When Insects Spread
DISEASE

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.

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

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.

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