Introduction

• This section describes the cholera surveillance strategies that implemented in surveillance units where there is a probable or confirmed cholera outbreak.
• The applicable surveillance strategies differ depending on whether there is community transmission or clustered transmission in a surveillance unit.

Definitions

Start date of a cholera outbreak

• The date of onset of symptoms of the first locally acquired suspected cholera case detected in a surveillance unit.

Suspected cholera case

• In the presence of a probable or confirmed cholera outbreak a suspected cholera case is any person:
  • with acute watery diarrhoea;
    or
  • who died from acute watery diarrhoea.

Community transmission

• There is community transmission if confirmed cholera cases are not all epidemiologically linked.
• By default, an outbreak is classified (and monitored) as community transmission unless clustered transmission has been demonstrated through case investigation.

Clustered transmission

• There is clustered transmission if confirmed cholera cases are all epidemiologically linked, based on the findings of case investigations.
• Clustered transmission is more likely to occur at the onset (or towards the end) of a cholera outbreak when the number of cholera cases is low.

End of a probable or confirmed cholera outbreak

• A probable or confirmed cholera outbreak can be considered over when, for a minimum of four consecutive weeks, all suspected cholera cases have a negative test result by RDT, culture, or PCR.

Classification of outbreaks: community transmission or clustered transmission

• In non-endemic countries (including countries on the path to eliminating cholera), it is recommended that a probable or confirmed cholera outbreak be further classified as either community transmission or clustered transmission.
• If this classification is not undertaken, the outbreak is classified as community transmission by default.
• Different surveillance strategies apply for monitoring community transmission and clustered transmission.

Monitoring the outbreak: community transmission

• In a probable or confirmed cholera outbreak with community transmission, surveillance aims to monitor the morbidity, mortality, and case fatality ratio to guide interventions and mitigate the impact and spread of the outbreak.

Detection and reporting of cases

• Standard data is collected on all suspected cholera cases detected in health facilities and community settings. See Appendix 2 for a template cholera case report form, Appendix 3 for a template cholera line list and Appendix 4 for a community-based surveillance template reporting form.
• Data is reported to local health authorities at least weekly (including zero reporting). More frequent reporting (i.e., daily) is recommended at the onset or towards the end of an outbreak when cases are sporadic.

Testing

• Test a subset of suspected cholera cases tested a according to a systematic sampling scheme (i.e., the sampling scheme is be consistent over time).
• If RDTs are available:
  • Test the first 3 suspected cases per day per health facility by RDT
    and
  • Collect stool samples from 3 RDT+ patients per week per surveillance unit and send them to a laboratory for testing by culture and/or PCR.
• If RDTs are unavailable:
  • Collect stool samples from the first 3 suspected cases per week per health facility and send them to a laboratory for testing by culture and/or PCR.
• Perform antimicrobial susceptibility testing (AST) on the first 5 confirmed cholera cases per surveillance unit. Then, perform AST on at least 3 confirmed cholera cases per surveillance unit per month.
• Conducting whole genome sequencing (WGS) on a subset of confirmed cholera cases is also encouraged. However, this is not required for public health intervention.
• Towards the end of an outbreak, test all suspected cases by RDT or culture or PCR.

Data analysis and interpretation

• Analyse and interpret data at least on a weekly basis.
• More frequent analysis (e.g., daily) is encouraged at the onset and towards the end of an outbreak to help ensure timely implementation of interventions to interrupt transmission, and to confirm the end of the outbreak.
• Conduct data analysis primarily at the level of the surveillance unit to inform targeted interventions.
• Analyse community-based and health facility-based surveillance data separately but interpret these data streams jointly.
• Include both weekly data for the last epidemiological week and cumulative data starting from the beginning of the calendar year (or the start date of the outbreak) in the analysis. Compare weekly values with those of the previous week(s).
• Include a description of cases by person, place, and time, as well as key morbidity and mortality indicators in the analysis. See below for more information about descriptive epidemiology and key indicators.
• Disseminate findings in weekly epidemiological reports to health authorities, health professionals, and other sectors. See Appendix 9 for an outline of an epidemiological report.

