The first cases of Ebola virus infection were reported in Zaire (now known as the Democratic Republic of the Congo [DRC]) in 1976. There were 318 cases and 280 deaths, an 88% case fatality rate. Transmission in this outbreak was traced back to the use of contaminated needles in an outpatient clinic at Yambuku Mission Hospital. Since then, frequent outbreaks have occurred in Central and Western Africa.
The most common species of Ebola virus responsible for outbreaks is the Zaire ebolavirus, the second most common species being the Sudan ebolavirus.
The Zaire ebolavirus was responsible for the outbreak that started in West Africa in 2014 and finished in 2016. It was first reported in March 2014, and is the largest outbreak since the virus was first discovered in 1976. Genetic sequencing has shown that the virus isolated from infected patients in the 2014 outbreak is 97% similar to the virus that first emerged in 1976. It is also responsible for smaller outbreaks in the DRC from 2017-2020. The Zaire ebolavirus has a reported case fatality rate of up to 90% in previous outbreaks. Direct comparison of case fatality rates between different Ebola treatment centers and outbreaks should be interpreted with caution as many variables can introduce bias and skew even large cohort data. The case fatality rate during the 2014 outbreak was up to 64.3% in hospital admissions, falling to 31.5% in some treatment centers in West Africa, and around 20% in patients managed outside West Africa.
In contrast to this, the Sudan ebolavirus has a lower case fatality rate of 53% to 65% in previous outbreaks, with the largest outbreak occurring in 2000 in Uganda (425 cases). There has only been one outbreak of Bundibugyo ebolavirus: in 2007 in western Uganda, and this outbreak had a case fatality rate of 25%.
Recent major outbreaks
2020: the eleventh outbreak in the DRC started on the 1st June in the Équateur province and was declared over on the 18th November, with a total of 130 cases and 55 deaths (case fatality rate 42%).
2018-2020: the world’s second largest outbreak in the north Kivu and Ituri provinces of the DRC in 2018 was declared over on the 25th June 2020, with a total of 3481 cases and 2299 deaths (case fatality rate 66%).
2018: small outbreak in the DRC with 54 cases and 33 deaths (case fatality rate 61%).
2014-2016: the world’s largest outbreak started in the DRC in 2014 and finished in 2016, with over 28,000 cases and 11,000 deaths (case fatality rate 46%).
The WHO declares an outbreak is over when no confirmed or probable cases are detected for a period of 42 days (i.e., twice the maximum incubation period) since the last potential exposure to the last case occurred; however, WHO recommends heightened surveillance and response activities during the 42-day period and for at least 6 months after.
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