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Viral Hemorrhagic
Fevers
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What
are viral hemorrhagic fevers? |
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Researchers wearing protective clothing
investigate the 1993 HPS outbreak
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Viral hemorrhagic
fevers (VHFs) refer to a group of illnesses that are caused by several
distinct families of viruses. In
general, the term "viral hemorrhagic fever" is used to describe
a severe multisystem syndrome (multisystem in that multiple organ systems
in the body are affected). Characteristically, the overall vascular
system is damaged, and the body's ability to regulate itself is impaired.
These symptoms are often accompanied by hemorrhage (bleeding); however,
the bleeding is itself rarely life-threatening.
While some types of hemorrhagic fever viruses can cause relatively mild
illnesses, many of these viruses cause severe, life-threatening disease.
The Special Pathogens
Branch (SPB) primarily works with hemorrhagic fever viruses that are classified
as biosafety level four
(BSL-4) pathogens. A list of these viruses appears in the SPB
disease information index. The Division of Vector-Borne Infectious
Diseases, also in the National Center for Infectious Diseases, works with
the non-BSL-4 viruses that cause two other hemorrhagic fevers, dengue
hemorrhagic fever and yellow fever.
How
are hemorrhagic fever viruses grouped? |
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VHFs are caused by
viruses of four distinct families: arenaviruses,
filoviruses, bunyaviruses, and flaviviruses.
Each of these families share a number of features:
- They are all RNA
viruses, and all are covered, or enveloped, in a fatty (lipid) coating.
- Their survival
is dependent on an animal or insect host, called the natural reservoir.
- The viruses are
geographically restricted to the areas where their host species live.
- Humans are not
the natural reservoir for any of these viruses. Humans are infected
when they come into contact with infected hosts. However, with some
viruses, after the accidental transmission from the host, humans can
transmit the virus to one another.
- Human cases or
outbreaks of hemorrhagic fevers caused by these viruses occur sporadically
and irregularly. The occurrence of outbreaks cannot be easily predicted.
- With a few noteworthy
exceptions, there is no cure or established drug treatment for VHFs.
In rare cases, other
viral and bacterial infections can cause a hemorrhagic fever; scrub typhus
is a good example.
What
carries viruses that cause viral hemorrhagic fevers? |
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Viruses associated
with most VHFs are zoonotic. This means that these viruses naturally reside
in an animal reservoir host or arthropod vector. They are totally dependent
on their hosts for replication and overall survival. For the most part,
rodents and arthropods are the main reservoirs for viruses causing VHFs.
The multimammate
rat, cotton
rat, deer mouse, house mouse, and other field rodents are examples
of reservoir hosts. Arthropod ticks and mosquitoes serve as vectors for
some of the illnesses. However, the hosts of some viruses remain unknown
-- Ebola and Marburg
viruses are well-known examples.
Where
are cases of viral hemorrhagic fever found? |
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Taken together, the
viruses that cause VHFs are distributed over much of the globe. However,
because each virus is associated with one or more particular host species,
the virus and the disease it causes are usually seen only where the host
species live(s). Some hosts, such as the rodent species carrying several
of the New World arenaviruses, live in geographically restricted areas.
Therefore, the risk of getting VHFs caused by these viruses is restricted
to those areas. Other hosts range over continents, such as the rodents
that carry viruses which cause various forms of hantavirus pulmonary syndrome
(HPS) in North and South America, or the different set of rodents that
carry viruses which cause hemorrhagic fever with renal syndrome (HFRS)
in Europe and Asia. A few hosts are distributed nearly worldwide, such
as the common rat. It can carry Seoul virus, a cause of HFRS; therefore,
humans can get HFRS anywhere where the common rat is found.
While people usually
become infected only in areas where the host lives, occasionally people
become infected by a host that has been exported from its native habitat.
For example, the first outbreaks of Marburg hemorrhagic fever, in Marburg
and Frankfurt, Germany, and in Yugoslavia, occurred when laboratory workers
handled imported monkeys infected with Marburg virus. Occasionally, a
person becomes infected in an area where the virus occurs naturally and
then travels elsewhere. If the virus is a type that can be transmitted
further by person-to-person contact, the traveler could infect other people.
