Epidemiological Surveillance

• Describe the outbreak (who is affected, where the outbreak is located and its evolution) to guide control measures.
• Monitor and evaluate the impact of the interventions implemented.

• Conduct field investigations in the affected areas including active case finding, explore possible sources of contamination and identify risk factors and transmission pathways. If possible, test sources of drinking water of patients for faecal contamination, or if the water is reported to be chlorinated, test for free residual chlorine (FRC).

• During the field investigations conduct active case finding and include an intervention component, such as distribution of water treatment products and ORS, and deliver key messages for health education. Interventions should be prioritized for the household and neighbours of individuals with cholera.

• Map the locations of homes and water sources where people have been found to have cholera to help identify areas as risk, target interventions and monitor disease spread.

• Implement prevention and control measures for suspected or potential exposure risks. If resources permit, a case-control or KAP study may help to confirm these. Do not wait to implement prevention and control measures.

Data Collection

• A standard line list should be available and used in all health facilities and the same data collected by all partners. Provide a template if necessary  (see appendix 5 – template of a cholera line list).

• Depending on country capacity, registers can be paper-based or electronic-based forms.

• Case information includes: name, age, sex, place of residence (at lowest recognized administrative unit), date of visit, dehydration status and treatment plan, hospitalization, outcome, date of discharge or death, stool sample taken, RDT result and laboratory confirmation (culture and/or PCR) and vaccination status. Additional information on exposures, occupation, pregnancy, malnutrition status, global positioning system (GPS) points of the house of the patients, etc. can also be included in the register.

• Data should be compiled daily and reported to the local health authorities through the established channels. Patient records and registers must be kept from the start of the epidemic to its very end.

• Ensure that community health workers are trained and integrated in the surveillance system and that they systematically collect and report suspected cases and deaths occurring in the community, especially in remote areas. Implement community-based surveillance in areas where necessary.

• Community health workers should record and report to the health facility or district health office the number of cases and deaths occurring in the community where people did not seek medical attention and consequently are not registered at the health facilities (see appendix 6 – weekly register for community-based surveillance).

Data Reporting

• Report the aggregated number of cholera cases and deaths — both registered at the health facility and occurring in the community — by age group (minimally under 5 years of age and 5 years of age or older) to the surveillance unit/health authorities’ office (district, regional or national).
• The reporting procedures may include paper reporting forms, electronic methods or telephone.

• Aggregate data at each health administrative level before reporting to the next highest health authorities (Figure 1).

• Ensure that partners (NGOs, international agencies, etc.) managing independent cholera structures also report to the local health authorities (at the district or national level).

• Compile, validate and analyse the data at each health administrative level to describe the outbreak, monitor trends and identify populations at risk, and guide preventive and control measures.

• Laboratories should also inform and update the surveillance unit about the number of positive cases and antimicrobial susceptibility patterns. This information should be shared as part of regular epidemiological updates.

• Ensure data exchange with other countries when there is a risk of cross-border transmission. High-risk districts often share borders with neighbouring countries.

Periodicity of Reporting

• In a previously unaffected area or area with no recently reported cases, immediately report (within 24 hours) any cholera alert to the next level health authority (district, regional or national) to conduct field investigations and confirm or rule out the outbreak.
• Community health workers must immediately report any alert to the nearest health facility or district health authority.
• In an area where a cholera outbreak has been declared, report on a daily or weekly basis the number of cases and deaths — both registered at the health facility and occurring in the community — to monitor the outbreak and to guide and adapt control measures.
• In an area where cholera is common, with year-round cases, report at least weekly the number of cases and deaths — both registered at the health facility and occurring in the community — in order to estimate basic surveillance indicators (incidence rates, AR and CFR), assess the impact of the interventions and help plan control strategies.
• Laboratories should immediately report the results of testing in the suspected cases to the health authorities, and to the health facility sending the sample, especially in a previously unaffected area.
• Once an outbreak is declared, the laboratory should periodically report to the health authorities the number of samples received, the number of samples tested and the test results by date and reporting area.

Figure 1. Flow of Information

Data Analysis

• Health authorities should collect and analyse data received by all sources in a timely manner to describe the situation, identify populations at risk and initiate the necessary preventive and control measures. Incidence rates, CFR and AR are key epidemiological indicators
• Health facilities should also conduct data analysis of the cases and deaths seen at the health facility.
• Produce regular (daily or weekly) epidemiological bulletins or situation reports to disseminate among partners at district and national levels (see appendix 7 – outline of the outbreak situation report).
• Include all sectors (health, WaSH, hygiene promotion, etc.) and partners (national authorities, national and international partners) in dissemination of outbreak information, to orient and adapt prevention and control activities. Partners should exchange information to guide actions
• If available, consider historical data of previous outbreaks to better interpret the analysis.
• Consider conducting epidemiological studies (such as KAP and case control studies) to identify risks factors and transmission patterns.

Descriptive Epidemiology

By Person

• Provide description of cases (suspected and confirmed) by age (under 5 years of age and 5 years of age or older) from community and health facilities to identify and describe the affected population. If population figures are available, the number of cases can be expressed as incidence rates (IRs) and attack rates (ARs).
• Include the number of hospitalized patients and proportion of cases by dehydration status or treatment plan applied.
• Provide the number of deaths (at the health facility and in the community) in a region or district over time. The risk of dying from cholera is usually expressed by case fatality rate (CFR), by dividing the number of deaths attributed to cholera by the total number cholera cases (suspected and confirmed).

