Water-related Diseases

Four ways water makes people ill

Water-related diseases are the infections whose spread depends on water: on drinking it, on washing with it, on living in it, or on the insects that breed in it. Because the routes are different, the measures that control them are different too, and that is the single most useful idea in this topic. The four classes are water-borne, water-washed, water-based and water-related insect-vector diseases.

Water-borne diseases are caught by drinking water contaminated with human or animal faeces. The pathogen leaves an infected person in the stool and enters the next person through the mouth, which is the faecal-oral route, and the water is simply the vehicle that carries it. Cholera (Vibrio cholerae), typhoid (Salmonella typhi) and most diarrhoeal disease are water-borne. A single contaminated well or standpipe can infect a whole neighbourhood, and the pathogen does not have to multiply in the water to be a danger.

Water-washed diseases are caught not from the water itself but from not having enough of it. Where people cannot wash, infection passes from skin to skin and from hand to mouth: trachoma, which scars the eyelids and causes blindness, scabies, and skin and eye infections. The remedy here is more water, not cleaner water, because the water is not carrying the pathogen; the shortage of it is what keeps the infection alive.

Water-based diseases are caused by parasites that spend part of their life cycle in water, with an aquatic intermediate host. Schistosomiasis (bilharzia) is the classic case: larvae released by freshwater snails burrow through the skin of anyone wading, washing or swimming in slow-moving fresh water, and the adult worms in the blood vessels release eggs that pass out in urine or faeces and restart the cycle in the water. Guinea worm is a water-based disease caught by drinking water that contains tiny water fleas, or copepods, carrying the larvae. Water-related insect-vector diseases are spread by insects that breed in or near water: malaria and dengue by mosquitoes, river blindness by blackflies that breed in fast-flowing rivers, and sleeping sickness by the tsetse fly. Here the bite, not the drink, is the route.

• Water-borne: the pathogen is in the drinking water and travels the faecal-oral route - cholera, typhoid, diarrhoeal disease.
• Water-washed: too little water for washing, so infection passes skin to skin and hand to mouth - trachoma, scabies.
• Water-based: a parasite with part of its life cycle in water and an aquatic host - schistosomiasis, guinea worm.
• Water-related insect-vector: spread by insects that breed in water - malaria and dengue by mosquitoes.
• Classify the disease first, because the class decides which control measures score.

The pathogens, and the conditions that cause outbreaks

The pathogens involved belong to four groups. Bacteria cause cholera (Vibrio cholerae) and typhoid (Salmonella typhi); viruses such as rotavirus cause much of the severe diarrhoea in young children; protozoa include Giardia and Entamoeba in water supplies and the malaria parasite Plasmodium in the blood; and helminths, or parasitic worms, include Schistosoma and the guinea worm Dracunculus medinensis. The group matters for control, because a very small dose of chlorine kills most bacteria and viruses but is much less effective against some protozoa such as Cryptosporidium.

An outbreak usually needs three things at once: a contaminated source, a population with no alternative supply, and a route from the source to the mouth. The conditions examiners expect are a contaminated or unprotected drinking water supply; no sanitation, or sewers that leak and cross the water mains; flooding and storms, which mix sewage with drinking water and wash latrines into wells; a refugee camp or a fast-growing informal settlement, where many people share few latrines and taps; conflict and displacement, which damage or destroy water and sewerage infrastructure and move people into crowded conditions; poverty, which decides whether a household can afford soap, fuel for boiling and the fare to a clinic; drought, which concentrates people and animals around a shrinking, polluted supply; and a lack of knowledge about hygiene and how the disease is spread.

Cholera is the disease that shows how quickly water-borne infection can move. It kills by dehydration rather than by the infection itself: profuse watery diarrhoea can remove several litres of fluid from an adult in a day, so a patient who was walking in the morning can be close to death by evening. That is why the speed of the response - clean water, rehydration and separating sewage from the supply - decides how many people die.