For instance, in 1996, a medical professional treating patients with Ebola
hemorrhagic fever (Ebola HF) in Gabon unknowingly became infected. When
he later traveled to South Africa and was treated for Ebola HF in a hospital,
the virus was transmitted to a nurse. She became ill and died. Because
more and more people travel each year, outbreaks of these diseases are
becoming an increasing threat in places where they rarely, if ever, have
been seen before.
How
are hemorrhagic fever viruses transmitted? |
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Viruses causing hemorrhagic
fever are initially transmitted to humans when the activities of infected
reservoir hosts or vectors and humans overlap. The viruses carried in
rodent reservoirs are transmitted when humans have contact with urine,
fecal matter, saliva, or other body excretions from infected rodents.
The viruses associated with arthropod vectors are spread most often when
the vector mosquito or tick bites a human, or when a human crushes a tick.
However, some of these vectors may spread virus to animals, livestock,
for example. Humans then become infected when they care for or slaughter
the animals.
Some viruses that
cause hemorrhagic fever can spread from one person to another, once an
initial person has become infected. Ebola, Marburg,
Lassa and Crimean-Congo hemorrhagic fever viruses
are examples. This type of secondary transmission of the virus can occur
directly, through close contact with infected people or their body fluids.
It can also occur indirectly, through contact with objects contaminated
with infected body fluids. For example, contaminated syringes and needles
have played an important role in spreading infection in outbreaks of Ebola
hemorrhagic fever and Lassa fever.
What
are the symptoms of viral hemorrhagic fever illnesses? |
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Specific signs and
symptoms vary by the type of VHF, but initial signs and symptoms often
include marked fever, fatigue, dizziness, muscle aches, loss of strength,
and exhaustion. Patients with severe cases of VHF often show signs of
bleeding under the skin, in internal organs, or from body orifices like
the mouth, eyes, or ears. However, although they may bleed from many sites
around the body, patients rarely die because of blood loss. Severely ill
patient cases may also show shock, nervous system malfunction, coma, delirium,
and seizures. Some types of VHF are associated with renal (kidney) failure.
How
are patients with viral hemorrhagic fever treated? |
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Patients receive
supportive therapy, but generally speaking, there is no other treatment
or established cure for VHFs. Ribavirin, an anti-viral drug, has been
effective in treating some individuals with Lassa fever or HFRS. Treatment
with convalescent-phase plasma has been used with success in some patients
with Argentine hemorrhagic fever.
How
can cases of viral hemorrhagic fever be prevented and controlled? |
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With the exception
of yellow fever and Argentine hemorrhagic fever, for which vaccines have
been developed, no vaccines exist that can protect against these diseases.
Therefore, prevention efforts must concentrate on avoiding contact with
host species. If prevention methods fail and a case of VHF does occur,
efforts should focus on preventing further transmission from person to
person, if the virus can be transmitted in this way.Because many of the
hosts that carry hemorrhagic fever viruses are rodents, disease prevention
efforts include
- controlling rodent
populations;
- discouraging rodents
from entering or living in homes or workplaces;
- encouraging safe
cleanup of rodent nests and droppings.
For hemorrhagic fever
viruses spread by arthropod vectors, prevention efforts often focus on
community-wide insect and arthropod control. In addition, people are encouraged
to use insect repellant, proper clothing, bednets, window screens, and
other insect barriers to avoid being bitten.
For those hemorrhagic
fever viruses that can be transmitted from one person to another, avoiding
close physical contact with infected people and their body fluids is the
most important way of controlling the spread of disease. Barrier nursing
or infection control techniques include isolating infected individuals
and wearing protective clothing. Other infection control recommendations
include proper use, disinfection, and disposal of instruments and equipment
used in treating or caring for patients with VHF, such as needles and
thermometers.
In conjunction with
the World Health Organization, CDC has developed practical, hospital-based
guidelines, titled Infection Control for Viral
Haemorrhagic Fevers In the African Health Care Setting. The
manual can help health-care facilities recognize cases and prevent further
hospital-based disease transmission using locally available materials
and few financial resources.
What
needs to be done to address the threat of viral hemorrhagic fevers? |
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Scientists and researchers
are challenged with developing containment, treatment, and vaccine strategies
for these diseases. Another goal is to develop immunologic and molecular
tools for more rapid disease diagnosis, and to study how the viruses are
transmitted and exactly how the disease affects the body (pathogenesis).
A third goal is to understand the ecology of these viruses and their hosts
in order to offer preventive public health advice for avoiding infection.
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