Why the Resurgence?

ABOUT 40 years ago, classic insect-borne diseases like malaria, yellow fever, and dengue were thought to have been nearly eradicated from large areas of the earth. But then the unexpected happened​—insect-borne diseases began to reemerge.

Why? For one thing, some of the insects and the microbes they carry have developed resistance to the insecticides and medicines used to control them. This natural process of adaptation has been given a boost, not only by overuse of insecticides but also by misuse of medicines. “In too many poor households,” states the book Mosquito, “people obtain the medicine, use just enough to ease their symptoms, and then hoard the remainder for the next wave of illness.” With such an incomplete cure, the stronger microbes may survive in a person’s body to produce a new generation of drug-resistant offspring.

A Change in the Climate

An important factor in the resurgence of insect-borne diseases is change​—in nature and in society. A case in point is global climate change. Some scientists expect a warming global environment to expand the range of disease-carrying insects into presently cooler climates. There is some evidence that this may already be taking place. Dr. Paul R. Epstein of the Center for Health and the Global Environment, Harvard Medical School, notes: “Both insects and insect-borne diseases (including malaria and dengue fever) are today being reported at higher elevations in Africa, Asia, and Latin America.” In Costa Rica, dengue has spilled over the mountains, which until recently confined the disease to the Pacific Coast, and it now blankets the entire country.

But warmer weather can do more. In some areas it transforms rivers into puddles, while in others it triggers rains and floods that leave behind stagnant pools. In both cases the standing water serves as a perfect breeding ground for mosquitoes. Hotter weather also shortens the mosquitoes’ breeding cycle, speeding up their reproduction rate, and it lengthens the season during which mosquitoes abound. In warmer weather, mosquitoes are more active. Hotter temperatures even reach inside the mosquito’s gut and intensify the reproduction rate of disease-causing microbes, thereby increasing the likelihood that  a single bite will cause infection. Yet, there are other concerns.

A Case Study in Disease

Changes in human society can also contribute to insect-borne disease. To understand how, we need to take a closer look at the role of insects. In many diseases an insect may be only one of several links in the chain of disease transmission. An animal or a bird can serve as a host for a disease by carrying insects on its body or by harboring microorganisms in its bloodstream. If the hosts can survive this way, they may also become a reservoir of the disease.

Consider Lyme disease, identified in 1975 and named for Lyme, Connecticut, U.S.A., where it was first observed. The bacterium that causes Lyme disease may have come to North America a hundred years ago with rats or livestock on ships from Europe. After a tiny Ixodes tick ingests the blood of an infected animal, the bacteria remain in the tick’s gut for the rest of its life. When the tick later bites another animal or a human, it can transmit the bacteria to the victim’s bloodstream.

In the northeastern United States, Lyme disease is endemic​—it has been present there for a long time. The main local reservoir of Lyme disease bacteria is the white-footed mouse. The mice also serve as hosts for the ticks, particularly ticks in their developing stages. Adult ticks prefer to make their home on deer, where they feed and mate. Once engorged with blood, the adult female tick drops to the ground to lay her eggs, from which larvae soon emerge to begin the cycle anew.

A Shift in Circumstances

Pathogens have coexisted with animals and insects for many years without causing disease in humans. But a change in circumstances can turn an endemic disease into an epidemic​—a disease affecting many people in a community. What changed in the case of Lyme disease?

In the past, predator animals helped to limit contact between deer ticks and man by controlling the deer population. When early European settlers cleared the forests to farm, the deer population dwindled even further and the deer’s predators also moved  on. But during the mid-1800’s, many farms were abandoned as agriculture moved westward, and the forest began to reclaim the land. The deer came back, but their natural predators did not. Thus, the deer population rebounded explosively, and so did the tick population.

Some time later, the Lyme disease bacterium arrived and took up residence for decades before emerging as a threat to humans. However, when suburbs began to be built at the forest’s edge, children and adults in far greater numbers began to enter the ticks’ domain. The ticks found humans to attach themselves to, and the humans got Lyme disease.

Disease in an Unstable World

The foregoing scenario represents only one of the many paths taken by disease and only one example of how the actions of man influence its emergence. “Almost all the new improved diseases owe their comebacks to human meddling,” writes environmentalist Eugene Linden in his book The Future in Plain Sight. A few other examples: The popularity and speed of modern travel can spread pathogens and their carriers around the globe. Damage to the habitats of creatures both large and small threatens biodiversity. “Pollution flows into the air and water,” notes Linden, “weakening the immune systems of animals and  humans alike.” He adds Dr. Epstein’s summation: “In essence human tampering with ecology has weakened the globe’s immune system, fostering conditions favorable for microbes.”

Political instability leads to wars that damage ecosystems and destroy the infrastructures that provide health care and food distribution. Along with that, the Biobulletin of the American Museum of Natural History points out: “Refugees, malnourished and weak, are often forced into camps whose crowded and unsanitary conditions expose people to a range of infections.”

Economic instability drives human migration, both across and within national borders, primarily into crowded urban areas. “Pathogens like crowded places,” explains the Biobulletin. As city populations explode, “often essential public health measures, such as basic education, nutrition, and vaccination programs cannot keep pace.” Overcrowding also places an extra burden on water, sewage, and waste-disposal systems, making sanitation and personal hygiene difficult while at the same time creating conditions that foster insects and other disease carriers. Nevertheless, the situation is not hopeless, as the following article will show.

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“Almost all the new improved diseases owe their comebacks to human meddling”

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West Nile Virus Invades the United States

West Nile virus, transmitted to man primarily by mosquitoes, was first isolated in 1937 in Uganda and later observed in the Middle East, Asia, Oceania, and Europe. The virus was not detected in the Western Hemisphere until 1999. Since then, however, more than 3,000 infections have been reported in the United States and more than 200 individuals have died.

