# Water-borne Diseases

> Water-borne diseases for FMGE Community Medicine: Bradley classification, cholera ORS, typhoid, hepatitis E, residual chlorine, water purification and filters.

- Canonical URL: https://prepelephant.com/topics/fmge/community-medicine/water-borne-diseases-fmge
- Exam / course: FMGE · Subject: Community Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Water-borne Diseases", PrepElephant, https://prepelephant.com/topics/fmge/community-medicine/water-borne-diseases-fmge

## Direct answer

Cholera, typhoid and hepatitis E ride the faecal-oral route through contaminated water, but water's role in disease is broader than that — Bradley's classification separates water-borne (faecal-oral), water-washed (skin and eye infections from scarcity), water-based (organisms living in water, like the guinea worm) and water-related vector (mosquitoes breeding in water) diseases. The public health response is safe water: disinfection to the standard residual chlorine of about 0.2 to 0.5 mg/L at the tap after 30 minutes of contact, filtration, and household methods like boiling — while clinical response to the biggest killer, cholera, is rehydration with WHO low-osmolarity oral rehydration salts (245 mOsm/L).

## What you must remember

- Bradley classification: water-borne — cholera, typhoid, hepatitis A and E, poliomyelitis; water-washed — scabies, trachoma (improve quantity and access); water-based — schistosomiasis, dracunculiasis; water-related vector — malaria, dengue, filariasis.
- Cholera: rice-water stool, painless diarrhoea with rapid dehydration; treatment is rehydration first (WHO ORS 245 mOsm/L with 75 mmol/L sodium for most, Ringer's lactate for severe), with doxycycline as an adjunct in severe cases to shorten excretion.
- Typhoid: blood culture in the first week is the gold standard; on Widal, a fourfold rise in paired titres matters far more than any single value; treat per current sensitivity patterns.
- Hepatitis E deserves special respect in pregnancy: infection in the third trimester can turn fulminant with maternal mortality quoted in double digits, a classic Indian exam fact.
- Guinea worm (dracunculiasis) is water-based; India's last case was in 1996 and WHO certified India guinea-worm-free in February 2000.
- Chlorination standard: about 0.5 mg/L applied for 30 minutes leaves a residual of 0.2 mg/L or more at the tap, tested by DPD or ortho-tolidine; bleaching powder carries 25-33% available chlorine; superchlorination with dechlorination serves outbreaks, and break-point dosing is where free residual begins to rise.
- Slow sand filtration is the superior bacterial filter (98-99% removal) because of the vital biological layer (schmutzdecke) that develops on the sand; rapid sand filters need prior coagulation with alum and remove far fewer bacteria on their own.
- Fluoride: about 1 mg/L protects teeth; above 1.5 mg/L causes fluorosis over years — dental mottling then skeletal fluorosis; the Nalgonda technique (alum plus lime) is India's known defluoridation method.

## Investigating a gastroenteritis outbreak

A district receives 60 acute diarrhoea cases from one ward in two days. Confirm the syndrome, count cases against a denominator, and draw the epidemic curve — a sharp common-source spike points to water. Inspect the supply: when was the overhead tank last cleaned, and what does an immediate field test of residual chlorine at three household taps reveal. Collect water samples in sterilised bottles for coliform count and stool samples in Cary-Blair medium for Vibrio cholerae. Meanwhile act: advise boiling or household chlorination, superchlorinate the implicated source, open an ORS corner with intravenous Ringer lactate for severe dehydration, and search for mild cases and carriers. The investigation ends with the classic chain — source identified, chain broken by disinfection, cases treated — and the proof of the fix is the residual chlorine reading at the same taps a week later.

## Numbers worth knowing

Four numbers carry most of the marks. Residual chlorine: 0.2 to 0.5 mg/L at the point of use after 30 minutes of contact — examiners ask the field target rather than the dose. ORS: the WHO reduced-osmolarity formula at 245 mOsm/L with sodium 75, glucose 75 and potassium 20, the global standard since 2003. Fluoride: 1.0 mg/L optimal, 1.5 mg/L the upper permissible limit in India, Nalgonda technique for defluoridation. Slow sand filter: 98-99% bacterial removal thanks to the schmutzdecke, versus the rapid filter's need for alum coagulation. Add two timed facts: guinea worm certification February 2000, and the typhoid rule — blood culture first week, Widal rising titre after the first week.

## Frequently asked questions

### What is Bradley's classification of water-related diseases?
Water-borne (faecal-oral: cholera, typhoid, hepatitis A and E), water-washed or water-scarcity (scabies, trachoma), water-based (schistosomiasis, dracunculiasis) and water-related vector-borne (malaria, dengue, filariasis).

### What is the composition and advantage of WHO low-osmolarity ORS?
Sodium 75, glucose 75, potassium 20, citrate base, total osmolarity 245 mOsm/L — it reduces stool output, vomiting and need for intravenous fluids compared with the old 311 mOsm/L formula.

### What residual chlorine should drinking water have at the consumer end?
About 0.2 mg/L or more after 30 minutes of contact time, tested by the DPD or ortho-tolidine method; absence of residual means recontamination or under-dosing.

### Why is slow sand filtration more effective than rapid sand filtration?
Because the schmutzdecke — a living biological film of algae, plankton and bacteria on the sand surface — traps and digests pathogenic bacteria, achieving 98-99% removal without chemicals, while rapid filters are primarily clarifiers needing coagulation and disinfection.

### What is the significance of hepatitis E in pregnancy?
Hepatitis E in the third trimester carries a high risk of fulminant hepatic failure with maternal mortality reported up to 15-25%, making it the most dangerous water-borne infection for pregnant women in India.