Waterborne diseases are a major cause of illness in developing countries and are responsible for sporadic and localised outbreaks of disease in the developed world. They are generated by a variety of organisms that exist in water
THE poor are most at risk from waterborne infections as they tend to be undernourished, lack proper sanitation and have little access to medicines. Children and the elderly are also especially vulnerable, because their immune systems are either undeveloped or worn out. Anyone who is suffering from a disease that damages the immune system will also be in particular danger.
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Cholera and typhoid were widespread in Europe and North America 100 years ago, but now have almost disappeared from the developed world, largely because of improved water supplies and sanitation. However, this has not happened to the same extent in developing countries. In Peru, for example, where there have been more than 500 000 cases of cholera since 1991, social expenditure has declined over the past 10 years and the supply of water to almost 90 per cent of the rural population is still grossly contaminated. The World Bank estimated in 1993 that diarrhoea and intestinal worm infections caused by poor water supplies and bad sanitation accounted for as much as 10 per cent of the entire disease burden of developing countries.
The United Nations, recognising that most of the disease was the result of polluted water supplies, set up a Decade of Water, 1981/1990. Its aim was to provide safe water supply and adequate sanitation for everyone. Over the decade, the number of people lacking a safe water supply in less well developed countries dropped from 1800 million to 1200 million. Although the number of people without lavatories remained at around 1700 million, against a continued overall population increase, this was significant progress.
The provision of clean drinking water is not always the complete answer to the problem. For example, in Bangladesh, where cholera is endemic, people drinking safe water are often just as much at risk of contracting cholera as those drinking polluted water if they continue to use polluted water for cooking, washing utensils and bathing. In developed countries, where people have efficient sewage treatment and clean water supplies, it is quite rare to contract a waterborne disease.
When sanitation does break down and sewage or untreated water accidently gets into the drinking water, the whole population fed by that supply is at risk. This happened in the world鈥檚 worst outbreak of cryptosporidiosis (see Box 1) which occurred in April 1993 in Milwaukee, in the US. Raw water from a river carrying spring run-off contaminated the distribution system for drinking water. Out of 800 000 people served by that system 370 000 became ill, 4400 were admitted to hospital and 40 are thought to have died. An outbreak of cryptosporidiosis occurred in Britain in 1989 in the Oxford area when at least 500 people became ill, but fortunately no one died from it. Both outbreaks happened when the protozoan Cryptosporidium contaminated the water supply.
During warfare and natural disasters 鈥� such as earthquakes and floods 鈥� the scale of the problem greatly increases when these occur in the developing world. With the breakdown of pumping stations and breakage of pipework, drinking water becomes mixed with human sewage. It is usually not long before the media report serious outbreaks of the diarrhoeal infections, shigella, salmonella, typhoid and cholera.
When water is scarce, treated sewage effluent is a valuable source of irrigation water. It may first be chlorinated to kill the pathogenic microorganisms and then stored in irrigation lagoons. The chlorinated effluent when released into the lagoons is usually pathogen-free, but it can soon become recontaminated by seepage from the surrounding area. Microbiologists found at one water treatment and irrigation plant in Libya that the droppings from a large population of birds living around the lagoon were the source of contamination. If irrigation water becomes polluted, pathogenic microorganisms are transferred onto the crops and into the food chain.
Springs of hope
Contaminated waters
WATER from springs, wells, aquifers and long-term ground water, is usually pure and wholesome (potable) and needs little, if any, treatment, such as disinfection (see Inside Science No. 42). However, large conurbations require huge amounts of water which has to come from lakes, rivers, reservoirs and treated, recycled effluent. These sources are invariably contaminated to a greater or lesser extent by bacteria, viruses and various parasites and have to be treated to get rid of potential pathogens.
If the provision of safe drinking water is a priority in promoting health, then the safe disposal of human excreta is another. All human faeces contain large numbers of microorganisms but when a proportion of the population suffers from diarrhoeal disease they excrete even greater numbers of bacteria. Without good sanitation, excreta and pathogens enter water from faeces. This is a particular problem where people are suffering from watery diarrhoea, such as Shigella dysentery and cholera. One person with cholera excretes 1013 bacteria each day, and since for cholera the infectious dose (the number of bacteria required to cause the disease) is around 106 bacteria, one person could theoretically infect up to 10 million people.
