WORLD HEALTH ORGANIZATION
Ebola virus disease
Key facts
- Ebola virus disease (EVD),
formerly known as Ebola haemorrhagic fever, is a severe, often fatal
illness in humans.
- EVD outbreaks have a case
fatality rate of up to 90%.
- EVD outbreaks occur primarily
in remote villages in Central and West Africa,
near tropical rainforests.
- The virus is transmitted to
people from wild animals and spreads in the human population through
human-to-human transmission.
- Fruit bats of the Pteropodidae
family are considered to be the natural host of the Ebola virus.
- Severely ill patients require
intensive supportive care. No licensed specific treatment or vaccine is
available for use in people or animals.
Ebola first appeared in 1976 in 2 simultaneous outbreaks, in Nzara, Sudan,
and in Yambuku, Democratic Republic of Congo. The latter was in a village
situated near the Ebola
River, from which the
disease takes its name.
Genus Ebolavirus is 1 of 3 members of the Filoviridae family
(filovirus), along with genus Marburgvirus and genus Cuevavirus. Genus
Ebolavirus comprises 5 distinct species:
- Bundibugyo ebolavirus (BDBV)
- Zaire ebolavirus (EBOV)
- Reston
ebolavirus (RESTV)
- Sudan ebolavirus (SUDV)
- Taï Forest
ebolavirus (TAFV).
BDBV, EBOV, and SUDV have been associated with large EVD outbreaks in Africa, whereas RESTV and TAFV have not. The RESTV
species, found in Philippines
and the People’s Republic of China,
can infect humans, but no illness or death in humans from this species has been
reported to date.
Transmission
Ebola is introduced into the human population through close contact with the
blood, secretions, organs or other bodily fluids of infected animals. In Africa, infection has been documented through the
handling of infected chimpanzees, gorillas, fruit bats, monkeys, forest
antelope and porcupines found ill or dead or in the rainforest.
Ebola then spreads in the community through human-to-human transmission,
with infection resulting from direct contact (through broken skin or mucous
membranes) with the blood, secretions, organs or other bodily fluids of
infected people, and indirect contact with environments contaminated with such
fluids. Burial ceremonies in which mourners have direct contact with the body
of the deceased person can also play a role in the transmission of Ebola. Men
who have recovered from the disease can still transmit the virus through their
semen for up to 7 weeks after recovery from illness.
Health-care workers have frequently been infected while treating patients
with suspected or confirmed EVD. This has occurred through close contact with
patients when infection control precautions are not strictly practiced.
Among workers in contact with monkeys or pigs infected with Reston ebolavirus, several infections have been
documented in people who were clinically asymptomatic. Thus, RESTV appears less
capable of causing disease in humans than other Ebola species.
However, the only available evidence available comes from healthy adult
males. It would be premature to extrapolate the health effects of the virus to
all population groups, such as immuno-compromised persons, persons with
underlying medical conditions, pregnant women and children. More studies of
RESTV are needed before definitive conclusions can be drawn about the
pathogenicity and virulence of this virus in humans.
Signs and symptoms
EVD is a severe acute viral illness often characterized by the sudden onset
of fever, intense weakness, muscle pain, headache and sore throat. This is
followed by vomiting, diarrhoea, rash, impaired kidney and liver function, and
in some cases, both internal and external bleeding. Laboratory findings include
low white blood cell and platelet counts and elevated liver enzymes.
People are infectious as long as their blood and secretions contain the
virus. Ebola virus was isolated from semen 61 days after onset of illness in a
man who was infected in a laboratory.
The incubation period, that is, the time interval from infection with the
virus to onset of symptoms, is 2 to 21 days.
Diagnosis
Other diseases that should be ruled out before a diagnosis of EVD can be
made include: malaria, typhoid fever, shigellosis, cholera, leptospirosis,
plague, rickettsiosis, relapsing fever, meningitis, hepatitis and other viral
haemorrhagic fevers.
Ebola virus infections can be diagnosed definitively in a laboratory through
several types of tests:
- antibody-capture
enzyme-linked immunosorbent assay (ELISA)
- antigen detection tests
- serum neutralization test
- reverse transcriptase
polymerase chain reaction (RT-PCR) assay
- electron microscopy
- virus isolation by cell
culture.
Samples from patients are an extreme biohazard risk; testing should be
conducted under maximum biological containment conditions.
Vaccine and treatment
No licensed vaccine for EVD is available. Several vaccines are being tested,
but none are available for clinical use.
Severely ill patients require intensive supportive care. Patients are
frequently dehydrated and require oral rehydration with solutions containing
electrolytes or intravenous fluids.
No specific treatment is available. New drug therapies are being evaluated.
Natural host of Ebola virus
In Africa, fruit bats, particularly species
of the genera Hypsignathus monstrosus, Epomops franqueti and Myonycteris
torquata, are considered possible natural hosts for Ebola virus. As a
result, the geographic distribution of Ebolaviruses may overlap with the range
of the fruit bats.
Ebola virus in animals
Although non-human primates have been a source of infection for humans, they
are not thought to be the reservoir but rather an accidental host like human
beings. Since 1994, Ebola outbreaks from the EBOV and TAFV species have been
observed in chimpanzees and gorillas.
RESTV has caused severe EVD outbreaks in macaque monkeys (Macaca
fascicularis) farmed in Philippines
and detected in monkeys imported into the USA
in 1989, 1990 and 1996, and in monkeys imported to Italy
from Philippines
in 1992.
Since 2008, RESTV viruses have been detected during several outbreaks of a
deadly disease in pigs in People’s Republic of China
and Philippines.
Asymptomatic infection in pigs has been reported and experimental inoculations
have shown that RESTV cannot cause disease in pigs.
