Rapid Health Assessment in Outbreaks of
Viral Haemorrhagic Fever (VHF),
including Yellow Fever


Table of contents

1. 1.1 1.2 2. 3. 3.1 3.2 3.3 3.4 4.
Assessment Purpose
Confirming an Outbreak of Viral Haemorrhagic Fever (VHF)
Assessing Impact on Health
Assessing Existing Response Capacity and Immediate Needs
Deciding on immediate needs

1. Assessment Purpose and Background

1.1 Assessment Purpose
  • confirm that an epidemic or potential epidemic of viral haemorrhagic fever (VHF) exists and estimate its geographic distribution
  • estimate its health impact
  • assess local response capacity and identify the most effective control measures
  • 1.2 Background

    a) General characteristics

    Viral haemorrhagic fevers (VHF) are caused by a number of viruses, some associated with insects or rodents, which may infect humans. These diseases cause special problems for public health services because of their epidemic potential, high case-fatality rates and the unusual difficulties arising in their treatment and prevention. Although the specific clinical profile of each viral illness may vary, there are two prominent features that may occur in all types of VHF during the most critical stage of the illness: The existence of a specific virus in a community tends to reflect the geographic distribution of its natural host. Nonetheless, human and natural environments are changing quickly: in this perspective, research should be considered an integral part of emergency preparedness against these epidemics.

    Several viral infections also have the potential for extensive nosocomial spread (spread within a health care facility), especially when safe barrier nursing procedures are not observed. In these conditions, case-fatality rates can often exceed 50%, and may reach 80% for several days.

    The table below lists the major VHFs which cause epidemics and their distribution.

    VHFs Causing Epidemics
    VHF Distribution Natural Host/Vector
    Lassa Fever Central/West Africa rodents
    Junin/Machupo/Guanarito/Sabia South America rodents
    Ebola/Marburg Africa unknown
    Crimean-Congo haemorrhagic fever (CCHF) Africa/Asia ticks
    Rift Valley fever Africa mosquitoes
    Dengue haemorrhagic fever Africa/Americas/Pacific/ Middle East/Australasia/ Caribbean/India mosquitoes
    Yellow fever Africa/South America mosquitoes
    Haemorrhagic Fever with Renal Syndrome (HFRS) Asia/Europe rodents

    b) The special concerns of yellow fever

    In Africa and South America, yellow fever has caused many serious epidemics, with high attack rates and mortality. However, while the clinical presentation of yellow fever may resemble other types of VHF, it is unique with respect to emergency preparedness and containment. Unlike other VHFs, timely vaccination against yellow fever, combined with vector control measures, interrupt transmission and prevent unnecessary cases and deaths.

    There are many examples in which the first recognition of a yellow fever epidemic occurred several months after the actual epidemic onset. The consequences of such late detection (e.g. delayed initiation of control efforts) underscore the need to consider yellow fever in a rapid assessment when an outbreak of VHF is reported or rumoured.

    An epidemic alert for an outbreak of VHF with yellow fever as a possible cause should be given when one of the following occurs: Early warning procedures should be established for early detection of VHFs. Routine health surveillance, and rapid reporting from hospitals are essential for detecting VHF outbreaks at an early stage.

    Hospital reports of increased incidence of fatal hepatitis, suspected cases of yellow fever and of VHF, deserve to be considered as early warning indicators. Ask: Because VHF outbreaks frequently occur in rural areas, their early detection is often missed by routine health surveillance. However when reported, such outbreaks assume high visibility with the media, often in a context of panic expressed by the medical and local communities affected.

    Therefore, the rapid assessment provides a valuable opportunity to allay anxiety at the community level and to give basic information on protective measures to prevent further spread.

    2. Preparedness for the Rapid Assessment

    Develop locally-adapted working case-definitions for VHFs and yellow fever, as well as guidelines that help health workers at all levels recognize suspicious trends and/or trigger an epidemic alert.

    The prompt diagnosis of the cause of a VHF outbreak requires analysis of a representative sample of specimens by a competent laboratory. Epidemic preparedness should give this utmost priority, as well as assessing capacity of national laboratories; identifying reference laboratories, and ensuring methods of diagnostic specimen transport.