Descriptive epidemiology

• By person
  • Cases
    • Number of suspected cholera cases
    • Number of cholera cases stratified by age group and sex. The following age groups should be considered: <2, 2-4, 5-14, 15-44, 45- 59, ≥60 years old.
  • Tests
    • Number of suspected cases tested by RDT or by culture or PCR
    • Number of suspected cases tested positive by RDT or by culture or PCR
  • Deaths
    • Number of cholera deaths in health facilities
    • Number of community cholera deaths
    • Number of cholera deaths stratified by age group and sex. The following age groups should be considered: <2, 2-4, 5-14, 15-44, 45- 59, ≥60 years old.
  • Severity & hospitalization
    • Proportion of cases hospitalized as inpatients
    • Proportion of cases by level of dehydration (at least severe dehydration)
• By place
  • Provide a spatial distribution of cases and deaths to describe the geographic extent of the outbreak, identify the areas most affected, and formulate hypotheses about sources of contamination and contexts of transmission.
  • Include other geographic variables or points of interest that might be associated with cholera transmission (e.g., water sources, major transportation routes, markets, etc.).
• By time
  • Plot cholera cases and deaths over time to monitor the outbreak dynamics (i.e. the epidemic curve of the number of suspected cholera cases by date of symptom onset or consultation/admission).
  • Important dates can be indicated alongside the epidemic curve to facilitate the interpretation of outbreak dynamics (e.g., date of the first reported case, changes in surveillance, declaration of the outbreak, response efforts including OCV campaigns, etc.)
• Key indicators
  • The following key morbidity and mortality indicators are monitored throughout the outbreak:
  • Incidence Rate (IR)
    • IR indicates the evolution of the outbreak and the speed of its spread; this indicator allows for a comparison of geographic units and time periods.
    • IR is calculated for a given time interval (e.g., week) and for a given geographic unit (e.g., surveillance unit) .
    • IR is often expressed per 1,000, 10,000, or 100,000 population.
    • Calculation:
      Numerator: Number of new (suspected and confirmed) cholera cases reported during a given time interval
      Denominator: Population during the same time interval
• Cumulative incidence rate
  • The cumulative incidence rate is the proportion of the population that has contracted cholera over a given time interval (for example, one year or the whole duration of the outbreak).
  • Often expressed as a percentage, it indicates the impact of the outbreak on the population.
  • Calculation:
    Numerator: Total number of (suspected and confirmed) cholera cases reported since the beginning of the outbreak or since the beginning of the year
    Denominator: Population at the beginning of the outbreak or at the beginning of the year
• Case fatality ratio (CFR)
  • The CFR is the proportion of health facility cholera deaths that occur among cholera cases (suspected and confirmed) during a specified time interval.
  • Often expressed as a percentage, the CFR is an indicator of adequate case management and access to cholera treatment.
  • A CFR >1 % is usually due to one or a combination of different factors such as poor access to health facilities, lack of healthcare-seeking behaviour, and/or inadequate case management.
  • Monitoring the CFR should be complemented by monitoring the number of community deaths.
  • Calculation:
    Numerator: Number of cholera deaths reported in health facilities during a given time interval
    Denominator: Number of (suspected and confirmed) cholera cases reported in health facilities within the same time interval
• Test positivity rate
  • The test positivity rate is the proportion of tests performed (stratified by testing method) that are positive, expressed as a percentage.
  • The test positivity rate should be reviewed along with the epidemic curve to interpret outbreak trends. For example, a low test positivity rate that coincides with an increase in suspected cholera cases may indicate a concomitant outbreak of diarrhoeal illness caused by a different pathogen, or issues with laboratory confirmation.
  • Calculation:
    Numerator: Number of positive test results (stratified by test method) Denominator: Number of tests performed (stratified by test method)

Detecting and investigating outbreak deterioration

• A deteriorating outbreak may be detected if, over at least two consecutive weeks, there is:
  • an increase in weekly cholera incidence;
  • a spatial extension of the outbreak;
  • an increase in the case fatality ratio (CFR) or in the number of community deaths;
  • a shift in the socio-demographic profile of cases.
• If a deteriorating cholera outbreak is detected, a field investigation should be conducted. The deterioration may be due to internal or external factors (e.g., overstretched response capacity, breakdown or failure of control measures, ill-targeted interventions, change in the drivers or context of transmission, etc.).
• Based on the findings of the field investigation, the response should be strengthened and adapted to control the outbreak more effectively (e.g., scaling up interventions, allocating additional capacity or resources for response, etc.).

