CHAPTER 11
Fear of being in water is rarely found in human infants and is a learned response in many cultures. In island, coastal, lakeshore or riverbank communities in warm climates, water is not feared but forms a playground, washing place and source of food. However, it is not only being in water that provokes fear and we have all been impressed by the power of water as it pounds coasts, swamps large ships, and cascades as white water in rivers and waterfalls. We also fear flooding, swamps and quicksands.
Flooding of rivers is a common feature and one that is beneficial to agriculture in regions where flooding is a predictable, seasonal event. However, if flooding of low-lying land damages homes and crops it is clearly less welcome, as is the incursion of the sea through barrages around coasts. Of course, flooding has helped to shape the landscape and it is a natural event, so we should not be surprised if our calculations of its likely impact sometimes go awry.
Several very important human diseases are associated with water. Micro-organisms thrive where effluent is swept up by floods and outbreaks of diseases like cholera and gastro-enteritis are common accompaniments to flooding [11.1.]. Many natural water bodies also contain disease-causing organisms. Drinking water supplies can become contaminated with Giardia [11.2.] and Cryptosporidium [11.3.], both protists that cause severe, but usually non-fatal, diarrhoea. Care is taken by water industries to monitor the presence of these pathogens and various harmful bacteria, to prevent them being passed into domestic supplies. However, drinking from streams is often a way of acquiring infections of these organisms and backpackers are familiar with this potential hazard.
Other diseases are spread by aquatic animals, or are caused by parasites that spend part of their life in water. Malaria is probably the best-known tropical disease [11.4., 11.5.] and is spread by mosquitoes. Larval mosquitoes are common in swamps and small organically-enriched pools (malaria means "bad air") and adult female flies emerge to go in search of a blood meal [11.6.], both male and female flies using plant sugars as a fuel for flight. By biting more than one person the mosquitoes transmit malarial parasites from those infected to those uninfected and thus ensure the spread of the disease. Global warming is likely to extend the range of malaria and it is difficult to control both the disease and its vectors.
River Blindness, or onchocerciasis [11.7.], is spread by blackflies. Adult female flies transmit nematode worms from one person to another and very small worms migrate to the surface of the body in part of the life cycle, causing eye lesions [11.8.] and subsequent blindness. Although the disease-causing organisms are very different, the mode of transmission of malaria and river blindness are similar as both result from the bites of female flies needing a blood meal to acquire sufficient nutrients to develop their eggs. Even when not carrying disease organisms, the persistent biting of mosquitoes, blackflies and "no-see-'ums" (biting midges) creates a major nuisance impact on humans [11.9.], their livestock, and other animals.
Schistosomiasis [11.10.] is another water-borne disease and is caused by trematode worms. As part of the life cycle, these worms invade snails living in streams and ditches and, after metamorphosis, produce stages that invade vertebrates venturing into the water. Humans are ready victims of schistosomiasis that, like river blindness, is a chronic disabling disease.
Many other examples can be given of tropical diseases associated with fresh waters but water-borne diseases are not confined to the tropics. Weil's disease is an often-fatal infection spread into water via the urine from rats living near rivers, ponds and canals. The parasites invade cuts on human skin and notices warning against the disease are becoming common. Other pathogenic organisms found in water are those causing Legionnaires' disease [11.11.], forms of permanent eye damage, and many other failures of organ systems.
The ability of humans to manipulate the environment has important long-term consequences, especially as we face the prospect of global warming and an increasing human population. Fresh waters are a significant strategic resource and the oceans provide effective buffers against all manner of changes. We must not damage these systems further.
If we are to plan effectively for the future it is clear that we need to understand much more about life in water.
11.1. http://www.who.int/hac/techguidance/ems/flood_cds/en/
11.2. http://www.cdc.gov/parasites/giardia/
11.3. http://www.cdc.gov/parasites/crypto/
11.4. http://www.who.int/mediacentre/factsheets/fs094/en/
11.5. http://www.cdc.gov/malaria/
11.6. http://bioweb.uwlax.edu/bio203/s2008/otto_lau2/my%20images/anopheles%20bite.jpg
11.7. http://www.who.int/topics/onchocerciasis/en/
11.8. http://www.icp.ucl.ac.be/~opperd/parasites/onch1.html
11.9. http://wiki.bugwood.org/Archive:Hazards/Biting_Flies
11.10. http://www.who.int/tdr/diseases-topics/schistosomiasis/en/index.html
11.11. http://www.hse.gov.uk/legionnaires/