    Most of the viruses (excluding dengue haemorrhagic fever) causing VHF are classified as ‘Biosafety Level 4’ pathogens. This biohazard requires analysis at special facilities which provide maximum containment.

    Attempts to isolate the virus should be undertaken only at approved high containment laboratories. Therefore, these should be identified in advance and contacts established with the nearest specialist laboratory to obtain details of precautions necessary for packing and transport of specimens.

    Serology can be carried out in standard laboratories only if it is possible to inactivate specimens and reagents.

    It is also necessary to:

    3. Conducting the Rapid Assessment

    The rapid assessment consists of confirming an outbreak of VHF and estimating its geographic distribution, assessing the impact on health, assessing the existing response capacity and immediate needs.

    3.1 Confirming an Outbreak of VHF and estimating its geographic distribution

    a) Initial case-definition

    As for all potential epidemics, this is best determined in advance, as part of emergency preparedness. Simple, workable case-definitions should be defined for suspect, probable and confirmed cases of VHF.

    Examples of case-definitions for VHF are: Yellow fever

    In a rapid assessment, it may be difficult to distinguish yellow fever from other haemorrhagic illnesses, or diseases such as malaria. However, to maximize case-detection at this early stage, it is often necessary to use a broad case-definition such as ‘jaundice, fatal or non-fatal’ to identify suspected cases.

    b) Confirm the increase in the number of cases.
    (See Rapid Health Assessment in Epidemics of Infectious Origin.)

    c) Case-finding and estimating geographic distribution
    (See Rapid Health Assessment in Epidemics of Infectious Origin.)

    It is important to recognize that there could be many asymptomatic or mild cases who are hospitalized with a non-specific febrile illness. In order to be thorough, case-finding efforts for VHF and yellow fever should not be limited to infectious wards but should include other hospital departments and health facilities.
    d) Collection of appropriate specimens for laboratory analysis and confirmation

    Because the definitive diagnosis of a VHF can only be made by serology or virus isolation, it is essential that appropriate specimens are collected during the rapid assessment.

    Key considerations in specimen collection are as follows. Specimens required:

    3.2 Assessing Impact on Health

    a) Collect information on a representative sample of cases

    When the cause of a VHF outbreak is not known, careful interviewing and physical examination of suspect, probable and confirmed cases is extremely important. These early clinical findings provide clues as to type of virus and source of infection. As a minimum, gather information on: Useful information on the mode of transmission can be gained by investigating the contacts of identified index cases. It is also important to ask about exposures to infected animal hosts (e.g. contact while slaughtering livestock).

    The definition of a ‘primary’ or ‘close’ contact is one or more of: The definition of a "possible" contact is: Whatever the method chosen, the characterization of the contact should include a clarification on the index case: Was he or she suspect, probable or confirmed?

    b) Analyze the information gathered

    (Refer to Rapid Health Assessment in Epidemics of Infectious Origin.)
    c) Assess vectors present

    One rapid assessment priority is to identify whether vectors are present in the affected area that may transmit VHF or yellow fever. It is not the purpose of a rapid assessment to carry out a detailed entomological survey - but rather to ask the following questions. The answers to these preliminary questions are critical for deciding on the need for further entomological studies and control measures for vectors and natural hosts.

    d) Assess disease in other vertebrate hosts
    3.3 Assessing Existing Response Capacity and Immediate Needs

    a) Assess local response capacity

    Local epidemic surveillance Response capacity of existing health services 3.4 Deciding on immediate needs

    To decide on immediate needs, ask: If the answer to both questions is "yes", then an emergency response is needed.

    State the following:
    • Is there an outbreak of some type of VHF?
    • If so, how many cases and deaths so far?
    • What is the geographic distribution?
    • Does it appear to be spreading?
    • What are the trends?
    • What is the clinical presentation?
    • Are signs and symptoms indicative of any specific type of VHF?
    • Where should specimens be sent for rapid analysis?
    • Is the etiologic agent responsible for the outbreak identified?
    • Have specimens being sent to reference laboratories?
    • What are the estimated geographic magnitude, size of population at risk and health impact in numbers of projected cases and deaths?
    Describe immediate needs
    Are outside resources such as the following needed
    • Drugs
    • Other supplies
    • Equipment
    • Manpower
    • Expert assistance
    • Logistics
    • Funding
    • Other