Epidemiology

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.[24] 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.[25]

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.[26] It has also been responsible for smaller outbreaks in the DRC since then. The Zaire ebolavirus has a reported case fatality rate of up to 90% in previous outbreaks.[4] Direct comparison of case fatality rates between different Ebola treatment centres 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,[18] falling to 31.5% in some treatment centres in West Africa,[27] and around 20% in patients managed outside West Africa.[28]

In contrast to this, the Sudan ebolavirus has a lower case fatality rate of 39% to 65% in previous outbreaks, with the largest outbreak occurring in 2000 in Uganda (425 cases).[4][29] There has only been one outbreak of Bundibugyo ebolavirus: in 2007 in western Uganda, and this outbreak had a case fatality rate of 25%.[6]

Recent outbreaks

  • 2022: an outbreak of Sudan ebolavirus disease in Uganda started on September 20, 2022 and was declared over on 11 January 2023, with a total of 142 confirmed cases and 55 deaths (case fatality rate 39%). This was the first outbreak caused by Sudan ebolavirus in Uganda since 2012.[29]​​​

  • 2022: one case was reported in the DRC on 21 August 21 2022 in the North Kivu province. The case, a 46-year old woman, died after being hospitalised for 23 days for symptoms thought to be related to her known co-morbidities.[30] No additional confirmed or probable cases were identified, and the outbreak was declared over on 27 September 2022.

  • 2022: the fourteenth outbreak in the DRC started on 23 April 2022 in the Équateur province and was declared over on 4 July 2022, with a total of 5 cases and 5 deaths (case fatality rate 100%). It was the third outbreak in the province in the last four years.[31]

  • 2021: the thirteenth outbreak in the DRC started on 8 October 2021 in the North Kivu province and was declared over on 16 December 2021, with a total of 11 cases and 9 deaths (case fatality rate 82%).[32]

  • 2021: a small outbreak was reported in Guinea on 14 February 2021 and was declared over on 19 June 2021, with a total of 23 cases and 12 deaths (case fatality rate 52%). This was the first outbreak in Guinea since the 2014-2016 West Africa outbreak.[33]

  • 2021: the twelfth outbreak in the DRC started on 7 February 2021 in the North Kivu province and was declared over on 3 May 2021, with a total of 12 cases and 6 deaths (case fatality rate 50%).[34]

  • 2020: the eleventh outbreak in the DRC started on 1 June 2020 in the Équateur province and was declared over on 18 November 2020, with a total of 130 cases and 55 deaths (case fatality rate 42%).[35]

  • 2018-2020: the world’s second largest outbreak in the north Kivu and Ituri provinces of the DRC in 2018 was declared over on 25 June 2020, with a total of 3481 cases and 2299 deaths (case fatality rate 66%).[36]

  • 2018: small outbreak in the DRC with 54 cases and 33 deaths (case fatality rate 61%).[2]

  • 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%).[2]

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.[37]

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