It is thus essential to separate the disposal of excreta from water used for drinking. This can be done on a small scale by building latrines or septic tanks where excreta are held and prevented from seeping into nearby water courses. Modern sewage plants combine a wide range of processes. Sewage treatment reduces the numbers of microorganisms present and removes solids so that rivers do not become silted up, thus reducing the amount of dissolved organic matter in effluent before it is released into surface waters.
Ways to the water
Faecal-oral route
WATERBORNE diseases are spread by the so called 鈥渇aecal-oral route鈥� where organisms are excreted in faeces and subsequently ingested by a new host.
Most waterborne pathogenic viruses, bacteria and protozoa are well adapted for growth in the intestines of humans and animals where they multiply. They are found in large numbers in domestic sewage, effluent from abattoirs and animal processing plants, all of which can contaminate water catchment areas. Other sources of microbial pollution for water catchments include birds, domestic and wild animals, farm slurries and agricultural run-off.
Waterborne diseases can also be acquired and spread by food, especially if it is uncooked, or inadequately cooked 鈥� notoriously shellfish, such as oysters, clams, cockles and mussels, which grow in polluted water and concentrate viruses and bacteria by their method of 鈥渇ilter feeding鈥�. In many parts of the world, crops are grown using contaminated irrigation water. It is a widely accepted practice in recycling.
The symptoms caused by waterborne diseases often include diarrhoea, vomiting, abdominal pain and dehydration, with the severity depending on the particular microorganism, the extent of exposure (number of organisms in relation to the infectious dose) and the age and health of the victim. The infectious dose of bacteria and viruses varies with the type of the organism and ranges from 200 cells for Shigella to 106 cells for Vibrio cholerae. The bacteria must first withstand the acid produced by the stomach. They are more likely to do so if the water is consumed with food or milk which neutralise the acid and 鈥渉ide鈥� the bacteria. Vomiting occurs when the bacteria cause inflammation of the mucosa lining the stomach. From the stomach the bacteria enter the small intestine where they multiply in alkaline conditions and in a rich supply of nutrients. In order to initiate infection the bacteria must attach to epithelial cells (enterocytes) that line the villi of the small or large intestine.
Survival in water
Most like it cold
THE ABILITY of pathogens to survive in water varies widely. Campylobacter, Leptospira and the viruses are unable to grow in water. Salmonella, Escherichia and Shigella can do so to a limited extent if temperatures are high enough and nutrients are available (as in organic-rich effluents and in phytoplankton blooms).
Vibrio cholerae manages to keep alive over winter in brackish estuarine and inshore coastal waters, and can multiply to infective levels in warmer months. The viruses and the bacteria Salmonella, Escherichia, Shigella and V. cholerae can survive after being concentrated by filter-feeding shellfish. Additionally, V. cholerae is adapted to colonise and multiply on the chitin found on the shells of crabs and planktonic crustaceans.
Legionella pneumophila, the cause of Legionnaire鈥檚 disease, is a naturally aquatic bacterium. Its ability to grow inside layers of slime in pipes, on plants and rocks, and in amoebae help it to survive low temperatures and from challenge by disinfectants.
The intestinal protozoa, Cryptosporidium, Giardia and Entamoeba, are all excreted as cysts which are resistant to a wide range of environmental factors, such as temperature, acidity and desiccation, and against many biocides.
The ability of waterborne pathogens to survive is best at low temperatures, low salinities and low intensities of light, and is enhanced by turbidity, planktonic blooms and uptake by filter feeders. Warm temperatures favour the survival of V. cholerae and L. pneumophila, which can grow in water. The main factors responsible for the death of bacteria and viruses in water is sunlight and salinity. Microbiologists find, for example, that bacteria released with sewage effluent into the clear warm waters of the Mediterranean and Pacific oceans are usually killed within minutes but similar bacteria released into the turbid waters around Britain are not.
Swimmers, surfers and canoeists who use British coastal waters, into which raw sewage is routinely discharged, are at risk of contracting minor infections of the intestines and the ear, nose and throat. There are, however, no recorded cases of these people catching life-threatening diseases from such activities. (Claims for people catching polio in the 1950s and more recently hepatitis A in this way are now considered to be ambiguous.)
Coastal waters in Britain are monitored for pollution levels once a fortnight between May and September, and designated bathing beaches should not exceed the 鈥渕andatory standards鈥� laid down by the European Commission 鈥� 20 per cent of Britain鈥檚 designated beaches do so at present. The National Rivers Authority is considering a similar scheme for characterising inland waters in Britain. This is because surfing, sailing and other water sports on coastal and inland waters continue to increase. However, almost all surface waters are subjected to agricultural run-off, sewage effluent and direct contamination by animals. So it is not surprising that they contain a wide variety of potentially pathogenic microorganisms.
