As humans alter land, infectious diseases follow

As people remake the world's landscapes, cutting forests, draining wetlands, building roads and dams, and pushing the margins of cities ever outward, infectious diseases are gaining new toeholds, cropping up in new places and new hosts, and posing an ever-increasing risk to human and animal health.

Writing this month (July 2004) in the journal Environmental Health Perspectives, an international team of experts warns that widespread changes in the global landscape are providing new opportunities for dozens of infectious diseases, including scourges like malaria, dengue fever, Lyme disease, yellow fever, cholera, influenza, foot and mouth, and hemorrhagic fevers.

"Evidence is mounting that deforestation and ecosystem changes have implications for the distribution of many other microorganisms, and the health of human, domestic animal and wildlife populations," according to the report compiled by the Working Group on Land Use Change and Disease Emergence, an international group of infectious disease and environmental health experts.

"Many of our current activities, primarily for economic development, have some major adverse health effects," says Jonathan A. Patz, the lead author of the report, and a University of Wisconsin-Madison professor in the Center for Sustainability and the Global Environment (SAGE) of the Nelson Institute for Environmental Studies and the department of population health sciences.

Indeed, a detailed understanding of the influence of human activities on the spread of pathogens, the report notes, is limited to only a few diseases. In the northeastern United States, for example, studies have documented that forest fragmentation, urban sprawl and the erosion of biodiversity have contributed significantly to the spread of Lyme disease.

A more global example is the AIDS virus, which scientists think may have first infected "bush meat" hunters given access to Africa's tropical forests by the growing network of logging roads in the continent's interior. The disease subsequently spread by human contact and has become a global tragedy through the ability of humans to travel the world with relative ease.

In scope, the issue is broad, affecting nearly every corner of the globe. The causes are as varied as the human activities that create the opportunities for pathogens to thrive, spread geographically and invade new hosts. It involves well-known and pervasive pathogens such as the parasite that causes malaria, a disease that claims more than 1 million lives annually, to diseases like SARS that are relatively new and, so far, limited.

"There is no single smoking gun," says Patz. "The causes are interwoven into current unsustainable development practices. "

The list of activities that contribute to the spread of infectious disease, according to the Environmental Health Perspectives report, is long and varied, ranging from seemingly innocuous pursuits like ecotourism and agriculture to war and civil unrest.

Even climate change or extremes, the report notes, can trigger a chain of events that manifests itself in the emergence of new diseases. An example cited in the report is the emergence of nipah virus in Malaysia and Singapore in 1999 when El Nino-fueled fires are thought to have driven fruit bats from their forest habitat to farms where the virus was transmitted to pigs and humans.

The report makes a series of recommendations to address the issue, including linking land use to public health policy, expanding research on deforestation and infectious disease, the development of policies to reduce "pathogen pollution," and the establishment of centers for research and training in ecology and health research.

"While there are many health crises around the world today, there are ongoing human activities that threaten natural resources key to sustaining the health of future generations," says Patz. "We need to look at the root causes of the spread of infectious disease, and many of these are related to habitat and ecosystem change."

Author: Terry Devitt - [email protected]