RATIONALE FOR SURVEILLANCE

This severe acute viral infection has the potential to produce epidemics, and as such surveillance mechanisms to detect outbreaks and to monitor control measures are critical in affected countries.

RECOMMENDED CASE DEFINITION

Clinical description

An illness of gradual onset with one or more of the following:

 

malaise, fever, headache, sore throat, cough, nausea, vomiting, diarrhoea, myalgia, chest pain, hearing loss, and

A history of contact with excreta of rodents or with a probable or confirmed case of Lassa fever.

Laboratory criteria for diagnosis

 

• Isolation of virus (only in laboratory of biosafety level 4) from blood, urine or throat washings or
• Positive IgM serology or seroconversion (IgG antibody) in paired serum specimens or
• Demonstration of Lassa virus antigen in autopsy tissues by immunohistochemistry or in serum by ELISA
• Positive PCR from serum or autopsy tissues

Case classification

Suspected: A case compatible with the clinical description.

Probable: A suspected case that is epidemiologically linked to a confirmed case.

Confirmed: A suspected case that is laboratory-confirmed.

Contact: A person having close personal contact with the patient (living with, caring for) or a person testing the laboratory specimens of a patient in the 3 weeks after the onset of the illness.

RECOMMENDED TYPES OF SURVEILLANCE

Endemic situation:

Immediate reporting of case-based data of suspected, probable or confirmed cases from peripheral level to intermediate and central levels.

All cases must be investigated, and contact tracing undertaken.

Routine monthly reporting of aggregated data from intermediate to central level.

Outbreak situation:

All suspected outbreaks must be reported centrally. Surveillance must be intensified with active case finding and contact tracing. Aggregated data on a daily/weekly basis to be submitted to intermediate and central level by investigation team.

The disease is endemic in Sierra Leone, Liberia, Guinea and regions of Nigeria. Outside these areas, compatible symptoms, with a history of travel to or arrival from one of these countries, should prompt investigation and reporting.

RECOMMENDED MINIMUM DATA ELEMENTS

Case-based data for reporting and investigation

 

• Case classification (suspected/probable/confirmed)
• Unique identifier, age, sex, place of residence for the three weeks before onset of illness
• Date of onset
• Hospitalization
• Outcome

Aggregated data for reporting

Endemic situation

 

• Number of cases (suspect/probable/confirmed) by geographical area and by outcome
• Contacts by geographical area, success of tracing and outcome

Outbreak situation

 

• Total number of cases by village, geographical area, onset date, hospitalization, outcome
• New cases and contacts identified since last report
• Total number of contacts by outcome
• New contacts identified and traced since last report

RECOMMENDED DATA ANALYSES, PRESENTATION, REPORTS

 

• Mapping number of cases by geographical area
• Percentage of contacts followed up
• Case-fatality rate

PRINCIPAL USES OF DATA FOR DECISION-MAKING

 

• Monitoring endemic disease over time
• Identification of risk groups or areas
• Identification of clusters/outbreaks
• Investigation of cases, contacts and source of infection

SPECIAL ASPECTS

Extreme biohazard is associated with sample collection and transport and with laboratory investigations.

CONTACT

Regional Offices

See Regional Communicable Disease contacts on section "Communicable disease contacts in Regional Offices".

Headquarters: 20 Avenue Appia, CH-1211 Geneva 27, Switzerland

Department of Communicable Diseases Surveillance and Response (CSR)

 

E-mail: arthurr@who.ch / outbreak@who.ch
Tel: (41 22) 791 2658/2636/2111
Fax: (41 22) 791 48 78