Mosquito Bytes

 

1. Mosquito alert
2. Whining in your ear
3. Malaria
4. Illnesses expand (Dengue)
5. Death to mosquitoes
6. Climate change=more disease?
7. Advice for the weary (repellent pictured)
8.Stop already! (Q & A)

 

 

 

 

 

 

A promising bunch of killers
Death to Mosquitoes (illustration of a dead mosquito) 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:


Public education
Explaining that some disease-carrying mosquitoes breed in tires and other artificial containers, including flower pots around the house.


Behavioral controls Covering beds with insecticide-soaked mosquito nets.


Chemical controls Making smarter, more focused use of less hazardous insecticides.


Biological controls Deploying organisms that eat or otherwise harm mosquitoes.

graphic of mosquitoes life cycle
Some public education and simple tips for reducing mosquito growth.
Both graphics courtesy Food and Environmental Hygiene Department (Hong Kong).
tipsheet about mosquito prevention

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.

each  egg type is long and spherical, with slight differences between each in color and grouping.
Some different varieties of mosquito eggs. From left to right: Anopheles species, Aedes species, Culex species Courtesy Safe Food and Public Health.

Can we get to the part where the mosquitoes die?
The spores kill the larvae before they can mature, and the larvae disintegrate, releasing the infectious agents -- the spores. Any other larvae in the same container of water will in turn become infected, because mosquito larvae are "filter feeders" -- in other words, they obtain food by straining it from the water.

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?
Say a skeeter just dies young. Say it's short on get-up-and-go. Either way, if it doesn't suck out that second blood meal, it can't infect anybody with disease. The ultimate test, Becnel says, is this: "Are mosquitoes in the field actually taking fewer blood meals?"

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?
Hardly. In their understandable fervor to destroy, disable, or disarm mosquitoes, scientists are looking at a bunch of other techniques:

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
When it comes to defeating malaria, the most promising techniques may involve neither genetic engineering nor biological control. In several studies in Africa, a plain, ol' mosquito net (treated with insecticide) has protected large numbers of people.

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?
So does the average mosquito... a fact that, surprisingly enough, explains how the bed-net scheme worked. Shiff says the engorged insect (a polite term for a bug that's too heavy with human blood to move) needs a place to rest to digest its nutritious repast. (Does it like to read the Mosquito Monthly after dinner?)

body brown. Blood visible in thin proboscis, and abdomen is swollen with blood. drinking amidst hairs on the person's handA female mosquito takes her blood mead from a human hand 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
Economy is important, even in the control of an epidemic disease like malaria, Shiff says. While international agencies no longer regularly fund malaria control, several African countries are considering using treated nets to alleviate the enormous toll malaria exacts from their populations. (According to the World Health Report, published in 1996 by the World Health Organization, the estimated direct and indirect cost of malaria in Africa was US $1.8 billion in 1995.) "I think these low-tech, community-based interactions are the way to go," Shiff says. "These results must be widely publicized so the ministers of health can see them."

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?

 

 

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