Details about the Organism

Introduction

Viral Haemorrhagic Fevers (VHF) are caused by a diverse group of viruses that cause an array of illnesses from relatively mild to rarely severe and life threatening. Viral survival in nature is dependent on the availability of specific natural hosts and this restricts the viruses to where their host species live. Humans are not the natural reservoir for any of these viruses. They become infected by contact with infected hosts or with arthropod vectors. However, under specific circumstances, an infected human can transmit some of the viruses to other humans (person-to-person transmission).

Classification of VHF viruses

Viruses associated with haemorrhagic fever are RNA viruses from four distinct virus families. Although a wide range of viruses are involved, they all share some common characteristics. They all have an RNA genome, and are enveloped. They are transmitted in three ways: by mosquitoes, via ticks or directly from their host as a zoonosis.

Deliberate release of Haemorrhagic Fever Viruses

In theory, VHF agents can be used as agents of bioterrorist attacks. However, in reality, it is very difficult to create a suitable conduit for transmission of these agents in a weaponised form. Laboratory testing on animal models and epidemiology of the Reston strain (Ebola virus) outbreak in Phillipino imported monkeys, show that some VHF agents can be spread by the airborne route, however to date this has not been seen in any of the human outbreaks studied.

This focuses on those VHF agents that are known to be readily capable of person-to-person spread. It assumes that even if aerosolisation does occur, it is the following four viruses that can be transmitted by person-to-person spread and could present a risk to public health in the UK.

These agents are:

• Lassa fever an arenavirus (and related arenaviruses, e.g. Sabia virus from Brazil)
• Crimean/ Congo haemorrhagic fever (CCHF) a bunyavirus,
• Ebola virus a filovirus
• Marburg virus a filovirus

The threat of infection by these pathogens is considered serious because:

• They can cause severe, rapidly fatal infection.
• Secondary cases may arise from contact with severely ill primary cases.
• The reputation of these diseases is such that they can induce public anxiety and disrupt everyday life in the population.
• Laboratory testing on animal models shows that some agents may be transmitted by aerosol, although this has not been seen in the human outbreaks studied.

Epidemiology

The agents of these VHFs are naturally endemic in different parts of the world, most notably Africa, parts of South America and some rural parts of the Middle East and Eastern Europe. However, environmental conditions in the UK do not support the natural reservoirs or vectors of any of these viruses, thus cases do not occur in the UK except as an imported disease. Such cases in travellers returning from endemic areas are rare: there have been nine cases of imported VHF (all Lassa fever) since 1980.

Other viruses, such as Rift Valley Fever virus, can cause VHF in humans, but although occasional reports indicate that person-to-person spread has occurred with some of these agents they do not present the same risk to public health.

Person-to-person transmission

All four main VHF viruses are transmitted from person-to-person in the same way: through direct contact with virus-infected body fluids such as blood, saliva, vomitus, stools and possibly sweat. Cross infection with multiple-use sharp instruments, such as lancets and needles, is associated with a high infection risk and a high fatality rate. Marburg, Ebola and Lassa virus have been shown to be present in the genital secretions of convalescents several weeks after illness; rare incidents of transmission from convalescent patients to sexual contacts have occurred with both Marburg and Lassa fever.

There is no evidence that close personal contact with a non-febrile, non-symptomatic, infected individual during the incubation period or convalescence results in transmission. Similarly, feverish patients who are well enough to care for themselves have never been shown to transmit infection to contacts in aeroplanes or other public transport or by other casual contact routes. Previous epidemics in Africa have resulted largely from secondary spread to health care workers and family contacts caring for the ill. Re-use of needles and syringes, inadequate infection control techniques, and unhygienic practices are the major routes for nosocomial transmission among hospital staff and patients. Contact with the body or body fluids of the dead in customary preparation for burial are also a recognised source of infection.

These viruses are not airborne, but may be transmitted over short distances of a meter or so by droplets of body fluids from infected patients, if the droplets come into contact with mucous membranes. There is also a potential risk to laboratory workers as small clouds of aerosolised viruses can be released in laboratory accidents, such as breakages of containers within centrifuges.

Infectious dose

The infectious dose of haemorrhagic fever viruses is unknown but is probably low (around 1-10 organisms).

Incubation period

The incubation period varies between 1 and 21 days. The infectivity period depends on viral type and mode of infection.

Incubation period for VHF viruses
Virus	Disease	Incubation Range
Arenaviridae Lassa	Lassa Fever	3 - 21 days
Bunyaviridae Nairovirus	Crimean-Congo Haemorrhagic Fever	1 - 12 days
Filoviridae Ebola Virus Marburg Virus	Ebola Haemorrhagic Fever Marburg Haemorrhagic Fever	2 - 21 days 3 - 16 days

 

Period of communicability

As a precaution, patients who have clinical symptoms are considered infectious. Lassa virus can continue to be in the blood during early convalescence when the fever has resolved. There are reports of late transmission events (92 days for Marburg) and Lassa fever virus can be shed in urine for several weeks or in semen for months after illness has resolved.

Organism Survival

No specific studies have been undertaken, but these are all RNA viruses with lipid envelopes. This renders them relatively susceptible to detergents as well as to low pH environments. Conversely, they are quite stable at neutral pH, especially in the presence of protein.

Last reviewed: 13 May 2011