Act as community medicine Prof checking answers Sheet Now answer theses for 5 marks frima park text book Water purification in small scale
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Water Purification on a Small Scale
Park's Textbook of Preventive & Social Medicine
(Model Answer - 5 Marks)
Small-scale (household/individual) water purification uses three principal methods, which may be applied singly or in combination.
(a) BOILING
Water must be brought to a "rolling boil" for 10-20 minutes. This kills all bacteria, spores, cysts, and ova, yielding sterile water. It also removes temporary hardness by driving off CO2 and precipitating calcium carbonate.
Limitation: Offers no residual protection against subsequent contamination. Water should be boiled and stored in the same container to minimise re-contamination.
(b) CHEMICAL DISINFECTION
Several agents are used:
| Agent | Key Details |
|---|---|
| Bleaching powder (CaOCl2) | Contains ~33% available chlorine; unstable on exposure to air/light/moisture. Target: free residual chlorine of 0.5 mg/L after 1 hour contact. |
| Chlorine solution | 4 kg bleaching powder (25% available Cl) + 20 L water = 5% chlorine solution. |
| High Test Hypochlorite (HTH/Perchloron) | Contains 60-70% available chlorine; more stable than bleaching powder. |
| Chlorine tablets (Halazone) | 0.5 g tablet disinfects 20 litres; convenient but costly. |
| Iodine | 2 drops of 2% ethanol solution per litre of clear water; contact time 20-30 minutes; useful for emergency disinfection. |
| KMnO4 | No longer recommended - not reliable against most organisms; alters colour, smell, and taste. |
The principle in all chlorination methods: achieve a free residual chlorine of 0.5 mg/litre at the end of 1 hour contact.
(c) CERAMIC FILTRATION
Water is passed through candle filters:
- Pasteur-Chamberland filter - porcelain candle
- Berkefeld filter - kieselguhr (infusorial earth) candle
- Katadyn filter - silver-coated surface; silver ions kill bacteria by oligodynamic action
Limitation: Remove bacteria but not filter-passing viruses. Candles clog with impurities and must be scrubbed weekly and boiled regularly. Not widely suitable under Indian field conditions.
(d) ULTRAVIOLET (UV) IRRADIATION
Water (up to 120 mm thick) is exposed to quartz mercury vapour arc lamps at 254 nm wavelength. Effective against bacteria, viruses, fungi, algae, protozoa, and cysts.
Advantages: Short exposure time, no foreign matter introduced, no taste/odour change, no harm from overexposure.
Disadvantages: No residual effect, no simple field test to assess efficacy, expensive apparatus. Water must be turbidity-free.
(e) MULTI-STAGE REVERSE OSMOSIS (RO)
Reduces total dissolved solids (TDS), hardness, heavy metals, and microorganisms (bacteria, viruses, protozoa, cysts), making water both chemically and microbiologically potable.
Stages typically include:
- Clarity cartridge - removes suspended particles (dust, mud, sand)
- Pre-carbon filter
- RO membrane
- Post-carbon filter
Examiner's Marking Scheme (5 marks)
| Component | Marks |
|---|---|
| Boiling - mechanism, duration, limitation | 1 |
| Chemical disinfection - at least 3 agents with details | 1.5 |
| Ceramic filtration - types, oligodynamic action, limitations | 1 |
| UV irradiation - wavelength, advantages, disadvantages | 0.5 |
| Reverse osmosis - principle and stages | 0.5 (bonus/extra credit) |
| Total | 5 |
Professor's Note: A complete 5-mark answer must cover boiling, at least 3 chemical agents with the 0.5 mg/L residual chlorine principle, and at least one filtration method with the oligodynamic action concept for Katadyn filter. UV and RO earn extra marks. Candidates who only list methods without mechanistic details lose 1-2 marks.
Source: Park's Textbook of Preventive and Social Medicine, Chapter on Water Supply
Rita virus vaccine
Rota virus vaccine
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All data in hand. Here is the full model answer:
Rotavirus Vaccine
Park's Textbook of Preventive & Social Medicine
(Model Answer - 5 Marks)
Background / Burden of Disease
Rotavirus is the most common cause of severe diarrhoeal disease in infants and young children worldwide. Vaccination is a key strategy within the Integrated Global Action Plan for the Prevention and Control of Pneumonia and Diarrhoea (GAPPD).
Types of Rotavirus Vaccines
Two live, oral, attenuated rotavirus vaccines were licensed in 2006:
| Feature | Rotarix™ (RV1) | RotaTeq™ (RV5) |
|---|---|---|
| Type | Monovalent, human rotavirus vaccine | Pentavalent bovine-human reassortant vaccine |
| Doses | 2 oral doses | 3 oral doses |
| Schedule | 2 and 4 months of age | 2, 4, and 6 months of age |
| First dose | From 6 weeks; no later than 12 weeks | Between 6-12 weeks |
| Interval between doses | At least 4 weeks | 4-10 weeks apart |
| Completion | By 16 weeks (no later than 24 weeks) | All 3 doses before 32 weeks |
| Route | Oral | Oral |
Both vaccines have demonstrated very good safety and efficacy profiles in large clinical trials and are now used in both industrialized and developing countries.
India - Universal Immunization Programme (UIP) Schedule
Under India's UIP (2020), the Rotavirus Vaccine (RVV) is:
- Given as 3 doses at 6 weeks, 10 weeks, and 14 weeks of age
- Dose: 3 drops, oral route
- Can be given up to 1 year of age
Contraindications
- Age >12 weeks for first dose - vaccination should NOT be initiated; higher risk of intussusception
- Severely immunocompromised infants
- History of intussusception
- Should not be used in catch-up vaccination campaigns where exact infant age is difficult to ascertain
Adverse Effects
| Adverse Effect | Detail |
|---|---|
| Intussusception | Most important - post-marketing surveillance detected ~1-2 cases per 100,000 infants vaccinated, shortly after the first dose in some settings |
| Mild GI symptoms | Transient, self-limiting |
The risk of intussusception is potentially higher when the first dose is given to infants >12 weeks of age - hence the strict age cut-off.
Key Points for Exam (5 Marks)
| Point | Marks |
|---|---|
| Two types - Rotarix (monovalent) and RotaTeq (pentavalent) with year of licensure | 1 |
| Schedules for both vaccines (doses, ages, intervals, completion age) | 1.5 |
| India UIP schedule - 6, 10, 14 weeks; oral; 3 drops | 1 |
| Contraindications - age >12 weeks for first dose, no catch-up | 0.5 |
| Adverse effect - intussusception risk (1-2/100,000) | 1 |
| Total | 5 |
Professor's Note: Many candidates confuse Rotarix (2 doses) with RotaTeq (3 doses) - this distinction is frequently tested. The intussusception risk and the strict upper age limit of 12 weeks for first dose initiation are high-yield examination points. The India UIP uses a 3-dose schedule (6, 10, 14 weeks) orally.
Source: Park's Textbook of Preventive and Social Medicine
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