Epidemiology

The first cases of Ebola virus infection were reported in Zaire (now known as the Democratic Republic of the Congo) 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. It was first reported in March 2014, and was 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] Over 28,000 cases (confirmed, probable, and suspected) were reported in this outbreak, with over 11,000 deaths.

An outbreak was announced in the Democratic Republic of the Congo (DRC) in August 2018. As of 27 August 2019, 2997 cases (2892 confirmed and 105 probable), including 1998 deaths (a case fatality rate of 67%), have been reported in the North Kivu and Ituri provinces of the DRC. Of the total number of cases, 58% were female and 28% were children <18 years of age.[27]Related to this outbreak, a cluster of cases have also been confirmed in Uganda.[28]This outbreak comes soon after a previous outbreak in the Equateur Province of the DRC (Bikoro, Iboko, and Wangata regions), with 54 cases and 33 deaths (a case fatality rate of 61%), was declared over. It has been confirmed that this latest outbreak is not related to the one in the Equateur Province, but is also due to the Zaire ebolavirus species. This latest outbreak is the tenth identified in the DRC to date.

The Zaire ebolavirus has a reported case fatality rate of up to 90% in previous outbreaks.[29] 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,[30] falling to 31.5% in some treatment centres in West Africa,[31] and around 20% in patients managed outside West Africa.[32]

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).[29] There has only been 1 outbreak of Bundibugyo ebolavirus: in 2007 in western Uganda, and this outbreak had a case fatality rate of 25%.[33]

WHO declares an outbreak over when a country has no new reported cases for 42 days (i.e., twice the maximum incubation period), provided that active surveillance is demonstrably in place and there is good diagnostic capacity.[34]

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