Diarrhoeal disease is not a rare tropical curiosity. It is one of the largest causes of death in children under five worldwide, and almost all of those deaths are preventable by safe water, adequate sanitation and handwashing. For a management question, that makes water and sanitation one of the cheapest public health investments available: the pathogens are ordinary, the route is well understood, and the interventions are known.

• A contaminated drinking water supply, or one with no protection between the source and the home.
• No sanitation, or sewers that leak into the water mains and the groundwater.
• Flooding or a storm, which mixes sewage, latrines and animal waste with drinking water.
• Crowding, as in a refugee camp or an informal settlement with few shared taps and latrines.
• Conflict and displacement, which destroy infrastructure and concentrate people.
• Poverty and a lack of hygiene education, which decide whether households can boil water, buy soap and reach a clinic.

Malaria: the Anopheles mosquito and the Plasmodium parasite

Malaria is caused by protozoa of the genus Plasmodium, and it is not passed directly from person to person: it needs a mosquito. The vector is the female Anopheles mosquito. Only the female bites, because she needs a blood meal to develop her eggs, while the male feeds on plant nectar and transmits nothing. When she bites a person who already has malaria, she takes up Plasmodium in the blood meal.

The parasite then develops inside the mosquito, and after roughly ten days its salivary glands hold the infective stage. When the mosquito bites the next person she injects saliva to stop the blood clotting, and the parasite goes in with it: that bite is how the malaria parasite is transmitted to humans. The new patient develops malaria, and if another female Anopheles feeds on that patient's blood she takes up the parasite and joins the cycle, so the parasite is transmitted to non-infected mosquitoes when they feed on the blood of infected humans.

The cycle therefore has two arrows, human to mosquito and mosquito to human, and every control measure cuts one of them. Personal protection such as nets and repellent breaks the mosquito-to-human arrow, because the mosquito never gets to bite. Treating the patient with drugs breaks the human-to-mosquito arrow, because a mosquito that bites a cured person takes up no parasite at all. Vector control reduces the number of mosquitoes, so it attacks both arrows at once.

Anopheles breeds in still, often small bodies of water: puddles, tyre tracks, borrow pits, irrigation canals, drains and uncovered water tanks. It bites mainly between dusk and dawn, which is exactly why a net hung over the bed works. Compare this with dengue, which is carried by Aedes aegypti: that mosquito bites during the day and breeds in small containers of clean water around houses, so a bed net gives very little protection against it and the control measures that matter are emptying and covering containers, clearing drains, and using repellent and window screens by day.

Controlling malaria: personal protection and vector control

Personal protection stops the mosquito reaching the person. Insecticide-treated nets are the most effective single measure: Anopheles bites mainly at night, so a net over the bed blocks the bite and kills mosquitoes that land on it, which protects the whole household and not only the sleeper. Insect repellent on exposed skin, together with long sleeves and trousers after dusk, reduces bites where a net cannot be used. Antimalarial drugs have two separate jobs: chemoprophylaxis, taken before infection by travellers and pregnant women to stop the parasite establishing itself, and treatment of a patient with a combination therapy that includes artemisinin, which cures the illness and also removes the parasites that a mosquito could pick up. A vaccine against malaria - RTS,S, and the newer R21 - has been recommended for children in areas of moderate to high transmission; it cuts severe disease and deaths but does not give complete protection.

Vector control attacks the mosquito itself, so it needs no cooperation from the person being bitten. To cover or drain breeding areas, puddles, borrow pits, blocked drains and irrigation canals are cleared, filled or drained, and water storage tanks are covered, so that no still water is left for the larvae. Spraying large areas with insecticides means indoor residual spraying of the inside walls of houses, where resting mosquitoes are killed, and fogging outdoors to knock down adults in flight. To sterilise male mosquitoes, large numbers of males are sterilised by radiation, or reared carrying the bacterium Wolbachia, and released so that wild females mate with them and lay eggs that never hatch. Biological control uses the mosquito's natural enemies instead of chemicals: larvivorous fish such as Gambusia are put into tanks and ponds to eat the larvae, and the bacterium Bacillus thuringiensis israelensis (Bti) is sprayed as a larvicide that kills mosquito larvae with very little effect on other wildlife.

