SCIENCE IN THE NEWS - Diseases Spread by Mosquitos（在线收听）

SCIENCE IN THE NEWS - Diseases Spread by Mosquitos
By Nancy Steinbach

Broadcast: Tuesday, January 25, 2005

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VOICE ONE:

This is SCIENCE IN THE NEWS, in VOA Special English. I'm Barbara Klein.

VOICE TWO:

 
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And I'm Bob Doughty. On our program this week, we tell about diseases spread by mosquitoes -- the most widely hated insects in the world.

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VOICE ONE:

Mosquitoes are very small insects. There are more than two thousand different kinds of mosquitoes. Female mosquitoes bite people to drink their blood. Male mosquitoes do not drink blood. They drink fluids from plants.

The female mosquito uses its long thin sucking tube to break the skin and find blood. The insect injects the victim with a substance that keeps blood flowing.

The female mosquito drinks the blood and uses it to produce eggs. One meal gives her enough blood to produce as many as two hundred fifty eggs. The mosquito lays them in any standing water.

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The eggs produce worm-like creatures in two days to a few months. However, some eggs can stay in water for years until conditions are right for development. The worm-like creatures feed on organisms in the water. After four to ten days, they change again, into creatures called pupas. The pupas rise to the surface of the water. Adult mosquitoes pull themselves out of the pupas and fly away.

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VOICE ONE:

The World Health Organization says mosquitoes cause disease and death for millions of people throughout the world. That is because mosquitoes can carry organisms that cause disease. However, the disease does not affect mosquitoes.

 
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W.H.O. officials expressed concern about the possible spread of disease after the major earthquake in the Indian Ocean last month. The earthquake produced huge waves that killed thousands of people. The waves destroyed many villages and left floodwaters in coastal areas. The officials have warned that the floodwaters could increase the risk of diseases spread by mosquitoes.

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The most important disease spread by mosquitoes is malaria. More than three hundred million people become infected with malaria each year. At least one million die from it every year. The disease is found in Africa, Asia, the Middle East and South America.

Malaria parasites enter a person's blood through a mosquito bite. These organisms travel to the liver. They grow and divide there. After a week or two, the parasites invade red blood cells and reproduce thousands of times. They cause the person's body temperature to rise. They also may destroy major organs. People with malaria may suffer kidney failure or loss of red blood cells.

VOICE ONE:

Some drugs are generally effective in preventing and treating malaria. They are designed to prevent the parasites from developing in the body. The most commonly used malaria prevention drugs are chloroquine, mefloquine and doxycycline.

People die from malaria because they are not treated for the disease or the treatment is delayed. Health officials are increasing efforts to reduce the number of deaths from malaria.

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Dengue fever is another disease that is carried by mosquitoes. The insects can survive in new and different environments. They can spread diseases to new areas. For example, experts say only nine countries had dengue fever before Nineteen-Seventy. Since then, the disease has spread to more than one hundred countries around the world.

The World Health Organization says about fifty million people suffer from dengue fever each year. There is no cure. Children may develop a kind of the disease that is not serious. They may have a high body temperature and some areas of skin may turn red.

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Older people suffer from dengue fever much more. They may develop reddish skin and lose their sense of taste. They also may have terrible pain in the head or behind their eyes. And they may experience pain in joints such as the elbow or knee. This kind of joint pain is the reason why dengue fever is sometimes known as breakbone fever.

The most severe kind of the disease is called dengue hemorrhagic fever. People who have this disease bleed from the nose or other openings in the body. Dengue hemorrhagic fever kills about five percent of all people it infects. The only treatment involves controlling the bleeding and replacing lost body fluids.

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Yellow fever is another disease carried by mosquitoes. There are no effective drugs against yellow fever. Doctors can only hope that a person's defense system is strong enough to fight the disease. The World Health Organization says there are an estimated two hundred thousand cases of yellow fever each year. It is found mainly in Africa, northern South America and the islands of the Caribbean Sea.

A virus causes yellow fever. A few days after a mosquito bite, the victim experiences high body temperature and pain in the head or muscles. Victims also may expel food they ate. Most patients improve after three to four days.

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However, fifteen percent of patients develop a more serious condition. High body temperatures re-appear and the body turns yellow in color. The victim bleeds from the nose, mouth, eyes or stomach. Half the people with this condition die within ten to fourteen days.

A vaccine can prevent yellow fever. The vaccine strengthens the body's defense system against the disease. Medical experts say the vaccine is safe and very effective. The protection continues for at least ten years and possibly for life.

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Mosquitoes also carry lymphatic filariasis, a disease commonly known as elephantiasis. The disease has already affected more than one hundred twenty million people. One third of those infected live in India. Another one third are in Africa. The others live in South Asia, the Pacific Ocean, or the western half of the world.

Mosquito bites spread the worms that cause elephantiasis. People usually begin to develop the disease as children. Many children never experience signs of the disease. But it may cause hidden damage to the body's lymphatic system and kidneys.

The worst signs of elephantiasis appear in adults. The signs are more common in men than in women. These include damage to the arms, legs, and reproductive organs. Two drugs are effective in treating the disease. Experts say that keeping the affected areas clean can decrease the damage and reduce the number of times that it takes place.

VOICE ONE:

Still another disease carried by mosquitoes is encephalitis. It causes an infection or swelling of the brain. Many different viruses cause different kinds of the disease. One virus lives naturally in birds and horses. Mosquitoes spread it to people. Mosquitoes in several Asian countries spread a kind of encephalitis known as Japanese encephalitis. A vaccine can prevent this sickness.

Other kinds include West Nile encephalitis, Saint Louis encephalitis and Eastern Equine encephalitis. Most healthy people infected with the virus show no signs. Or they become sick for only a day or two. But those with weak natural defenses may develop a severe infection. They may suffer from high body temperature, headache, shaking and even death.

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VOICE TWO:

Experts have learned many things about mosquitoes. For example, the insects can smell carbon dioxide in the breath of a person or animal from as far away as sixty meters. Mosquitoes often like the blood of animals better than the blood of people.

Mosquitoes like dark colors. They do not bite women who are bleeding during their fertility period. But they do bite pregnant women. Many kinds of mosquitoes are most active in the early morning and early at night. They eat mostly at night.

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Experts say the best way to prevent the diseases carried by mosquitoes is not to be bitten by one. There are several ways to prevent mosquito bites. Do not keep standing water anywhere near your home.

Remove all containers that could provide a place for mosquitoes to live. Stay in an enclosed area when mosquitoes are most active. Wear clothes that cover most of the body. Other ways to prevent mosquito bites are to put anti-insect chemicals on the skin, clothing and sleeping areas. Also, place special nets treated with insect poison on windows and over the bed at night.

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VOICE TWO:

This SCIENCE IN THE NEWS program was written by Nancy Steinbach. Cynthia Kirk was our producer. The engineer was Eva Nenicka. I'm Bob Doughty.

VOICE ONE:

And I'm Barbara Klein. Join us again next week for more news about science in Special English on the Voice of America.