When Insects Spread
DISEASE

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!

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. The following article will discuss why.