Every one of these measures has a limitation, and that is where the marks are. Mosquitoes develop resistance to insecticides, so a spray programme loses effectiveness over a few years and the chemical has to be rotated or changed, while the same chemicals kill bees and other insects and must be applied by trained staff with protective clothing. Nets need replacing and, unless they are long-lasting types, re-treating, and a torn net lets mosquitoes in. Resistance to antimalarial drugs develops where treatment is used carelessly or courses are not finished. The vaccine reduces severe disease but does not replace nets. Draining and covering every small breeding site in a large irrigated area is impossible to sustain, and abandoned drains silt up and become breeding sites again. The sterile-male method needs huge numbers of insects to be reared and released continuously, and its effect fades as soon as it stops. Biological control rarely removes a mosquito population completely, and an introduced fish can damage other species in the pond. And none of these measures touch a mosquito that bites outdoors or in the daytime.

The conclusion that examiners reward is that no single measure wins. Combined programmes have succeeded where single measures failed: Sri Lanka was certified malaria-free by the World Health Organization in 2016 and China in 2021, in both cases after decades of combining diagnosis and treatment, vector control and surveillance in which every case was investigated and the local breeding sites were attacked.

Controlling cholera: handwashing, sanitation, safe water and vaccination

Handwashing with soap attacks the faecal-oral route at the point where most infections are actually picked up. Hands touch the latrine, the soil, the nappy and the floor, and then touch food, water and the mouth. Washing at the two critical times, after using the latrine and before preparing or eating food, removes the pathogens before they reach the mouth, and it costs almost nothing. Its limitation is that it needs soap, a regular supply of clean water and the habit to be kept up; where the water supply fails, handwashing collapses with it.

Adequate sanitation and sewage treatment keep human waste out of the water in the first place, which is the most permanent answer to cholera. That means latrines or sewers, sewage carried away in pipes rather than ditches, and treatment works that remove or kill the pathogens before the effluent is returned to a river. It is the strategy that removes the source of an outbreak rather than the symptoms, but it is also the most expensive: it needs capital, water, electricity, skilled operators and continuing maintenance, and a fast-growing informal settlement usually grows faster than the pipe network can follow.

A potable water supply means water that is safe to drink, and there are two routes to it. In a piped supply the water is treated at the works and the mains are kept under pressure so that nothing seeps in; this is why the four treatment stages of screening, sedimentation, filtration and chlorination matter to disease as well as to supply. Where there is no piped supply, the treatment happens at the household: boiling kills bacteria, viruses and protozoa reliably and needs no chemicals, while chlorination with tablets or a measured dose kills bacteria and viruses, leaves a residual that protects the water in a container, and is cheap and quick. Both have limits. Boiling consumes fuel or firewood every day, costs time and money, and stored boiled water can be recontaminated by dirty hands, cups or a dirty container; chlorine is less effective in cloudy water, is less effective against some protozoa, and does nothing to the copepods that carry guinea worm larvae, which is why filtering rather than chlorinating is used against that disease.

Vaccination completes the set. The oral cholera vaccine is given in campaigns in areas at high risk and during outbreaks; it reduces the chance that a vaccinated person becomes ill and dies while the water and sanitation are being repaired. It is not a solution on its own. Protection is not complete, does not last indefinitely, and for full effect the two-dose course must be completed, which is difficult for people who have been displaced. The pattern is the same for malaria: a vaccine limits the harm, but it does not remove the pathogen from the environment.

Treatment: oral rehydration and what it does not do

Cholera and severe diarrhoeal disease kill by removing water and salts from the body, so the treatment that saves lives is to put those back. Oral rehydration solution (ORS) is a mixture of clean water, salt and sugar, prepared from a sachet to a fixed recipe or, in an emergency, from a measured amount of sugar and salt in clean water. It is given by mouth, in small sips, as much as the patient will take, and continued as the diarrhoea continues.