A sporting chance
Don鈥檛 risk your health
ESTUARIES are not covered by European Commission or national directives even though sewage effluent is often discharged into them, and windsurfers, canoeists, water skiers and jet skiers extensively use them. For example, water sports enthusiasts are increasingly using the estuary of the river Lune, in Lancaster. Unfortunately they tend to do so at high tide which coincides with the release of sewage effluent into the estuary. Consequently, they risk water exposure to high levels of microorganisms associated with faeces.
In the past, water sports were almost exclusively a summer pastime, when bacteria in sewage effluent are most exposed to solar radiation. Nowadays such sports continue throughout the winter, when bacteria survive for longer in coastal waters. Water skiers falling into the Lune estuary are more likely to receive an infective dose of Campylobacter (500 bacteria) in the winter than in the summer. It is perhaps time for Britain鈥檚 recreational waters to be monitored all year round.
Swimming pools are normallly treated with chlorine and are usually microbiologically safe. However, there have been several reports of Shigella, Giardia and Cryptosporidium infections being contracted from swimming pools becoming contaminated by faeces from children suffering diarrhoea.
1: Some dangerous enemies
(A) Diarrhoeal families
Salmonella typhi, which is responsible for typhoid fever, crosses from the intestine into the blood and lymph systems where it is taken up by phagocytic cells, a type of white blood cell. S. typhi is resistant to the destructive activity of these cells and is transported by them to susceptible parts of the body, such as the spleen. The main symptons are a high fever, enlarged spleen and diarrhoea. If untreated, up to 40 per cent of victims die. The main reservoirs of infection are the human intestine and gall bladder.
Vibrio cholerae, the cause of cholera, produces a toxin, choleragen, which although it does not kill enterocytes disrupts water uptake in the body and causes sodium and chloride ions to be released in the intestine. This loss of water and electrolytes results in extreme dehydration which is the major cause of death in cholera.
We are currently experiencing the seventh worldwide cholera epidemic (pandemic), which began in 1961, and the WHO calculates that in Asia and Africa there are 5.5 million cases and 120 000 deaths annually. In 1991 cholera appeared in Peru and is spreading across South America. A new cholera strain, first reported in Madras in 1992, has spread to other parts of India, Bangladesh and Thailand and threatens to begin the eighth pandemic.
Traveller鈥檚 diarrhoea
IN THE developing world a number of other enteric bacteria are important waterborne causes of diarrhoea in young children. They are also responsible for many cases of so-called 鈥渢ravellers鈥� diarrhoea鈥�. In the developed world they are mainly associated with foodborne disease and their role in causing waterborne enteritis is confined to accidental contamination of drinking water tainted with sewage. There have been numerous reports of backpackers and solders on manoeuvres contracting these diseases from mountain streams. These 鈥渙ther鈥� bacteria include Campylobacter, the non-typhoid salmonellae, Shigella, and various types of Escherichia coli, Aeromonas and Yersinia.
Protozoa
THE THREE main genera of waterborne enteric protozoa pathogenic to humans (Giardia, Entamoeba and Cryptosporidium) live in a wide variety of animals and their cysts are excreted in faeces. They live independently within the host. Their cysts are resistant and can survive for extensive periods in water.
Giardia lamblia, which is the cause of an unpleasant long-lasting diarrhoea (giardiasis), is a flagellate protozoan, and the most common protozoal parasite reported worldwide. It lives in the small intestine and attaches to small hollows in the intestinal wall by its ventral sucking disks. It multiplies, disrupts water uptake by the microvilli and causes a watery diarrhoea. The infection inhibits the absorption of fats and vitamins and causes the production of foul-smelling stools. Giardia is the most commonly identified pathogen in waterborne outbreaks in the US but does not seem to be a significant agent in waterborne illness in Britain.
Entamoeba histolytica is the main cause of amoebic dysentery (amoebiasis) in humans. The amoebae multiply in the large intestines and adhere to enterocytes causing ulceration and the production of watery, bloody diarrhoea. Infection is associated with poverty, poor hygiene and poor water quality, mainly in tropical regions. Waterborne outbreaks occasionally occur in the US and Britain but they have declined in the past 20 years. Amoebiasis is typically associated with subtropical and tropical countries where water supplies are often unsafe and sewage disposal haphazard.