By Time

• Provide description of cases and deaths over time to monitor the evolution and magnitude of the epidemic; data are usually presented as a histogram epidemic curve, plotting the number of cases by date of visit or date of onset of symptoms.
•  The risk of death from cholera is represented in the epidemic curve by the CFR for each time period (i.e. daily, weekly or monthly). This is usually shown as a line superimposed over the bar graph.

By Place

• Provide geographic distribution of cases by place of residence (per village, district and region) to identify affected areas at higher risk and to monitor outbreak extension.
• If possible, collect GIS points and create maps of patients’ households and water sources to help identify high-risk areas. The collection of this type of data can be conducted by WaSH/environmental teams, volunteers, community health workers and others who are conducting health promotion or following up on the distribution of hygiene kits at patients’ homes.

Incidence Rate (IR)

• The IR shows the speed at which new cases occur within a given period of time (usually per week) in a given area or a specific population (such as an age group).
• IRs can be expressed per 100 (percentage), per 1000, per 10 000 persons, or even more in case of small numbers of cases.
• IRs indicates the evolution of the epidemic and the rapidity of its spread. It can be compared between groups and with other areas since incidence is adjusted by population.

Attack Rate (AR)

• AR is the cumulative incidence of cholera over a defined period of time (usually the duration of an epidemic) in a defined area and population.
• AR is usually expressed as a percentage and can be calculated by age and area.
• AR indicates the impact of the epidemic in the population.
  • In rural communities with low population density, the AR might vary (0.1–2%)
  • In crowded places (such as urban settings, refugee camps), the AR tends to be higher (1–5%).
  • In settings with no immunity and poor water and sanitation conditions, AR can exceed 5%.

Case Fatality Rate (CFR)

• CFR is the proportion of cholera deaths among total number of cases within a specified period of time, expressed as a percentage.
• Deaths occurring at the CTUs/CTCs and in the community should be recorded and analysed separately. Calculate CFR at health facilities and in the community.
• CFR, calculated with deaths and cases registered in a given health structure, is an indicator of adequate case management and access to treatment. The death of a person from cholera at any time after arrival at a health facility is considered to be an institutional death.
• Cholera CFR can reach 50% if adequate treatment is not provided for patients with severe dehydration. With adequate and appropriate treatment, no one should die of cholera. However, a treatment centre with a CFR of less than 1% is considered to be well run.
• High CFR may indicate:
  • poor access to treatment – patients arrive late in the progression of the disease (with severe dehydration) due to factors including long distances from care centres with no means of transport; cultural barriers, beliefs or misinformation on when and where to go for treatment; or costs of care;
  • inadequate case management due to factors including lack of properly trained health professionals, lack of supplies and overwhelmed facilities; and/or bias of surveillance where deaths are more accurately recorded than numbers of cases (for example, outpatient cases are not recorded)
• If CFR is high, an assessment of the treatment structure, including early access to care, should be conducted to identify the causes and implement corrective measures (see section 6 – cholera treatment facilities).

Laboratory Surveillance

• Once an outbreak is declared, record and report any person presenting with suspected cholera.
  • Remember: In areas where a cholera outbreak has been declared, a suspected cholera case is defined as any person presenting with or dying from acute watery diarrhoea.
• There is no need to confirm all suspected cases in the laboratory.
• Conduct laboratory confirmation by culture or PCR of suspected cholera cases for each new area (district, province or region) affected by the outbreak (see section 2 – outbreak confirmation).
• Perform periodic sampling of suspected cases to monitor the outbreak, to determine the antimicrobial susceptibility profile and to carry out continuous monitoring of strains.
  • If RDT is available, send samples from patients with positive RDTs to the laboratory. Alternatively, send sample from patients with severe dehydration.
  • The number of samples collected and tested depends on the laboratory capacity and the extent and magnitude of the outbreak.
  • Ideally, submit a minimum of five samples (from patients randomly selected) per week per inpatient health facility.
  • On the confirmed isolates, perform antimicrobial susceptibility testing to guide antibiotic treatment, however, this is not required for individual cases. At least one isolate per week per affected area should be tested to monitor the antimicrobial susceptibility patterns.
  • Results of antimicrobial susceptibility should be reported to healthcare workers so they can adapt the antibiotic treatment accordingly                                                                            (see section 7 – case management in treatment facilities)
• In a situation of large or nation-wide outbreak, select a representative number of health facilities or CTUs/CTCs (sentinel sites) for collection and shipment of samples for testing. Sentinel sites should be selected to represent the main affected areas.
  • Contact international reference laboratories for external quality control and for shipment of specimens for further characterization, such as DNAbased molecular testing.
• For additional information, see:
  • GTFCC. Technical Note—Introduction of DNA-based identification and typing methods to public health practitioners for epidemiological investigation of cholera outbreaks. June 2017.
  • GTFCC. Interim guidance on cholera surveillance. 2017

Environmental Surveillance

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. Water testing should focus on FRC levels and basic tests for faecal contamination. The benefits of environmental sampling, such as long-term monitoring and strain identification, are primarily of research interest or elimination monitoring and thus beyond the scope of this document

• See Technical Note on environmental surveillance (under development):

For additional Information

1. Interim Guidance Document on Cholera Surveillance. Global Task Force on Cholera Control-Surveillance Working Group. June 2017.Click here
2. Technical Guidelines for Integrated Disease Surveillance and Response in the African Region. Second Edition. 2010.Click here
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 2017Click here
4. Managing a cholera epidemic. Chapter 2. Outbreak investigation. MSF. August 2017.Click here
5. Cholera outbreak: assessing the outbreak response and improving preparedness. Global Task Force on Cholera Control. 2010Click here