Communicable Disease Control: Outbreak Investigation

This section covers:

• The steps in outbreak investigation including the use of relevant epidemiological methods

• The Seven steps of an outbreak investigation
• Sources of outbreaks
• Potential outbreaks - key facts

The Seven steps of an outbreak investigation

1. Preliminary assessment in outbreak investigation
   (to confirm existence of outbreak)
   • is there an outbreak? Review cases, particularly with regard to time/place/person (age, sex, occupation), numbers, place of residence
   • confirm numbers
     • interview cases if possible
     • discuss laboratory results and repeat if necessary
     • consult experts at the Health Protection Agency's: Centre for Infection (CfI) or more locally at HPU
   • is further investigation needed?
     • review literature and surveillance data
     • are there still cases occurring? How serious?
   • form Outbreak Control Team (OCT) if prelim assessment suggests actual outbreak, ensuring the following are represented in the membership:
     • Consultant in Communicable Disease Control (CCDC)
     • Environmental Health Officer (EHO)
     • microbiologist
     • press officer
     • secretary
       and consider:
     • regional epidemiologist
     • CDSC Specialist
     • water board
     • Infection Control Nurse (ICN)

     NB:

     Outbreak Type	Outbreak control team consider including:
     food or waterborne	Environmental Health Officer (EHO)
     water alone	Relevant water company
     NHS premises	Infection Control Nurse/Infection Control Doctor Senior Manager
     More than one PCT	CDSC and Regional Epidemiologist
     zoonotic disease	state veterinary service/DEFRA
     occupational lookbacks	occupational health department

   • generate initial hypothesis
   • initiate immediate control measures as necessary, for example:
     • Environmental Health Officer to visit and sample
     • stop symptomatic food handlers from working
     • clean/disinfect premises
     • recall product
     • treat patients
     • offer prophylaxis to contacts
        

2. Case definition and identification

   • define 'cases' in terms of time/place/person/symptoms/lab results
   • define population at risk
   • case identification:
     • ask known cases
     • check routine surveillance data, notifications and laboratory results
     • email other CCDC's
     • other health workers/Environmental Health Officers (EHOs)
     • public alert
        

3. Descriptive study

   • collect data
   • analyse data
   • draw epidemic curve
   • generate hypothesis - try and link in some way to source e.g. use 'trawling' questionnaire
      

4. Analytical study of an outbreak

   • cohort study for food outbreaks where the population is known e.g. wedding reception => attack rate and relative risk (RR)
   • case-control study to sample large exposed population => odds ratio (OR); use trawling questionnaire
   • Purpose:
     • to test 'hypothesis'
     • to attempt to overcome bias
        

5. Verify hypothesis

   • by microbiology using food and environmental samples: Environmental Health Officer to visit site to sample and educate
   • by veterinary investigation
      

6. Institute control measures

   • remove source:
     • consider closing outlet
     • isolate and treat cases
     • destroy/treat food
   • protect persons at risk:
     • improve hygiene
     • prophylaxis
   • prevent recurrence
     • recommendations
     • guidelines
        

7. Communicate

   • During outbreak: information to public and professionals
   • After outbreak: report, for those involved and wider audience, also make recommendations for the future

   NB: always record outbreak details with times and dates

   • may need to do multiple levels of investigation at the same time
      

Sources of outbreaks

Point source - explosive (primary cases will cluster within the range of the incubation period) e.g. Salmonella outbreak at wedding reception

Continuing source - e.g cases of S. agona PT15 by week of onset in England and Wales

Point source and person to person spread - e.g. E.Coli 0157 in nursery

Potential outbreaks - key facts

Food/waterborne gastro-enteritis

• involve Environmental Health Officer in investigation e.g. to find source
• collect stool specimens if possible from all sporadic cases; as many as possible in outbreak
• Prevention
  • hand washing and no towel sharing
  • disposal of soiled material (use of plastic gloves/sealing; washing towels and linen on a 'hot' cycle)
  • disinfect toilet area and taps daily
  • education of personal hygiene and hygiene in preparation of food
• Exclusions
  • All cases must be excluded from work or school until free from diarrhoea and vomiting for 48 hours as a minimum
  • In the case of an 'at risk' group, exclude for longer especially if they are and review the need for microbiological clearance:
    • food handlers
    • staff of health care facilities
    • children under five years
    • people with learning disabilities
• In the case of 'non risk' groups there is no microbiology follow up
  • most food poisoning - no screening of contacts necessary, except in the case of Typhoid and E.coli OI57

Exposures to infected water

Check:

• occupational exposure
• recreational exposure
• domestic exposure
• consumption of water, vegetables, fish (in areas of infected water)

© Sarah Anderson, Gayatri Manikkavasagan 2008