Spreading diseases linked to environmental decline

Posted: 22 February 2005

Scientists are linking a rise in new and previously suppressed infectious diseases with the dramatic environmental changes now sweeping the planet.

Loss of forests, road and dam building, the spread of cities, the clearing of natural habitats for agriculture, mining and the pollution of coastal waters are promoting conditions under which new and old pathogens can thrive.

Experts cite the case of the highly pathogenic Nipah virus which until recently was found normally in Asian fruit bats. An often-fatal disease in humans, its emergence in the late 1990s is being linked with a combination of forest fires in Sumatra and the clearance of natural forests in Malaysia for palm plantations.

Bats, searching for fruit, were forced into closer contact to domestic pigs giving the virus its chance to spread to humans via people handling swine.

Rising temperatures

Climate change may aggravate the threats of infectious diseases in three ways experts suggest. Firstly by increasing the temperatures under which many diseases and their carriers flourish, for example, the geographic range and seasonality of two of the world’s most serious mosquito-borne infections, malaria and dengue fever, are very sensitive to changes in climate.

And secondly by further stressing and altering habitats. Neissseria meningitidis, a common cause of meningitis, can be spread many miles in the dusty conditions that occur following prolonged drought in the Sahel.

Thirdly, climate change may increase the number of environmental refugees who are forced to migrate to other communities, even countries. This in turn will also favour the spread of diseases from one location to another where the population may be more susceptible.

The findings from the UN Environment Programme's latest Global Environment Outlook Year Book is based on new research by some of the leading experts in the field, including Tony McMichael of the Australian National University, Bernard Goldstein of the University of Pittsburgh and Jonathan Patz of the University of Wisconsin.

Cause for concern

Overall scientists and disease specialists believe that intact habitats and landscapes tend to keep infectious agents in check, whereas damaged, altered and degraded ones shift the natural balance thereby triggering the spread to people of new and existing diseases.

Many leading experts are now convinced that ecological disruption, dramatic environmental change, and poor handling of human and animal wastes are playing an important part.

Other phenomena also favour the spread of infectious diseases, including international travel, technological change and the globalization of trade in agricultural and other products.

Professor McMichael argues that the emergence of many infectious diseases 5,000 to 10,000 years ago was a result of humans coming into increasing contact with animals as people established settlements.

The main cause of long-distance spread of infectious diseases, from around 500 years, ago was through war and conquest during the period of European exploration and imperialism in Latin America, the Caribbean, Africa, Asia and the Pacific.

Today the changing pattern of infectious diseases is as much due to environmental change as to trade, travel, migration and social conditions, according to Professor McMichael.

Environmental change

The Year Book report links the emergence of many other old and new diseases with environmental change. Like malaria, Japanese encephalitis and dengue hemorrhagic fever are transmitted to humans by mosquitoes, which also thrive in standing water.

Increasing level of rubbish and solid wastes in developing countries, a result of increasing consumerism, poor collection and refuse handling services, fly tipping, lack of re-cycling schemes and inadequate disposal site, are aggravating the problem.

Discarded plastic bags, old tins and car tyres offer, when filled with rainwater, perfect new breeding opportunities for disease carrying insects.

Increased and unplanned urbanisation, lack of proper waste water management schemes in many developing country cites and population growth, are also important factors in the spread of these diseases, like Tuberculosis, Bubonic Plague and Hantavirus pulmonary syndrome

Pollution problem

The GEO Year Book also links yellow fever, Kyasanur Forest disease and Ebola with deforestation and its knock-on effects.

Land use change, in the form of agriculture, is linked with the rise and spread of diseases like Western and Venezuelan equine encephalitis and Typhus.

Chemicals and antibiotics in farm animal wastes are helping to make disease-causing bacteria more resistant to drugs with implications for infections such as Hepatitis and some diarrheal diseases.

Meanwhile air pollution from transport and factories is linked with increased incidence of respiratory infections. Pollution of coastal waters from raw untreated sewage is a key factor in cholera outbreaks worldwide.

Related links:

The Global Environment Outlook Year Book 2004/2005 (PDF format)

United Nations Environment Programme

More detail on the diseases mentioned including symptoms, geographical spread and control measures can be found at the World Health Organization site and at the Centers for Disease Control and Prevention site.