Ebola virus disease, also called Ebola haemorrhagic fever, occurs in humans and primates.
The virus is in the Filoviridae family, members of which cause severe haemorrhagic fever. Five strains of Ebola virus have been identified, four of which are known to cause disease in humans.
The natural reservoir is unknown though the strongest evidence points towards fruit bats; non-human primates are a secondary host and develop fatal symptoms, so are unlikely to be the reservoir.
Ebola virus strains are genetically stable. This means the virus is unlikely to evolve into a form that spreads more easily and makes it more likely that future vaccines will be effective in preventing infection. The genetic material is a strand of RNA but the virus relies for its integrity on its outer envelope. This makes it vulnerable to disinfectants and drying.
Infectivity is not very high – in the West African outbreak, one case has been typically leading to two, by contrast with a disease such as measles where one case may lead to 18 cases in an outbreak – but high enough to create an epidemic.
First emerging in 1976, disease outbreaks have been occurring with increasing frequency in the last two decades.
Outbreaks primarily occur in villages close to rainforests in Central and West Africa and are likely to initiate via contact with animals carrying the virus. Infection has been documented through the handling of infected chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines.
Symptoms & mortality
In historical outbreaks, the mortality rate has varied between 25 and 89% (average 67%: see this table for all data). The mortality rate in the current outbreak is 39% (from these data).
The virus has an incubation period of 2-21 days before the development of symptoms. A person is not infectious during the asymptomatic period.
Early symptoms develop suddenly and include fever, headache, muscle pain, sore throat, weakness – are similar to many other diseases so recognising infection early is difficult. Tests exist for early diagnosis when there is reason to believe a person is infected.
Later symptoms include diarrhoea, vomiting, rash, impaired kidney and liver function, stomach pain, internal and external bleeding.
Ebola is not airborne and is only transmitted by direct contact with the blood, secretions, organs or other bodily fluids of infected persons or animals, including via objects e.g. needles or clothing.
Those at highest risk of infection are health workers, family members and others in close contact with ill or deceased patients.
Sexual transmission (from men) can occur after recovery from the disease as the virus is maintained for up nine months in semen.
Pig farms can help amplify infection when farms are in the territory of fruit bats since pigs carry the virus without symptoms and can infect humans.
This is a Factsheet issued by the Science Media Centre to provide background information on science topics relevant to breaking news stories. This is not intended as the ‘last word’ on a subject, but rather a summary of the basics and a pointer towards sources of more detailed information. These can be read as supplements to our Roundups and/or briefings.