The protozoan Cryptosporidium parvum is of increasing concern to the water industry in developed countries and has been responsible for extensive outbreaks. On ingestion, the oocysts release sporozoites that grow and infect enterocytes in the small bowel. This results in a cholera-like, watery diarrhoea, vomiting and anorexia.
Viruses
THERE ARE six groups of viruses that cause gastroenteritis and all have been isolated from contaminated drinking water and recreational waters. They are: rotaviruses, Norwalk-like agents, caliciviruses, astroviruses, other small round structured viruses (SRSV), and enteric adenoviruses. The symptoms they cause are similar and may include acute vomiting and mild to severe diarrhoea.
(B) Non-diarrhoeal diseases
THESE include the bacterial diseases legionellosis and leptospirosis and the viral disease, hepatitis A.
Legionella pneumophila causes legionellosis. There is little evidence that legionellae in their natural habitat pose significant risks to human health. But they multiply up into infectious doses in warm, artificial aquatic habitats such as heat exchangers, condensers in air conditioning units, cooling towers and shower heads. They leave these habitats in aerosols and infect the lungs of humans. The bacteria are able to survive and grow within phagocytic cells, multiplying in the lungs causing bronchopneumonia and tissue damage.
Leptospiras live in the kidneys of wild and domestic animals. The bacteria are excreted in urine and contaminate rivers, ponds and canals. In inland waters they can survive for long periods, threatening water-sports enthusiasts. The species which infects rats, Leptospira icterohaemorrhagiae, is the most dangerous to humans, causing Weil鈥檚 disease. It is acquired through cuts and abrasions as well as via the mucous membranes. The symptoms are normally flu-like and the patient recovers quickly. In severe cases the liver and kidneys are infected, resulting in jaundice and acute renal failure. Between 5 and 10 per cent of patients die. In tropical and subtropical regions, where submerged crops such as rice and taro grow, the disease is common. The species found in cattle, L. hardjo, is less dangerous but more common, especially among farm workers in developed countries.
Infectious hepatitis is caused by the Hepatitis A virus. It is an acute self-limiting infection of the liver. The virus is acquired from contaminated water or food and replicates in the intestinal lining from where it enters the blood and spreads to the liver. It multiplies in the liver, causing jaundice, and returns to the intestine by way of the bile duct. Large numbers of virus particles are released in faeces.
2: The numbers game
IT IS not feasible to test water for individual disease-causing organisms because different species may be mixed in low densities that are difficult to separate out and detect. So microbiologists test for indicator organisms, which may not themselves be pathogenic but are present in large numbers in faeces. If an indicator organisms is present, then faecal material and possibly pathogens are also likely to be present.
Most water tests are based on a count of the numbers of Escherichia coli, which is present in large numbers in human and animal faeces. Two tests are carried out. In the first, the total or presumptive coliform count (the number of coliform bacteria capable of growing at body temperature 鈥� 37掳C) are counted. This is a good, general measure of the level of pollution from a wide variety of sources but, as it includes coliforms from non-human sources, a simple count of all coliforms present in a water sample is not precise enough. So a second test is done in which the numbers of coliforms capable of growing at 44掳C are counted. As only coliform bacteria from human faeces can grow at 44掳C, this test, the 鈥渇aecal coliform count鈥� measures pollution by human waste. Regulations for the European Union and the US state that drinking water should contain no faecal coliforms at all and no presumptive coliforms in 95 per cent of the samples (100 ml) tested. The use of indicator species leaves a wide margin for error, as it is indirect and has been criticised for not directly counting viruses or individual pathogens. New methods are being developed involving molecular technology and DNA fingerprinting, which will not only test for the presence of specific bacterial pathogens and viruses, but will also detect the viable bacteria at stages at which they cannot be cultured, thus allowing scientists to identify more than by existing tests.
Further Reading
Foodborne Illness, edited by W. M. Waites and J. P. Arbuthnot (Edward Arnold, 拢9.99, 1991).
Cholera, edited by D. Barau and W. B. Greenoughill (Plenum Medical, 1992).
Recreational Water Quality Management: Volume 1 Coastal Waters, edited by D. Kay; Volume 2 Fresh Waters, edited by D. Kay and R. Hanbury (Ellis Horwood, 1992/1993).