Diseases, insects and environment

HealthcareDisease

by Professor Malcolm Molyneux.

When you come to think about it, a surprising number of different diseases can be spread between people by insects. Some of these diseases will be discussed in this issue of Footsteps. They are among the most important and serious diseases in the world, especially in areas which are not yet industrially developed.

Insects are part of our environment, the natural world which surrounds us, and they are therefore greatly affected by things that change this environment. Environmental changes can influence diseases by affecting how insects behave or survive. This also means that we can attack insect-borne diseases by controlling the environment in which we live.

Changes in the natural environment, and the ability of insect vectors (insects which spread diseases) to react to these changes can have an important effect on the spread of disease.

Agricultural developments Irrigation schemes create new breeding sites for some insects, especially the anopheline mosquitoes that spread malaria. When farmers move into forests to hunt or to clear and farm new ground, they may find tsetse flies that carry sleeping sickness parasites from wild game, or mosquitoes carrying the yellow fever virus from monkeys. In Thailand it is the gem miners working in the forests who are at particular risk of malaria, because mosquitoes live in the forested mining areas.

War and social change Uganda and the new Democratic Republic of the Congo are examples of countries where war, by changing the environment, caused epidemics of insect-borne disease. As a direct result of war, lantana thickets and other bushes were left to grow wild, providing ideal breeding grounds for tsetse flies. Cases of sleeping-sickness increased and hundreds of people died. Clearing the excess vegetation greatly reduced the problem.

Population movements In highland areas it is too cold for malaria parasites to grow inside mosquitoes, so the disease is not spread (even though there may be many mosquitoes around). People living there do not get malaria, and therefore do not build up any resistance or immunity to it. When individuals or whole populations move from high to low ground, as happened in Ethiopia during the 1980s, malaria is spread in the new, warm environment and there are epidemics of the disease, which can be severe or fatal. By recognising the danger in circumstances like these, it is possible to take precautions against outbreaks of disease.

Economic hardship When people lack basic resources it can be difficult to maintain the fight against insect-borne diseases. For example, in South America the bugs that transmit chagas disease can be kept away by cementing cracks in hut walls, or by replacing thatch roofs with metal. However, these changes cost people money, and without them the conditions remain for the insects to flourish. Similarly, treated bed nets can protect against many insect-borne diseases, but lack of resources to buy or re-treat the nets can limit the effectiveness of this approach.

Begin where you are

By understanding how the environment influences insects and the diseases they carry, we may all be able to play a part in reducing the spread of those diseases. A useful community exercise is to hold a group meeting to discuss the following questions…

  • What insect-borne diseases are causing major problems in our community?
  • Are there any things we could do locally to reduce the spread of these diseases amongst us?
  • What are the obstacles or difficulties that prevent us from taking such steps?
  • Where could we get either advice or help to change things for the better?

The pages that follow may give you some ideas about what you might usefully do to reduce the effects of insect-borne diseases in your own community. Many such measures are readily available and may be possible even with very limited resources. There is a proverb of the Achewa people in Malawi: ‘Konza kapansi kuti kamwamba katsike,’ meaning ‘Attend to the problem at your own doorstep, and more distant goals will then be easier to reach.’ This is a good message for environmental control – we do not have to wait for governments and national programmes to act. There is a lot we can do for ourselves, ‘on our own doorstep’.

Professor Molyneux is co-director of the Wellcome Trust Clinical Research Centre and Malaria Research Project in the College of Medicine, PO Box 30096, Chichiri, Blantyre 3, Malawi. He is an honorary professor in the College of Medicine, University of Malawi, and in the School of Tropical Medicine, University of Liverpool, UK.