Levels of prevention

Levels of prevention

• Classify any intervention by the stage it acts on: before risk factors exist, before disease starts, before disease shows, or after disease is established.

• Five modes of intervention: health promotion and specific protection (primary); early diagnosis and treatment (secondary); disability limitation and rehabilitation (tertiary).
• Prevention paradox: a small risk reduction across a whole population prevents more cases than intensive treatment of the few at high risk, though each individual gains little (population versus high-risk strategy).

Exam trap Primordial prevention targets the risk factor (childhood obesity); primary prevention targets the disease in people already exposed (statins, vaccines).

Natural history and disease frequency

• Pre-pathogenesis (susceptibility), then pathogenesis (subclinical, then clinical), then recovery, disability or death. The iceberg phenomenon: undiagnosed and subclinical cases far outnumber visible ones (hypertension, diabetes, TB).
• Prevalence counts old plus new cases at a time; incidence counts only new cases over a period. Prevalence ≈ incidence × duration.
• Endemic: constant usual presence; epidemic: clearly more cases than expected; pandemic: epidemic across countries or continents; sporadic: scattered cases; outbreak: a localised epidemic.

Transmission, R₀ and herd immunity

• Incubation period: infection to symptoms. Latent period: infection to infectiousness. Communicable period: time a person can transmit.
• Basic reproduction number (R₀): average number of new cases from one case in a fully susceptible population (measles about 12–18, polio 5–7, influenza about 2). The effective R falls as immunity and control measures increase; an epidemic shrinks once it is below 1.
• Herd immunity threshold = 1 − 1/R₀: the share that must be immune so that each case infects fewer than one other person.

• Because no vaccine is 100% effective, the coverage needed is higher than the threshold (coverage = threshold ÷ vaccine efficacy).
• Secondary attack rate = new cases among contacts ÷ susceptible contacts × 100.
• Chain of infection: agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host; breaking any link controls disease. Isolation separates the sick; quarantine restricts healthy contacts for the longest incubation period.
• Epidemic curves: point source (one sharp peak within one incubation period: food poisoning at a wedding); continuous common source (plateau: contaminated water supply); propagated (person to person: successive waves one incubation period apart).

Remember Herd immunity threshold = 1 − 1/R₀: R₀ of 4 needs 75%, measles needs over 90%.

Surveillance

• Passive: routine reports from facilities; cheap but under-reports. Active: staff search for cases (polio acute flaccid paralysis surveillance, outbreaks). Sentinel: selected sites (influenza, HIV). Syndromic and environmental (sewage sampling for poliovirus).
• Purpose: detect outbreaks, follow trends, evaluate programmes, set priorities. Pakistan uses Integrated Disease Surveillance and Response and Lady Health Workers; the International Health Regulations (2005) require reporting of public health emergencies of international concern to WHO.

Outbreak investigation

• Prepare for fieldwork, then verify the diagnosis and confirm that an outbreak exists (cases exceed expected numbers); these two come first and are done together.
• Define a case (person, place, time; confirmed, probable, possible), find cases and list them.
• Describe by time (epidemic curve), place (spot map) and person; form hypotheses; test them (cohort study for a defined group such as wedding guests, case-control otherwise); add laboratory and environmental studies.
• Start control measures as early as possible, without waiting for the end; maintain surveillance and communicate findings.
• Mass treatment can control institutional outbreaks, such as permethrin 5% cream for scabies in a school or hostel.

Safe water

• Safe water and sanitation prevent faecal–oral disease: typhoid, cholera, hepatitis A and E, polio and diarrhoea. Typhoid spreads through water or food contaminated by patients or carriers, so it is the classic disease controlled by safe water supply and sanitation.
• Household methods: boiling (most reliable), chlorination, filtration, solar disinfection. Community: storage and sedimentation, slow or rapid sand filtration, then chlorination.
• Chlorination: free residual chlorine of 0.5 mg/L after a contact period of one hour (at least 30 minutes) shows the water is safe; about 0.2 mg/L at the tap. Bleaching powder contains about 33% available chlorine.
• Horrock's apparatus is the field kit for estimating the dose of bleaching powder needed to disinfect a given volume of water (for example a well).
• Bacteriological quality: coliforms and E. coli indicate faecal contamination and must be zero per 100 mL of drinking water. Groundwater in parts of Pakistan carries arsenic and fluoride.

ORS, food and milk hygiene

• Reduced-osmolarity ORS (245 mOsm/L: sodium 75, glucose 75 mmol/L) works through sodium–glucose cotransport (SGLT1) in the small intestine, which stays intact even in cholera; water follows the absorbed solute. Lower osmolarity reduces stool output and vomiting.
• Pasteurisation heats milk to kill pathogens without sterilising it. Flash (HTST): about 72 °C for 15 seconds, then rapid cooling below 10 °C. Holder method: 63–66 °C for 30 minutes. UHT: about 135–150 °C for 1–2 seconds. It kills *M. bovis*, *Brucella*, *Salmonella*, *Listeria* and *Coxiella* (the most heat-resistant pathogen, which sets the standard); spores survive. The phosphatase test checks adequacy.

Occupational and environmental health

• Lung cancer is the commonest occupational cancer. Also important: indoor biomass smoke, outdoor smog (PM2.5), carbon monoxide, noise, overcrowding (TB, meningococcus), vector control (Aedes breeds in clean stored water) and climate events such as the 2022 Pakistan floods.

Population pyramids and health indicators

• A broad base and narrow top (Pakistan) means high birth rates and short lives (high mortality, young population). A column or inverted shape means low fertility and an ageing population (Japan).
• Infant mortality rate (deaths under 1 year per 1,000 live births) is the most sensitive indicator of socioeconomic development; neonatal mortality covers under 28 days; maternal mortality ratio is per 100,000 live births. DALY = years of life lost + years lived with disability; QALY weighs survival by quality.

How FCPS asks about this

• Example of primordial prevention: preventing childhood obesity; screening is secondary, rehabilitation tertiary.
• Herd immunity with R₀ of 4 / measles R₀ of 15: 75% / about 93% (1 − 1/R₀).
• Field kit to estimate bleaching powder dose: Horrock's apparatus; residual chlorine 0.5 mg/L after one hour.
• Flash pasteurisation: about 72 °C for 15 seconds; kills pathogens, not spores.
• Mechanism by which ORS replaces fluid: sodium–glucose cotransport.
• Stone cutter with eggshell hilar calcification: silicosis; at risk of tuberculosis. Shipyard worker with pleural tumour: asbestos.
• Population pyramid with broad base and narrow top: high birth rate and short life expectancy.