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Facts on Ebola virus: human and animal outbreaks

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Ebola virus hemorrhagic fever is a zoonotic disease transmitted to humans by direct contact with infected live or dead animals, and more specifically with their body fluid. Genetic and antigenic characterization of Ebola virus isolates during human outbreaks has led to the identification of four subtypes — Ebola Sudan, E. Zaire, E. Ivory Coast and E. Reston. Ebola Reston originates in Asia and has never been reported to cause human disease, but the other three subtypes circulate on the African subcontinent and are pathogenic for humans, causing a specific febrile hemorrhagic disease. After an incubation period of about 12 days, victims rapidly develop high fever, diarrhoea, vomiting, respiratory disorders and haemorrhaging. Death ensues within a few days. The case fatality rates are about 80% with E. Zaire and 50% with E. Sudan (Pourrut & al., 2005).


Chronology of the outbreaks


The first recorded outbreak of Ebola occurred in Sudan (due to the so called Ebola Sudan subtype), near the border with the Democratic Republic of the Congo, between June and November 1976. A second outbreak occurred in the same region 3 years later, between July and October 1979. Meanwhile (August and November 1976), an outbreak due to E. Zaire occurred in DRC, near the borders with Sudan and the Central African Republic. This previously unknown disease was named for the river Ebola, which flows past Yambuku (epicentre of the outbreak in DRC). E. Zaire made a second but restricted appearance (only one case recorded) in DRC in June 1977.


After a 15-year period in which no further cases were recorded, Ebola re-emerged in 1994 for a 3-year period. This new phase was marked by the identification of a new subtype, E. Ivory Coast, and by an escalation of outbreaks due to E. Zaire. Ebola outbreak was recorded in the chimpanzees of Tai National Park (Ivory Coast) in June 1994 (Formenty et al., 1999), and an ethnologist became ill after autopsying a chimpanzee found dead in the Tai forest. It was the first and only human case observed in West Africa, and the only case clearly attributed to E. Ivory Coast. 25% of the 43 chimpanzees in the studied community were recorded to have died from the virus (Formenty et al., 1999). Regarding E. Zaire, the first outbreak of this period occurred in the town of Kikwit, about 500 km from Kinshasa (DRC), while three further outbreaks occurred in northeast Gabon: in Mekouka between 1994 and 1995, Mayibout in early 1996, and Booué between 1996 and 1997. Tough there is no proofs; those outbreaks in humans are suspected to be linked to a drastic decline recorded in great ape abundance in the Minkebe forest (Huijbregts & al., 2003). Indeed, Lahm (2000) reported a decrease of 90% in gorilla and 98% in chimpanzee abundance compare to her previous observations in the same area - before the 1994 and 1996 Ebola epidemics.


Since 2000, multiple outbreaks of E. Zaire occurred in a relatively limited area: the border region of Gabon and the Republic of Congo. Successive outbreaks occurred in Mekambo and Makokou, in Gabon (October 2001); Ekata and Olloba villages, in the Republic of Congo (November-December 2001). Then, the district and village of Mbomo, RC (entrance of Odzala National Park) suffered 3 successive outbreaks: March 2002; December 2002-May 2003; and October-December 2003. The source of these outbreaks are well documented, and all (some of which had multiple sources) occurred after people had handled animal carcasses found in the forest (mainly gorillas, chimpanzees and duikers). In Lossi Sanctuary (South of Mbomo), 91% of the individually known gorillas in Bermejo’s study groups (143 individuals in total) disappeared from October 2002 to January 2003; and 95.8% of individuals in the newly monitored groups were killed from October 2003 to January 2004 (Bermejo & al., 2006). By extrapolating from more wide-ranging transect surveys they conducted, Bermejo and her colleagues conclude that in a 2700 km2 region surrounding the Lossi Sanctuary, roughly 5000 gorillas have succumbed to the current epidemic. Further north, inside Odzala National Park, Ebola broke out at Lokoué study site in December 2003. The epidemic lasted for almost a year, and killed about 95% of the some 377 identified gorillas that formerly frequented the clearing (Caillaud et al., 2006). Devos & al., (submitted) reported that both gorillas and chimpanzees nest encounter rate decreased in the surrounding forest by 80-85%.


This period also saw a resurgence of E. Sudan, in Uganda between October 2000 and January 2001, and in Sudan, close to the previous outbreak sites of 1976 and 1979, in May-June 2004.


In brief


Since the discovery of Ebola virus in 1976, there have been 13 human outbreaks in Africa (nine due to E. Zaire and four to due to E. Sudan) and two isolated cases (due to E. Zaire and E. Ivory Coast). These outbreaks took place during three distinct periods (three between 1976 and 1979, four between 1994 and 1997, and six between 2000 and 2004). In total, Ebola virus infected about 1850 people and caused nearly 1300 deaths. The different Ebola virus subtypes showed a certain geographic pattern, E. Ivory Coast affecting West Africa, E. Sudan east Africa, and E. Zaire central Africa. The recurrent emergence of E. Zairesince the mid-1990s in Gabon and the Republic of Congo have been the most monitored and brought clues on the link between humans and wild animal mortality. In each case of human outbreak, simultaneously, nearby ape populations have experienced massive declines in numbers, with ape populations in certain forests falling by more than 90% in just a few years or months (Huijbregts et al., 2003; Walsh et al., 2003; Bermejo et al., 2006; Caillaud et al., 2006; Devos et al., submitted). Four populations of monitored apes are known to have been affected and/or decimated by the virus (chimpanzees of the Tai forest; both apes of the Minkebe forest, Lossi Sanctuary and Lokoué bai). Figures given are alarming as for example, the Lossi outbreaks only, killed about as many gorillas as survive in the entire eastern gorilla species (Gorilla beringei). Yet Lossi represents only a small fraction of the western gorillas killed by E. Zaire in the past decade: because of the difficulty of outbreak detection in wild animals (local populations must ideally be monitored before, during and after an epidemic event to be able to detect changes in numbers), it is very likely that other die-offs amongst tropical forest mammals occurred but haven’t been recorded, which means that the now acknowledged facts are most probably an under-representation of the real impact of Ebola virus on wild animals. The fear is that the spread of the virus continues among wild animal populations. Scientists still doesn’t know why and how Ebola virus has emerged so explosively in recent years: is the virus dormant in a natural reservoir – a forest species that could carry the virus without getting deathly ill? Bats have been suspected as such (Leroy et al., 2005). And if so, what are the environmental parameters that lead to its outbreak? (habitat disturbance, climate change,…?) Or whether the virus is mostly spread from an infected ape to its contacts?

Although both mechanisms of spread probably play a role, evidence are now growing that apes are indeed passing the virus to each other (Walsh et al., 2007): within social group, between social groups, and even between species. Threat on ape populations living in contiguous forest is thus very strong as ape-to-ape transmission act as an amplifier of Ebola outbreaks. Control measures need to be taken as soon as possible to avoid those once abundant and widely distributed ape species to become reduced to tiny remnant populations.


 (Map from Pourrut & al., 2005: Human outbreaks for the different Ebola virus subtypes)

Map from Pourrut et al.



(Map from Vogel, 2006)

Map from Vogel


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Last modified : August 28, 2007