Whenever animals (including humans), plants, and microbes are introduced
into new places, they can disrupt ecosystems
in ways that increase the potential for infectious disease outbreaks. Such changes can be difficult to predict and even more daunting to prevent. The term “invasive species” is widely used to describe plants and animals that, when introduced to and established in new environments, spread aggressively. Invasions of disease-causing microbes play out in similar ways.
When humans move into new environments, microbes that occur in the native wildlife population without causing apparent ill effects may adapt and jump to people.
The edges or transition zones between two adjacent ecological systems appear to be “hot spots” for disturbance-induced disease emergence. Examples of such transition zones include the border of human settlements, as well as natural transitions between forests and plains, shorelines, and tree lines. Because of the massive expansion of human settlements into natural, uninhabited ecosystems, these ecological transition zones now dominate much of the geography of the world’s tropical developing regions.
When humans move into new environments, microbes
that occur in the native wildlife population without causing apparent ill effects may adapt and jump to people. Scientists believe that the Ebola virus
—first identified in a western equatorial province of Sudan and in a nearby region of Zaïre (now the Democratic Republic of the Congo) in 1976—naturally resides in the rainforests on the African continent and in the Western Pacific. Laboratory observation has shown that bats experimentally infected with Ebola do not die, and this has raised speculation that these mammals may play a role in maintaining the virus in the tropical forest. Closer to home, Lyme disease
surfaced when abandoned farmland in the Northeastern United States reverted to fragmented forest land—a perfect home for deer and the deer tick that carries the bacterium associated with Lyme disease.
The clearing and settlement of tropical rainforests has exposed woodcutters, farmers, and ecotourists to new vector
-borne diseases. Deforestation also creates new habitats
for pathogens and vectors. In South America, for instance, epidemic malaria
has broken out in recently clear-cut areas where mosquitoes now thrive.
The construction of large dams can cause profound ecological changes that increase vector-borne diseases. In tropical and subtropical nations the development of large water projects has been associated in some areas with a rise in malaria and schistosomiasis, both parasitic infections.
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