Most infected people are never aware of the infection, although some may develop flulike symptoms. But a small percentage develop serious illness, including encephalitis and spinal meningitis. There is as yet no preventive vaccine or specific treatment available for West Nile virus. The U.S. Centers for Disease Control and Prevention warns that West Nile virus may also be acquired through organ transplants or a blood transfusion from an infected donor. “There currently is no way to screen blood for the West Nile virus,” reported Reuters news service in 2002.

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CDC/James D. Gathany

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How Can You Protect Yourself? Some Dos and Don’ts

Awake! consulted residents of insect-infested and disease-ridden regions around the world for tips on staying healthy. You may find their advice to be helpful in your area.

Cleanliness​—Your First Line of Defense

Keep your home clean

“Cover food-storage containers. Keep cooked food covered until it’s served. Clean up food spills right away. Don’t leave dishes unwashed overnight or throw food garbage outdoors to be disposed of in the morning. Cover or bury it, since insects and rodents come out at night looking for food. Also, a thin layer of concrete over a dirt floor makes it easier to keep the home clean and insect free.”​—Africa.

“Store fruit or anything that attracts insects away from the house. Keep farm animals​—goats, pigs, chickens—​out of the house. Cover outdoor toilets. Bury animal droppings quickly or cover with lime to ward off flies. Even if neighbors don’t do these things, you may be able to keep insects to a manageable level and also set a good example.”​—South America.


Leaving food or garbage uncovered is like inviting insects to dine with you

Personal hygiene

“Soap is affordable, so wash hands and clothes often, especially after contact with people or animals. Avoid touching dead animals. Avoid touching your mouth, nose, and eyes with your hands. Clothes should be washed regularly even if they still appear clean. However, some fragrances attract insects, so avoid soaps and hygienic products that are scented.”​—Africa.

Preventive Measures

Eliminate mosquito breeding sites

Cover water tanks and washtubs. Eliminate all open containers that catch water. Do not let water stand in potted plants. Mosquitoes can breed in any puddle lasting longer than four days.​—Southeast Asia.

Minimize exposure to insects

Avoid the insects’ favorite feeding times and places. The sun sets early in the tropics, so many daily activities take place in the dark, when many insects are more active. Sitting and sleeping outside increase your risk when insect-borne disease is prevalent.​—Africa.


Sleeping outdoors in mosquito country is like inviting mosquitoes to dine on you

  Wear clothes that minimize exposed skin, especially when in the woods. Apply insect repellent to your clothes and skin, always following the directions on the label. Examine yourself and your children for ticks after spending time outdoors. Keep your pets healthy and free of insects.​—North America.

Minimize contact with farm animals, as insects can spread disease from them to humans.​—Central Asia.

Use mosquito nets​—preferably impregnated with insecticides—​for all members of the family. Use screens over windows, and keep the screens in good repair. Seal openings under eaves where insects can enter. Such preventive measures cost some money, but you will lose even more money if you must take a child to the hospital or if a wage earner becomes too sick to work.​—Africa.


Insecticide-impregnated mosquito nets are cheaper than medicine and hospital bills

Eliminate insect hiding places from your home. Plaster over walls and ceiling, and patch cracks and holes. Cover the underside of thatched ceilings with insect-proof cloth. Remove clutter​—such as piles of paper or cloth or clusters of pictures on the walls—​where insects hide.​—South America.

Some people regard insects and rodents as houseguests. They are not! Keep them out. Use insect repellents and insecticides​—but only according to instructions. Use fly traps and flyswatters. Be inventive: One woman made a cloth tube, filled it with sand, and put it in the space under the door to keep insects from entering.​—Africa.


Insects should not be our houseguests. Evict them!

Preventive medicine

Keep your resistance up by getting proper nutrition, rest, and exercise. Reduce stress.​—Africa.

Travelers: Inform yourself in advance of the latest information on risks. Data is available from public health departments and government Internet sites. Before you travel, take preventive treatment appropriate for the area you will visit.

If You Feel Sick

Get medical attention quickly

Most diseases are easier to cure when diagnosed early.

Beware of misdiagnosis

Seek doctors who are familiar with vector-borne diseases and with tropical diseases if appropriate. Tell your doctor all your symptoms and where you have traveled, even in the past. Use antibiotics only if necessary, and finish the course of treatment.


Insect-borne diseases can mimic other illnesses. Give your doctor a complete history

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Globe: Mountain High Maps® Copyright © 1997 Digital Wisdom, Inc.

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Do Insects Spread HIV?

After more than a decade of investigation and research, entomologists and medical scientists have found no evidence that mosquitoes or any other insects transmit HIV​—the AIDS virus.

In the case of mosquitoes, for instance, the insect’s mouth parts are not like a syringe with a single opening through which blood could be reinjected. Rather, mosquitoes draw blood in through one passage while delivering saliva through another. Then, explains Thomas Damasso, an HIV specialist with the District Health Management Team in Mongu, Zambia, the mosquito’s digestive system breaks down the blood, destroying the virus. HIV is not found in insect feces. And unlike malaria parasites, HIV does not get into the mosquito’s salivary glands.

To acquire HIV, a person must be exposed to a large number of infectious particles. If a mosquito’s meal is interrupted and it flies directly to another victim, any amount of blood that might remain on its mouth parts would be too small to be significant. According to experts, even swatting a mosquito filled with HIV-positive blood over an open wound would not cause HIV infection.

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CDC/James D. Gathany

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The deer tick (shown enlarged at right) spreads Lyme disease to humans

Left to right: Adult female, adult male, and nymph, all shown actual size

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All ticks: CDC

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Floods, unsanitary conditions, and human migration contribute to the spread of insect-borne diseases

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