Key Facts

Outbreak investigation is a core public health competency. Rapid identification, investigation, and control of outbreaks prevents further cases and protects public health. The principles apply to all infectious disease outbreaks.

Steps of Outbreak Investigation

1. Confirm the outbreak: verify diagnosis, confirm excess above expected baseline
2. Establish case definition: clinical and epidemiological criteria (confirmed, probable, possible)
3. Identify and count cases: active case finding, line listing
4. Describe: person (age, sex, risk factors), place (mapping), time (epidemic curve)
5. Generate hypotheses: from descriptive epidemiology, interviews, environmental assessment
6. Test hypotheses: analytical study (cohort or case-control), microbiological investigation
7. Implement control measures: source control, transmission prevention, population protection
8. Communicate: risk communication to public, media, healthcare professionals
9. Evaluate and report: outbreak report, lessons learned

Outbreak Team Members

• Consultant in Communicable Disease Control (CCDC)
• Microbiologist
• Environmental Health Officer
• Infection prevention and control team
• Communications officer
• Other specialists as needed (food safety, water, veterinary)

Legal Framework

• Health Protection (Notification) Regulations 2010: list of notifiable diseases
• Public Health (Control of Disease) Act 1984 (amended 2008): powers to investigate and control
• UK Health Security Agency (UKHSA) provides national coordination

Epidemic Curve Patterns

• Point source: all cases exposed at same time; bell-shaped curve; incubation period determines timing
• Continuous source: ongoing exposure; plateau pattern; cases continue until source removed
• Propagated (person-to-person): successive waves with peaks separated by incubation period
• Intermittent source: irregular pattern; intermittent exposure

Common Outbreak Scenarios

• Norovirus in care home: rapid onset vomiting and diarrhoea; highly infectious; environmental contamination
• Food-borne Salmonella: cohort exposed at common event; identify food vehicle using attack rates
• Measles in unvaccinated community: propagated spread; contact tracing and MMR catch-up
• TB cluster: linked cases identified through contact tracing; molecular typing confirms transmission
• Legionella: environmental source (cooling towers, water systems); no person-to-person spread

Red Flags for Outbreak Declaration

• Unusual clustering of cases (time, place, person)
• Disease more severe than expected
• Unusual organism or resistance pattern
• Cases in healthcare workers
• Bioterrorism potential (anthrax, smallpox, plague)

Descriptive Epidemiology

• Line listing: spreadsheet of all cases with demographics, symptoms, exposure history
• Attack rate calculation: cases from exposed group / total exposed × 100
• Epidemic curve: plot cases by date of onset
• Spot map: geographic distribution of cases

Analytical Studies

• Cohort study (for defined populations, e.g. wedding guests): compare attack rates in exposed vs unexposed
• Case-control study (for open populations): compare exposures in cases vs matched controls
• Calculate RR (cohort) or OR (case-control) for suspected exposures

Microbiological Investigation

• Clinical samples: stool, blood, respiratory samples for culture, PCR, serology
• Environmental samples: food, water, surfaces
• Molecular typing: whole genome sequencing (WGS) to confirm linked cases
• Reference laboratory: UKHSA provides specialist typing services

Environmental Investigation

• Food safety inspection of premises
• Water sampling and testing
• Review of food handling, storage, cooking processes
• Inspection of cooling towers (Legionella)

Control Measures

Source control:

• Remove contaminated food from sale
• Close implicated food premises
• Treat contaminated water supply
• Decontaminate cooling towers
• Isolate infectious cases

Transmission prevention:

• Infection control measures (hand hygiene, PPE, environmental cleaning)
• Exclusion of cases from work/school (48-hour rule for GI infections)
• Cohorting of cases in healthcare settings
• Enhanced cleaning protocols

Population protection:

• Post-exposure prophylaxis (e.g. rifampicin for meningococcal contacts, MMR for measles contacts)
• Vaccination campaigns
• Public health messaging
• Contact tracing

Communication

• Clear, timely communication to public, media, and healthcare professionals
• Risk communication principles: acknowledge uncertainty, provide actionable advice, maintain trust
• UKHSA and local authority issue joint communications

Notification

• All suspected notifiable diseases must be reported to local authority proper officer
• Immediate notification by phone for urgent cases
• Written notification within 3 days
• UKHSA Second Generation Surveillance System (SGSS) for laboratory reports

• Rapid outbreak investigation and control reduces secondary cases and saves lives
• UK has robust surveillance and response systems through UKHSA
• Most food-borne outbreaks are self-limiting; control measures prevent further cases
• Healthcare-associated outbreaks (norovirus, C. difficile) cause significant morbidity and mortality in vulnerable patients
• Vaccine-preventable outbreaks (measles) are entirely avoidable with adequate coverage
• Lessons from outbreaks improve future preparedness and policy

Selected UK Notifiable Diseases

Attack Rate Calculation Example

Chicken salad RR = 75%/8% = 9.4 (strong association)