When Insects Spread
DISEASE

from www.watchtower.org/library/g/2003/5/22/article_01.htm

INSECT-BORNE DISEASE
A Growing Problem

IT IS BEDTIME in a Latin-American home. A mother lovingly tucks her young son in and bids him good night. But in the dark a sleek, black kissing bug, less than an inch [less than 3 cm] long, slips out of a crevice in the ceiling over the bed. It drops undetected onto the sleeping child's face and almost imperceptibly pierces the soft skin with its beak. As the bug gorges itself on blood, it also discharges its parasite-laden waste. Without waking, the boy scratches his face, rubbing the infected feces into the wound.

The kissing bug

As a result of this one encounter, the child contracts Chagas' disease. Within a week or two, he gets a high fever and his body swells. If he survives, the parasites may take up residence in his system, invading his heart, nerves, and internal tissues. As many as 10 to 20 years may pass without symptoms. But then he may develop lesions in his digestive tract, experience cerebral infection, and ultimately die of heart failure.

This fictionalized account realistically depicts how Chagas' disease can be contracted. In Latin America, millions may be at risk of receiving this kiss of death.

Man's Multilegged Companions

"Most of the major fevers of man are produced by micro-organisms that are conveyed by insects," states the Encyclopædia Britannica. People commonly use the term "insect" to include not only true insects—six-legged creatures such as flies, fleas, mosquitoes, lice, and beetles—but also eight-legged creatures such as mites and ticks. Scientists list all of these under the larger category of arthropod—the largest division in the animal kingdom—which includes at least a million known species.

The vast majority of insects are harmless to man, and some are very beneficial. Without them, many of the plants and trees that people and animals depend on for food would not be pollinated or bear fruit. Some insects help to recycle waste. Many insects feed exclusively on plants, while certain ones eat other insects.

Of course, there are insects that annoy man and beast with their painful bite or simply by their presence in vast numbers. Some also wreak havoc on crops. Worse, however, are insects that spread sickness and death. Insect-borne diseases "were responsible for more human disease and death in the 17th through the early 20th centuries than all other causes combined," states Duane Gubler of the U.S. Centers for Disease Control and Prevention.

Presently, about 1 out of every 6 people is infected with a disease acquired through insects. Besides causing human suffering, insect-borne disease imposes a heavy financial burden, especially on developing countries—those that can least afford it. Even a single outbreak can be costly. One such incident in western India in 1994 is said to have drained billions of dollars from the local and world economies. According to the World Health Organization (WHO), the world's poorest countries will be unable to advance economically until such health problems are brought under control.

How Insects Make Us Sick

There are two main ways that insects serve as vectors—transmitters of disease. The first is by mechanical transmission. Just as people can track dirt into a home on unclean shoes, "houseflies may carry on their feet millions of microorganisms that, in large enough doses, can cause disease," says the Encyclopædia Britannica. Flies can pick up contamination from feces, for example, and pass it on when they land on our food or drink. In this way humans contract such debilitating and deadly illnesses as typhoid, dysentery, and even cholera. Flies also help to spread trachoma—the leading cause of blindness in the world. Trachoma can blind by scarring the cornea—the clear part of the eye in front of the iris. Worldwide, some 500,000,000 humans suffer from this scourge.

Cockroaches, which thrive in filth, are also suspected of mechanically transmitting disease. In addition, experts link a recent steep rise in asthma, especially among children, to cockroach allergies. For instance, picture Ashley, a 15-year-old girl who has spent many nights struggling to breathe because of her asthma. As her doctor is about to listen to her lungs, a cockroach falls out of Ashley's shirt and runs across the examination table.

Diseases on the Inside

When insects harbor viruses, bacteria, or parasites inside their bodies, they can spread disease a second way—by passing it on through a bite or other means. Only a small percentage of insects transmit disease to humans in this way. For instance, although there are thousands of species of mosquitoes, only those of the genus Anopheles transmit malaria—the world's second-deadliest communicable disease (after tuberculosis).

Still, other mosquitoes transmit a host of different maladies. WHO reports: "Of all disease-transmitting insects, the mosquito is the greatest menace, spreading malaria, dengue and yellow fever, which together are responsible for several million deaths and hundreds of millions of cases every year." At least 40 percent of earth's population are at risk for malaria, and about 40 percent for dengue. In many places, a person can contract both.

Of course, mosquitoes are not the only insects that carry disease inside them. Tsetse flies transmit the protozoa that cause sleeping sickness, afflicting hundreds of thousands of people and forcing whole communities to abandon their fertile fields. By transmitting the organism causing river blindness, blackflies have robbed some 400,000 Africans of sight. Sand flies can carry the protozoa that cause leishmaniasis, a group of disabling, disfiguring, and often fatal diseases that presently afflict millions of people of all ages around the world. The ubiquitous flea can host tapeworms, encephalitis, tularemia, and even plague—generally associated with the Black Death, which in just six years wiped out a third or more of the European population during the Middle Ages.

Lice, mites, and ticks can convey various forms of typhus, besides other diseases. Ticks in temperate lands around the world can carry potentially debilitating Lyme disease—the most common vector-borne illness in the United States and Europe. A Swedish study revealed that migratory birds can transport ticks thousands of miles, possibly introducing the diseases they carry to new regions. "Ticks," says the Britannica, "surpass all other arthropods (except mosquitoes) in the number of diseases they transmit to humans." In fact, a single tick can harbor as many as three different disease-causing organisms and can transmit all of them in just one bite!

Many insects carry diseases inside their bodies		Blackflies carry river blindness
		Mosquitoes carry malaria, dengue, and yellow fever
		Lice can convey typhus
		Fleas host encephalitis and other diseases
		Tsetse flies transmit sleeping sickness

A "Vacation" From Disease

It was only as recently as 1877 that insects were scientifically shown to transmit disease. Since then, massive campaigns to control or eliminate disease-carrying insects have been carried out. In 1939 the insecticide DDT was added to the arsenal, and by the 1960's insect-borne disease was no longer regarded as a major threat to public health outside Africa. Emphasis shifted away from controlling the vectors to treating emergency cases with drugs, and interest in studying insects and their habitats waned. New medicines were also being discovered, and it seemed that science could find a "magic bullet" to deal with any illness. The world was enjoying a "vacation" from infectious disease. But the vacation was to end.

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.

original article located at http://www.watchtower.org/library/g/2003/5/22/article_01.htm
Copyright © 2004 Watch Tower Bible and Tract Society of Pennsylvania

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