Unbalancing Act

Disease Outbreaks: Epidemics Unleashed

Human actions are dramatically changing environments around the globe—some making great habitats for mosquitoes and other disease-spreading animals. All this can spark new epidemics.

Mountain gorillas face extinction due to habitat loss. And warfare has raged over much of the apes' range. Now, scientists have discovered gorillas endure yet another problem—human illnesses. Conservationists and ecotourists in Uganda unintentionally spread Giardiasis to some of the apes.

Did you know that gorilla babies in the zoo get some of the same vaccinations (shots) as you?
Source: SNZP, Jessie Cohen ie Cohen

As happens in the wild, people took bathroom breaks in the woods. Some of the people carried the Giardiasis parasite, so it wound up in their waste too. Rain apparently washed contaminated waste into streams. Gorillas who drank from the streams then became infected.

A new initiative, The Consortium for Conservation Medicine, brings together wildlife veterinarians, conservation biologists, and other scientists to study the links between wildlife and human health.
Malaria-carrying mosquito, a disease vector, in action. Source: WHOMalaria-carrying mosquito, a disease vector, in action.
Source: World Health Organization

Clearing a forest, building a dam, even irrigating a field-these human actions change the environment in small, or sometimes dramatic ways. Landscapes are transformed. Water flows to new areas. Species arrive. Species depart.

These environmental changes often encourage outbreaks of infectious diseases. Why? Pathogens, or organisms that cause illness, may spread more easily, and disease vectors, that carry the disease, may behave in new ways. On the flip side, a few diseases may decrease when the environment changes.

Some pathogens move directly from one person to another. The blast of a sneeze, for example, sends microbes sailing towards the next person. Other pathogens live in water, making people sick when they drink, swim, or bathe in the contaminated water.

But many pathogens are spread by insects, animals, and other living things called disease vectors. These taxis of the disease world do not themselves get sick but carry the microbe from person to person. The mosquito that carries the malaria parasite, for example, never gets sick with malaria. But when it drinks the blood of an infected person or animal, the mosquito carries the parasite to the next person it bites.


This UN chart shows the spread of infectious diseases
Source: UNEP


When the environment is changed, vectors may have sudden bursts in population, move to new places, or change their behavior. The result in some cases: Increased incidence of infectious diseases, including many of the world's most persistent killers.

video_iconGet a quick overview of major diseases afflicting our world.
Options: 56k | 220k (RealPlayer required)
Source: Journey to Planet Earth


Here are summaries of some of these diseases. To get the complete story, click on the name of each disease:


    • Environmental links: Deforestation often creates new mosquito habitats. People moving to deforested areas then come in contact with the disease.

    • Source and vector: Caused by blood parasites. Spread by mosquitoes.

    • Symptoms: Chills, fever, coma, blood-vessel blockages.

  • Scope: Primarily tropical, but found in 100 different countries. Half a billion people each year are infected (1 out of every 12 people on Earth). About 2 million die. Half of world's population lives in malaria areas.


Source: World Health Organization



    • Environmental link: Spiny rats and sandflies thrive in deforested areas.

    • Source and vector: Caused by protozoans. Spread by sand flies.>

    • Symptoms: The disease has two basic forms: 1) Skin sores and open wounds; 2) High fever, infected bone marrow, enlarged spleen and liver, anemia (more serious).

  • Scope: Found in 90 countries, with the highest incidents in Bangladesh, Brazil, Central America, India, and southern Texas.


Leishmaniasis from a sandfly bite. Source: NIH
Source: NIH



    • Environmental link: Deforested areas often become grazing lands for cattle, which carry the disease. Their waste—and the parasites—can contaminate streams and other water sources.

    • Source: Caused by parasites. Spread via contaminated water.

    • Symptoms: Diarrhea, cramps, fever. Not usually fatal.

  • Scope: Found mostly in developing world but can strike U.S. and other industrialized nations.


    • Environmental link: Dams, irrigation systems, and other water-control projects create more habitats for freshwater snails, which serve as hosts for young blood flukes.

    • Source and vector: Caused tiny worms called blood flukes. Larvae mature and emerge from snails and escape into the water. They burrow into the skin of swimmers, bathers, or farm workers wading in irrigation ditches without boots.

    • Symptoms: Fever, cough, rash, itchy skin, tenderness around liver, blood in stool or urine (in more serious cases).

  • Scope: Found in many areas, particularly the subtropics and tropics. 200 million people worldwide are infected. Of these, 20 million are severely infected.


    • Environmental link: Deforestation reduces other mammal populations, so "kissing bugs" prey on humans instead.

    • Source and vector: Caused by parasites. Spread by "kissing bug" beetles, which feed on the blood of humans and other mammals.

    • Symptoms: Heart failure, damage to digestive system, swelling around the eye. Symptoms may not appear for 10-20 years after infection.

  • Scope: South America and Central America, where it kills 50,000 people a year. Scientists estimate that between 16 and 18 million people are currently infected.


Eliminating habitats for bugs that transmit para<span>sites</span> causing Chagas'. Source: WHO/TDR Covering cracks in walls eliminates habitats for bugs that transmit parasites causing Chagas'
Source: World Health Organization/TDR



    • Environmental link: Rainforest monkeys are a reservoir for yellow fever. Mosquitoes pick up the disease from them, and then spread it to migrant farmworkers and other people who move into—and clear—rainforests.

    • Source and vector: Caused by virus. Spread by mosquitoes. The word for the virus, Arbovirus, comes from "Arthropod-Borne Virus."

    • Symptoms: Fever, jaundice (yellowish skin), muscle pain, nausea, bleeding from mouth and nose, kidney failure.

  • Scope: Found in 50 countries, mainly in Africa and Latin America. About 200,000 cases reported each year, with some 30,000 deaths.


    • Environmental link: Roads and suburban developments cause deforestation and habitat fragmentation, upsetting the balance of nature. The number of species drops, and mice—who can live virtually anywhere and carry the disease—thrive.

    • Source and vector: Caused by bacteria. Spread by ticks that live on mice; can also live on deer and other mammals.

    • Symptoms: Usually a rash, shaped like a bull's eye, followed by fever and muscle aches. Can result in long-term joint and muscle pain, and memory loss. Early treatment with antibiotics is important.

  • Scope: Temperate forests of North America, Europe, and Asia (in the U.S., mostly in the Northeast; also in the upper Midwest and along the West Coast). Not found in tropical climates. Approximately 15,000 cases of Lyme disease occur each year in the U.S.


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