Monitoring the outbreak: clustered transmission

• In a probable or confirmed cholera outbreak with clustered transmission, surveillance aims to rapidly detect, confirm, investigate, and respond to cluster(s) of cholera cases to interrupt transmission before it spreads in the community.

Detection and reporting of cases

• Standard data is collected on all suspected cholera cases detected in health facilities and community settings. See Appendix 2 for a template cholera case report form, Appendix 3 for a sample cholera line list and Appendix 4 for a community-based surveillance template reporting form.
• Standard data on suspected cholera cases should be reported to health authorities within 24 hours.

Testing

• Test all suspected cholera cases.
• If RDTs are unavailable:
  • Collect stool samples from all suspected cholera cases and send them to a laboratory for testing by culture and/or PCR.
• If RDTs are available:
  • Test all suspected cases by RDT and
  • Collect stool samples from all RDT+ patients and send them to a laboratory for testing by culture and/or PCR.
• Perform antimicrobial susceptibility testing (AST) on the index confirmed (first confirmed) case at a minimum.
• Performing whole genome sequencing (WGS) on at least one confirmed cholera case is encouraged, particularly if the cluster origin is uncertain.
• Confirmation of toxigenicity may also be warranted if there is no established epidemiological link to a confirmed cholera case or source of exposure in another country.

Case and field investigation

• To document epidemiological links between cases, conduct case investigations on all confirmed cholera cases (at a minimum) and on any suspected cases for which laboratory specimens were not collected (specimen collection should then occur as part of the investigation). See Appendix 5 for a template cholera case investigation form.
• Conduct case investigations without waiting for laboratory results on suspected cases.
• Field investigation may also be undertaken to further guide the response.

Data analysis and interpretation

• Analyse daily surveillance data and case investigation findings.
• The principles for analyzing surveillance data are generally similar to those applied in an outbreak with community transmission. However, in clustered transmission, a more granular data visualization or description is useful for guiding highly targeted response measures.

Environmental Surveillance

• During a suspected or confirmed cholera outbreak, the primary objective of environmental testing is to determine if sources of drinking water (surface and stored) are contaminated with faecal matter.
• If the water source is part of a chlorination system or program, testing of free residual chlorine (FRC) is warranted. Drinking water quality monitoring should focus on FRC levels and basic tests for faecal contamination (such as testing for E.coli).
• Environmental sampling to detect outbreak strains of V. cholerae does not serve an immediate public health purpose, other than in unusual circumstances where cholera is rare or unknown and a single source seems likely.
• The benefits of testing for V. cholerae, such as long-term monitoring and strain identification, are primarily of research interest or elimination monitoring and thus beyond the scope of this document.

Additional resources:

1. Public Health Surveillance for Cholera – Guidance Document 2024. Global Task Force on Cholera Control. April 2024. https://www.gtfcc.org/wp-content/uploads/2024/04/public-healthsurveillance-for-cholera-guidance-document-2024.pdf
2. World Health Organization - Regional Office for Africa. Technical guidelines for integrated disease surveillance and response in the WHO African Region – Booklet four. 3rd ed. 2019. https://iris.who.int/bitstream/handle/10665/312364/WHO-AFWHE-CPI-02.2019-eng.pdf
3. Interim Technical Note Introduction of DNA-based identification and typing methods to public health practitioners for epidemiological investigation of cholera outbreaks. Global Task Force on Cholera Control. June 2017 https://www.who.int/cholera/task_force/GTFCC-Laboratory-supportpublichealth-surveillance.pdf?ua=1
4. Managing a cholera epidemic. Chapter 2. Outbreak investigation. MSF. August 2017. https://medicalguidelines.msf.org/en/viewport/CHOL/english/management-of-a-cholera-epidemic-23444438.html
5. World Health Organization. Early warning alert and response (EWAR) in emergencies: an operational guide. 2022. https://www.who.int/publications/i/item/9789240063587