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close this bookCommunicable Disease Control in Emergencies - A Field Manual (WHO - OMS, 2003, 223 p.)
close this folderCHAPTER 4: OUTBREAK CONTROL
View the document(introduction...)
View the document4.1 PREPAREDNESS
View the document4.2 DETECTION
View the document4.3 CONFIRMATION
View the document4.4 RESPONSE
View the document4.5 EVALUATION



At this stage, the OCT should:

· meet daily to update the team on outbreak developments;

· review the human, logistical (stores, stocks, etc.) and financial resources available to manage the outbreak;

· investigate reported cases to assess pathogen, source and transmission;

· make sure that clinical workers report suspected cases to the team immediately;

· make sure that clinical workers are using standard treatment protocols;

· quantify cases by time and place;

· produce spot maps and epidemic curves; and

· implement control measures.

Collection and analysis of descriptive data and development of hypotheses

The systematic recording of data on cases and deaths (time, place and person) in an outbreak is essential to ensure accurate reporting. These data are necessary to form a hypothesis of the pathogen involved and its source and route of transmission, and to measure the effectiveness of control measures. This process is summarized in the six key questions: Who? What? When? Where? Why? How?

A simple, clear, easily understood case definition must be used consistently from the beginning of an outbreak and must be placed conspicuously at the top of each case reporting form. This case definition, the outbreak case definition, may have to be adapted from the surveillance case definition. The syndromic definitions often used by the surveillance system for early detection may not be specific enough in the event of an outbreak and could lead to an overestimation of cases. In most outbreaks, basic epidemiological data on time, place, person and basic laboratory confirmation are sufficient for the design and implementation of effective control measures.

Cases may be placed in two categories: suspected or confirmed. A suspected case is one in which the clinical signs and symptoms are compatible with the disease in question but laboratory confirmation of infection is lacking (negative or pending). A confirmed case is one in which there is definite laboratory evidence of current or recent infection, whether or not clinical signs or symptoms are or have been present. Once laboratory investigations have confirmed the diagnosis in the initial cases, the use of a clinical/epidemiological case definition is sufficient. There is no need to continue to collect laboratory specimens from new cases for the purposes of notification.

During an epidemic, data should be analysed rapidly to determine the extent of the outbreak and the impact of actions taken to date (Fig. 4.3).

Fig. 4.3. Collection and analysis of descriptive data

The following steps should be taken by members of the outbreak response team in charge of the epidemiological investigation.

· Define the extent of the outbreak in time, place and person:

- when did the cases occur -dates of onset (e.g. epidemic curve)?
- where do patients live (e.g. spot map)?
- who are they (e.g. tables of age, immunization status)?

· Measure the severity of the outbreak:

- how many patients were hospitalized?
- how many patients suffered complications?
- how many patients died as a proportion of all cases (case fatality rate)?

· Draw an epidemic curve, i.e. a graph showing cases by date of onset or by date of report. This helps to demonstrate where and how an outbreak began, how quickly the disease is spreading, the stage of the outbreak (start, middle or end phase) and whether control efforts are having an impact (Fig. 4.3).

· Draw a graph or table of age distribution and immunization status of cases; this should be constructed from the line listing of cases. This information is used for identifying cases that were not preventable (e.g. those developing measles before the scheduled age of immunization). If population data are available, calculate age-specific attack rates.

· If appropriate, estimate the vaccine efficacy. In the case of a vaccine-preventable disease such as measles, vaccine efficacy and the proportion of cases that were vaccine-preventable should be calculated. Using immunization history data one can tabulate those immunized but not protected (vaccine failures) and those who failed to be immunized.

· Draw a spot map. A map of the camp or community should be marked with the location of all cases and deaths. The outbreak investigation team can use this map to identify areas with clusters of disease. Further investigation of these areas may reveal the source of infection or modes of transmission. Even when a camp is involved, it is essential that the effect on the local community outside the camp is documented (this may be the source) and the local health authorities assisted in controlling the outbreak if it has spread.

· Provide summary data of the outbreak, by calculating the following basic epidemiological indices set out in Table 4.9.

Table 4.9. Basic epidemiological indices

The case fatality rate (CFR) is the percentage of cases that result in death

· Count the number of patients who died of the disease
· Divide by the total number of cases of the disease
· Multiply the result by 100

The weekly attack rate is the number of cases per 10 000 people per week

· Divide 10 000 by the population of the camp
· Multiply the result by the number of cases that occurred in a given week

The age-specific weekly attack rate is the number of cases per 10 000 people in one age group (e.g. > 5 years)

· Calculate the number of persons in the age group (> 5) in the camp
· Count the number of cases in the age group for the chosen week
· Divide 10 000 by the number of persons

· Multiply the result by the number of cases in that group

Follow-up of cases and contacts

For each case, information should be collected on name, age, location, date of onset and outcome; for some diseases, additional information on immunization status, water source and duration of disease may be collected.

A rumour registry must be established to record rumours of cases systematically. One site should be dedicated to this activity. The registry must have close links to home visitors and the local community and its existence must be widely advertised. It should be carefully maintained and used to provide material for the team.

Active case finding may be required, depending on the infectiousness of the disease and the risk to the population. Contact tracing may also be required, particularly in the case of outbreaks of viral haemorrhagic fever. The OCT must define what constitutes a contact, specify the period of risk and agree on the method of follow-up, e.g. active contact tracing.

