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SCIENCE IN THE NEWS - August 28, 2001: Diseases Spread by Mosquitoes - 2001-08-27


VOICE ONE:

This is Sarah Long.

VOICE TWO:

And this is Bob Doughty with Science in the News, a VOA Special English program about recent developments in Science. Today 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 winged 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 a blood vessel. The insect injects the victim with a substance that keeps blood flowing. This substance makes the skin around a mosquito bite uncomfortable for several days. The female mosquito drinks the blood and uses it to produce eggs.

One meal gives a female mosquito enough blood to produce as many as two-hundred-fifty eggs. The mosquito lays them in any standing water. This includes small containers near peoples’ houses.

VOICE TWO:

The eggs produce worm-like creatures in two days to a few months. However, some mosquito eggs can stay in water for years until the conditions are right for hatching. 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 top of the water. The 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 when they bite a person, mosquitoes can also inject organisms that cause disease. Mosquitoes are not affected by the disease.

The most important disease spread by mosquitoes is malaria. As many as five-hundred-million people suffer malaria each year. About two-million people die from the disease each year. The disease is found in South America, Africa, the Middle East and Asia.

Malaria parasites enter a person’s blood through the mosquito bite. The parasites 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 severe fevers and may destroy major organs. People with malaria may suffer kidney failure or the loss of red blood cells.

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. International health organizations are increasing efforts to reduce the number of deaths from malaria.

VOICE TWO:

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 until Nineteen-Seventy. Since then, people in more than twenty-nine countries in Asia and the Caribbean have developed the disease.

The World Health Organization says about fifty-million people around the world suffer from dengue fever each year. There is no cure. Children may develop a kind of the disease that is not serious. Their skin may become covered with red spots and they may have a high body temperature.

Older people suffer from the disease much more. They may develop red spots on their skin. They also may have terrible headaches. They may lose their sense of taste. And they may experience pain behind their eyes and 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 body openings such as the nose. The disease kills about five percent of all people who get it. The only treatment involves controlling the bleeding and replacing lost body fluids.

VOICE ONE:

Another disease carried by mosquitoes is yellow fever. There are no effective drugs against the disease. Doctors can only hope that a person’s defense system is strong enough to fight the infection. World Health Organization officials say about two-hundred-thousand people suffer from yellow fever each year. It is found mainly in Africa, the Caribbean and South America.

The disease is caused by a virus. A few days after a mosquito bite, the victim experiences high body temperature, muscle pain, headache, nausea and vomiting. Most patients improve after three to four days.

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

A vaccine medicine can prevent yellow fever. 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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VOICE TWO:

Mosquitoes also carry the disease lymphatic filariasis, commonly known as elephantiasis. The disease affects more than one-hundred-twenty-million people in more than eighty countries. These include countries in South Asia, Africa, South America and the Pacific Islands. 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 the disease may cause hidden damage to the lymphatic system and kidneys.

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

VOICE ONE:

Still another disease carried by mosquitoes is encephalitis. It is 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 medicine 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 only slightly sick for a day or two. But those with a weak defense system 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 having their monthly period of bleeding. But they do bite pregnant women. Many kinds of mosquitoes are most active in the early morning and evening hours. They eat mostly at night.

VOICE ONE:

Medical 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 permit standing water anywhere around the house.

Remove all containers that could provide a place for mosquitoes to live. Stay indoors 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. And place special nets treated with insect poison on window screens and over the bed at night. Another way is to build a house for flying animals called bats on your property. Bats eat thousands of mosquitoes each night.

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VOICE TWO:

This SCIENCE IN THE NEWS program was written by Nancy Steinbach. It was produced by George Grow. This is Bob Doughty.

VOICE ONE:

And this is Sarah Long. Join us again next week for more news about science in Special English on the Voice of America.

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