One of the biggest ecosystem changes to occur during the past 200 years is deforestation, especially in biodiverse tropical forests, and conversion of land to agriculture. The massive expansion of human settlements into natural, uninhabited ecosystems has led to ecological transition zones that dominate much of the geography of the world’s tropical developing regions.
World population growth and its attendant demands on new living space and natural resources are factors strongly linked to disease emergence. Plotting where increasing population density, economic growth, land use change, and wildlife biodiversity coincide gives scientists a way to map “hotspots” where the next emerging infectious disease is likely to originate. They can be found in tropical developing countries such as Brazil, the Democratic Republic of the Congo, Indonesia, Malaysia, and Thailand.
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.
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 Ebola
—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 Ebola can replicate in bats experimentally infected with the virus, and this has raised speculation that these mammals may play a role in maintaining the virus in the tropical forest.
The World Health Organization recognized the 2014 outbreak of Ebola in West Africa as a public health emergency of international concern. This epidemic also deviated from the way the virus had traditionally behaved. It represented the first time the virus ventured into urban areas, infecting people in three capital cities across a wide geographic region. It also spread from Guinea to a neighboring country, Nigeria, via a symptomatic traveler. In addition, it was the first time cases were documented in the United States, transmitted first via a symptomatic traveler and then through human-to-human contact. As of May 2016, the total number of reported cases was 28,616, with 11,310 deaths, in three West African countries—Guinea, Liberia, and Sierra Leone. Previously, deaths from Ebola occurred in Mali, Nigeria, and Senegal in West Africa, as well as 1 case in Spain and 4 cases in the United States.
Another recognized global challenge is Lyme disease
, which 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. To add to this, centuries of hunting mean that there are no top predators—wolves—to help regulate the deer population.
The clearing and settlement of tropical rainforests has exposed woodcutters, farmers, and ecotourists to new zoonotic, often 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.
Although the construction of large dams provides numerous benefits to local communities, their presence can also 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.