispersing the parasites to new habitats. Since the adult mosquitoes must feed their internal parasites, they're neither as vigorous nor as long-lived as normal mosquitoes.
I like the idea of making mosquitoes
miserable. Would that help?
The effort is a "classic biocontrol," Becnel says, an attempt to artificially introduce agents that prey on the target. "These are pests that have no natural enemies," he says, "so the more enemies you can introduce, the lower their population will be." And the beauty of this approach is this: it should be able to sustain itself without human help in the field.
Is that the only biocontrol on the horizon?
Fungus. The Lagenidium giganteum fungus, developed to control mosquitoes in California farm fields, is already approved for use in pastures, soybean and rice fields. These fields are major sources of mosquitoes.
Transplanting genes of natural insecticides. Scientists at the National University of Singapore have moved genes from Bacillus sphaericus to a more hardy bacterium. B. sphaericus kills mosquitoes, but it dies quickly under sunlight outdoors. The scientists want to use the bioengineered bug as a hardier delivery system to carry the same safe, natural-killer punch to mosquitoes.
Blue-green algae. In a related effort, genes from a strain of Bacillus thuringiensis, another well-known insect killer, have been moved into a blue-green alga. While scientists at University of Memphis try to persuade the transgenic alga to produce higher amounts of toxin, researchers in Hawaii are figuring out how to grow the plant commercially. (See "Mauling Mosquitoes Naturally" in the bibliography).
Cheap ways to prevent malaria
Clive Shiff, a malaria expert at the Johns Hopkins School of Public Health, was part of a project that sold the treated nets to parents of 20,000 children in Tanzania. (By selling the nets, the researchers hoped to give the technique more staying power with the families, Shiff explains. He adds that the money went into village operating funds.)
All children received anti-malarial drugs when the six-month study began, and whenever they showed signs of the disease. The treatment group slept under treated nets, but the control group did not. Otherwise, there was no difference among them.
Like to nap after dinner?
Photo by Dept. of Bio., U. of Alberta, (c) BIODIDAC.
If that resting place happens to be an insecticide-treated net, it can become the bloodsucker's final resting place. And since malaria can only spread if a mosquito feeds on an infected person and then on a healthy person, the chain of infection is broken.
The technique was cheap: the nylon nets cost about $4, and the pyrethroid insecticide, made from plants, 15 to 25 cents. The insecticide must be reapplied about every six months.
Cheaper is better
As for the families in his study, he says 50 to 60 percent continue using the nets (that's down somewhat from 75 to 80 percent who used them during the study. See "Changes in Weight Gain..." in the bibliography.
So how might climate change affect mosquito-borne diseases tomorrow?
©2002, University of Wisconsin, Board of Regents.