Feature

Scientists link environment with infectious disease spread

In recent years we have all been affected by the increased spread of viruses, diseases and parasites. As humans dominate more and more of the world we become an ever larger target for disease. Find out how you can reduce the size of this 'human target' by lobbying your government to provide better services, by not littering and a host of other simple solutions.

A rise in the incidence of new and previously suppressed infectious diseases is being linked by scientists with the dramatic environmental changes now sweeping the planet. Deforestation, road and dam building, urban sprawl, natural habitats destruction for agriculture, mining and the pollution of coastal waters are promoting conditions under which new and old pathogens can thrive.

A case in point is the highly pathogenic Nipah virus which until recently was found in Asian fruit bats. In the late 1990s it emerged as an often-fatal disease in humans. This has been 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.

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, and secondly by further stressing and altering habitats.

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.

Thirdly, climate change may increase the number of environmental refugees who are forced to migrate to other communities, even countries.

These are among the findings from the United Nations Environment Programme (UNEP) in its latest "Global Environment Outlook Year Book 2004/2005" under the section "Emerging Challenges-New Findings".

The report is based on new research by 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.

Klaus Toepfer, UNEP's Executive Director, said: "It promises to be a momentous year for the United Nations as a whole and UNEP in particular. In September, nations will gather in New York for a meeting of the General Assembly to evaluate how far we have gone in achieving the Millennium Development Goals."

"A Task Force, appointed by Secretary General Kofi Annan, has concluded that the environment is the cornerstone upon which the Goals are likely to stand or fall. The report on the rise of infections underlines this fact. MDG 6 calls on the global community to reverse the spread of HIV/AIDS, malaria and other diseases", he said.

Mr. Toepfer added: "If environmental degradation is not checked then, it is clear from these new findings that this will be harder and tougher to achieve. There are implications for many of the other goals, from poverty eradication to the delivery of universal primary education for all. People who are sick are less able to work and children who are ill find it harder to attend and concentrate at school."

The issue of environmental degradation and a rise of many new and old infectious diseases is a complex, sometimes subtle, one that is causing increasing concern among scientists and disease specialists.

Overall, it seems 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. Environmental disruption and change, and the poor handling of human and animal wastes are also to blame. International travel, technological change and the globalization of trade in agricultural and other products favours the spread of disease.

In a paper in the Philosophical Transactions of the Royal Society, Prof. 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 Prof. McMichael.

Lyme Disease

Lyme disease is a bacterial disease occurring in North America, Europe and Asia. Symptoms include headaches, rashes, fatigue and muscle and joint pains. In some cases, there can be cardiac and mental problems. Lyme disease is transmitted by ticks, no bigger than a pin-head that normally live on deer. The bacteria that cause the disease are found to a large extent on mice.

Studies in DutchessCounty, north of New York City, have linked the high incidence of Lyme disease there with changes in the forest habitats, as well as social factors.

Firstly, forest fragmentation has led to a loss of rodent predators such as wolves and birds of prey, which, in turn, has led to an increase in the white-footed mouse population. Secondly, more recent patchy reforestation has increased the numbers of deer and thus the numbers of ticks. Finally, middle-class suburban sprawl and the increasing use of these woodland areas for recreation such as camping and hiking have brought humans into greater contact with the larger number of infected ticks.

The rate of Lyme disease in DutchessCounty in the past decade has been running at 400 cases per 100,000 people. There were an estimated 23,000 infections in the United States in 2002.

Malaria

The expansion of mining and other extractive industries, deforestation and road building can increase the habitat for malaria carrying mosquitoes leading to an increase in the incidence of the disease. Migration of workers into previously inaccessible areas spreads the disease.

In Sri Lanka gem mining results in shallow pits which favour mosquitoes. Mercury, used in small scale gold mining, results in pollution which increases human susceptibility to malaria.

The World Health Organization (WHO) recently estimated that 6% of malaria cases during the last 25 years result from climate change.

Schistosomiasis (bilharzias)

This disease leads to chronic ill health and is a major health risk in China and Egypt, and many developing countries, including a number in Africa.

Specific freshwater snails, serve as reservoirs for the disease. Aquatic changes associated with dams like the Aswan Dam in Egypt and irrigation schemes on the Senegal River, increase snail populations and, thus, cases of Schistosomiasis.

Overfishing in Lake Malawi has led to decreases in snail-eating fish triggering an increase in the disease-carrying snails, according to a paper by Dr. Jonathan Patz and others published in "Environmental Health Perspectives".

The "GEO 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 recycling schemes and inadequate disposal sites - 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 urbanization, lack of proper waste-water management schemes in many developing country sites and population growth are also important factors in the spread of these diseases.

Tuberculosis, Bubonic Plague and Hantavirus pulmonary syndrome are linked with unplanned urbanization.

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 diarrhoeal 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. - UNEP

More information:

The "Global Environment Outlook Year Book 2004/2005" can be found at http://www.unep.org/gc/gc23/other_publications.asp. 

Other issues in the "Year Book" include the impact of climate change on ocean circulation; the changing face of the Earth as seen from space, including the spread of greenhouses in Spain; state of the environment reports from the regions and a look back at significant environmental developments in 2004.

More details on the diseases mentioned, including symptoms, geographical spread and control measures, can be found on the WHO Web site and at the Centres for Disease Control and Prevention site 

The UNEP Governing Council Web site can be accessed at www.unep.org/gc/gc23/ 

Science in Africa - Africa's First On-Line Science Magazine

Copyright  2002, Science in Africa, Science magazine for Africa CC. All Rights Reserved

Terms and Conditions