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( By WHO - OMS, 1999 )

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4 - Background

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.

While the specific clinical profile of each viral illness may vary, there are two prominent symptoms that may appear in all types of VHF during the most critical stage of the illness:

- bleeding, with the risk of severe haemorrhage from both cutaneous and internal sites; and
- the development of shock, which may be irreversible.

The existence of a specific virus in a community tends to reflect the geographical distribution of its natural host. Nevertheless, human and natural environments are changing rapidly so 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. Under these conditions, case-fatality rates can often exceed 50% and may reach 80% for several days.

Table 3 lists the major VHFs that cause epidemics and shows their distribution.

Table 3. Viral haemorrhagic fevers causing epidemics



Natural host/vector

Lassa fever

Central/West Africa



South America





Crimean-Congo haemorrhagic fever (CCHF)



Rift Valley fever



Dengue haemorrhagic fever

Africa/Americas/Pacific/western Asia/Australasia/Caribbean/India


Yellow fever

Africa/South America


Haemorrhagic fever with renal syndrome (HFRS)



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, interrupts transmission and prevents unnecessary cases and deaths.

There are many examples of yellow fever epidemics that were identified as such several months after the actual epidemic onset. The consequences of this 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:

· one case is confirmed in a community with abundant vector mosquitos;

· a single case of yellow fever is diagnosed by serology or virus isolation, or suggested by histopathology;

· hospital reports show increased incidence of fatal hepatitis, suspected cases of yellow fever and of VHF.

Early warning procedures such as routine health surveillance and rapid reporting from hospitals are essential for detecting VHF outbreaks at an early stage.

The questions below should be addressed as part of these early warning procedures:

· Where are the high-risk areas for past and potential VHF and yellow fever epidemics? At-risk populations? Based on past experience, when are the high-risk seasons?

· What is the likely health impact of an epidemic of VHF or yellow fever (number of cases, hospitalizations, and deaths)?

· What early signs would signal a VHF or yellow fever “epidemic alert”? Can or could they be detected earlier through improved epidemic surveillance and reporting?

· Does routine health surveillance include rural areas, where VHF outbreaks frequently occur?

When VHF outbreaks are reported, they receive heavy media coverage, often in the context of the panic such outbreaks arouse in the local medical services and communities affected. Rapid health assessments will provide factual evidence on the existence and extent of an outbreak. This information can be provided to the media so that the potentially affected population and the medical authorities can make informed decisions.

In this way, the rapid assessment offers a valuable opportunity to allay the community’s anxiety and to provide basic information on protective measures to prevent the disease’s further spread.