Further investigation/epidemiological studies

In some outbreaks, routine data do not give sufficient information about items such as the source of the outbreak, risk factors, local characteristics of the causative agent (e.g. resistance, serotype) or mode of transmission. Further investigation, such as case control studies or environmental assessments (e.g. vector breeding sites), may be required to identify the source of this outbreak, risk factors in respect of severity, or modes of transmission. This may need the participation of external agencies with skills in epidemiological investigation or in specific diseases.


The data gathered in the course of these investigations should reveal why the outbreak occurred and the mechanisms by which it spread. These in turn, together with what is known about the epidemiology and biology of the organism involved, will make it possible to define the measures needed to control the outbreak and prevent further problems.

An outbreak may be controlled by eliminating or reducing the source of infection, interrupting transmission and protecting persons at risk. In the initial stage of an outbreak in an emergency situation, the exact nature of the causative agent may not be known and general control measures may have to be taken for a suspected cause. Once the cause is confirmed, specific measures such as immunization can be undertaken. These disease-specific measures are detailed in Chapter 5.

Control strategies fall into four major categories of activity.

1. Prevention of exposure: the source of infection is reduced to prevent the disease spreading to other members of the community. Depending on the disease, this may involve prompt diagnosis and treatment of cases using standard protocols (e.g. cholera), isolation and barrier nursing of cases (e.g. viral haemorrhagic fevers), health education, improvements in environmental and personal hygiene (e.g. cholera, typhoid fever, shigellosis, hepatitis A and hepatitis E), control of the animal vector or reservoir (e.g. malaria, dengue, yellow fever, Lassa fever) and proper disposal of sharp instruments (e.g., hepatitis B).

2. Prevention of infection: susceptible groups are protected by immunization (e.g. meningitis, yellow fever and measles), safe water, adequate shelter and good sanitation.

3. Prevention of disease: high-risk groups are offered chemoprophylaxis (e.g. malaria prophylaxis may be suggested for pregnant women in outbreaks) and better nutrition.

4. Prevention of death: through prompt diagnosis and management of cases, effective health care services (e.g. acute respiratory infections, malaria, bacterial dysentery, cholera, measles, meningitis).

Selection of control measures depends on:

· feasibility (technical/operational)
· availability (stockpiles)
· acceptability
· safety (of operators and population)
· cost.

Patient isolation

The degree of isolation required depends on the infectiousness of the disease. Strict barrier isolation is rarely indicated in health facilities, except for outbreaks of highly infectious diseases such as viral haemorrhagic fevers. The isolation room must be in a building separate from other patient areas and access must be strictly limited. Good ventilation with screened doors is ideal, but fans should be avoided as they raise dust and droplets and can spread aerosols. Biohazard warning notices must be placed at the entrances to patients' rooms. Patients must remain isolated until they have fully recovered.

During outbreaks, isolation of patients or of those suspected of having the disease can reinforce stigmatization and hostile behaviour of the public toward ill persons. The establishment of isolation rules in a community or in a health facility is not a decision to be taken lightly, and should always be accompanied by careful information and education of all members of the involved community. Every isolated patient should be allowed to be attended by at least one family member. Provided that enough supplies are available, designated family attendants should receive barrier nursing equipment, and be instructed on how to protect themselves when in contact with the patient.

Every outbreak requires a response specific to the disease. Control measures for the main communicable diseases encountered by displaced populations are described under disease-specific sections in Chapter 5.

Biohazardous materials

Safe disposal of body fluids and excreta is essential, especially in the case of highly contagious diseases. This may be achieved by disinfecting with bleach or by incineration. If contaminated material has to be transported, it should be placed in a double bag.

The threat of infection from body fluids of patients with diseases such as cholera, shigellosis or viral haemorrhagic fevers is serious, and strict procedures for disposal of hazardous waste must be maintained. Laboratory specimens and contaminated equipment should also be carefully sterilized or disposed of. When possible, heating methods such as autoclaving, incineration or boiling can be used to disinfect. Proper disposal of sharp objects such as needles is essential.

Table 3.10 outlines the general precautions to be taken in relation to isolation cases.

Table 3.10. General precautions to be taken for isolation of cases in outbreaks

Isolation measure

Contagious-ness of cases

Route of transmission

Type of protective measure


Standard precautions


Direct or indirect contact with faeces, urine, blood, body fluids and contaminated articles

Hand-washing, safe disposal of contaminated articles

Most infectious diseases except those mentioned below

Enteric isolation


Direct contact with patients and with faeces and oral secretions

Contact precautions

Cholera, shigellosis, typhoid fever Gastroenteritis caused by rotavirus, E. coli, hepatitis A

Respiratory isolation


Direct contact with patients or oral secretions and droplets

Separate room, masks, contact precautions

Meningococcal meningitis, diphtheria, measles

Strict isolation

Very high

Airborne, and through direct contact with infected bloods, secretions, organs or semen

Separate room, biohazard notification

Viral haemorrhagic fevers

See Section 2.3.6 for procedures for the disposal of the dead.

Prompt diagnosis and effective case management

There are two steps in this process: timely presentation to the health facility and effective diagnosis and treatment by the clinical workers. Home visitors and health educators can play an important role in ensuring that the community is aware of the symptoms and signs a disease, and that they know that effective treatment is available at the health facility. The second step is the use of standard treatment protocols by clinical workers well trained in their use. The early diagnosis of a disease is important, not only to avoid serious sequelae and death in the patient but also to prevent further transmission.