It works because of the way the gut absorbs sodium. Glucose in the solution is taken up by the intestine, and sodium is absorbed with it; where sodium goes, water follows, so the fluid taken by mouth is drawn back into the blood even while the gut is losing fluid. This simple mechanism turned a disease that killed a large share of its patients into one that can be treated anywhere, without electricity, refrigeration or a trained doctor, and it is why rehydration is regarded as one of the most cost-effective medical treatments there is. A sachet and clean water cost very little, and a parent can give the solution at home.

Its limitations are what make it a treatment rather than a control measure. ORS does not kill the pathogen: the patient stops being dehydrated but the bacteria are still in the gut and still being shed in the stool, so the patient can infect the water supply and other people for days. It cannot be given to a patient who is already too weak to swallow or who is vomiting everything back, and those patients need intravenous fluids in a clinic. It has to be mixed with genuinely clean water, and it must be mixed in the right proportions: too little salt is ineffective and a strong solution can itself harm the patient, especially a child. And it depends on supplies and knowledge reaching the household, which is exactly what a collapsed health system, a flooded road or a poor family may not have.

Because of those limits, rehydration sits beside the preventive measures rather than replacing them. In an outbreak the response is combined: treat the sick at once with ORS and intravenous fluids, chlorinate or boil the water and distribute it, stop the use of contaminated sources, build emergency latrines, promote handwashing, and vaccinate. Treatment and prevention are asked about together precisely because neither is enough alone.

Weighing up the strategies: benefits, limitations and a judgement

The four control strategies for water-related disease differ in cost, in speed, in how long they last and in how much they depend on people behaving differently. That is the basis on which an exam answer should judge them. A piped, treated supply and a sewer network protect everyone who is connected, permanently and without any effort from the household, but they are the most expensive option and take years to build. Household treatment by boiling or chlorination is cheap and available immediately, but it has to be repeated every single day and depends on fuel, chemicals and habit. Handwashing and hygiene education are almost free and reach places no pipe will reach, but they depend entirely on behaviour and on water being available. Vaccination and oral rehydration act fast and save lives during an outbreak, but neither stops transmission, and both consume scarce health budgets.

Vector control has the same shape of argument. Draining, covering and clearing breeding sites is a permanent physical change, needs no cooperation from residents and no chemicals, and reduces mosquitoes for as long as it is maintained - but small sites such as puddles, water tanks, discarded containers and irrigation channels are almost impossible to keep free of water, and they refill with the next rain. Spraying is quick and covers a large area, but it must be repeated, it is expensive, insects become resistant, and it damages other insects. Nets work every night where Anopheles is the vector, but they are useless against day-biting Aedes. Sterilising male mosquitoes and biological control are targeted and use no chemicals, but they must be sustained continuously and introduced organisms can affect species that were not the target.

Water-based and water-washed diseases need their own answers, because the water is not being drunk. Schistosomiasis is controlled by treating patients with praziquantel, killing the freshwater snails with molluscicides, providing safe piped water and latrines so that people do not have to wade to wash or defecate into the water, and by health education that warns children away from the shallow water where the larvae are. The drug cures the individual but reinfection is quick if the snails and the contaminated water remain, and molluscicides kill other water animals and are costly to apply across a whole lake or irrigation system. Guinea worm is controlled by filtering drinking water through a fine cloth, treating ponds with a larvicide that kills the copepods, supplying safe water, and containing every case so that a patient cannot contaminate a pond. It is the strongest example of success through a combined, targeted programme: cases fell from an estimated three and a half million a year in the 1980s to a handful in recent years.

A judgement. The measures that protect the most people for the longest are the unglamorous ones - a safe water supply, adequate sanitation and handwashing - because they remove the pathogen from the environment rather than treating its victims, and they protect the whole population, not only those who use them. Where money and time are short, the sensible order is to stop the immediate outbreak with treatment, chlorination and vaccination, then to make the physical changes in the order of their cost per person protected: latrines and handwashing first, then a treated piped supply and sewers. The most effective programmes use several measures at once, because every one of them has a failure point - fuel runs out, a net tears, resistance develops, a drain refills - and the failure of one measure is covered by the others.