Wednesday 14 January 2015

The source of the December 2013 Guinean Ebola outbreak.

In December 2013 cases of the haemorrhagic Virus Ebola began to be reported from the village of Meliandou near Guéckédou in the Republic of Guinea. As is typical of the disease, it quickly overwhelmed local healthcare facilities, where unprepared staff where among the first infected. Since this initial outbreak the disease has spread to Sierra Leone, Liberia, Nigeria, Senegal, USA, Spain, and Mali, with 17 145  reported cases and 6070 recorded deaths, as of 3 December 2014; the worst outbreak of the disease since records began. The outbreak has been confirmed to be of the Zaire Ebola Virus, EBOV, the most lethal of the five known Ebola Viruses, which typically kills around 88% of people who become infected but do not receive treatment.

Ebola Viruses are thought to be zoonotic in origin (i.e. present in a population of animals, with only occasional, opportunistic infections of Humans) as its high fatality rate prevents the establishment of a permanent viral reserve in the Human population. The animal vector for the disease is still not known; non-Human Primates and Duikers (small, forest-dwelling Antelopes) have both been suggested as possible vectors for the disease, as these animals are also susceptible to the Virus, however these animals suffer levels of mortality similar to that seen in Humans, making them unlikely permanent hosts for the disease.

Bats have also been suggested as a possible vector for the disease, and several species have been shown to be able to survive Ebola infections (notably the Fruit Bat Epomophorus wahlbergi and the Insectivorous Bats Chaerephon pumilus and Mops condylurus), while Ebola RNA has been recovered from three species of Fruit Bat (Epomops franqueti, Hypsignathus monstrosus, and Myonycteris torquata) and antibodies to the Virus from a wide range of Bats (Epomops franqueti, Hypsignathus monstrosus, Myonycteris torquata and Mops condylurusagain, plus Eidolon helvum, Epomophorus gambianus, Micropteropus pusillus, Rousettus aegyptiacus, and Rousettus leschenaultia). Fruit Bats are widely eaten as Bushmeat in Africa, and it has been suggested that the preparation and consumption of Fruit Bats may be the cause of Ebola outbreaks in Human populations.

In a paper published in the journal EMBO Molecular Medicine on 30 December 2014, a team of scientists led by Almudena Marí Saéz of Institute ofTropical Medicine and International Health at the Charité UniversitätsmedizinBerlin describe the results of a survey of the wildlife around Meliandou with the intention of determining the vector for the Virus.

Sampling and investigation locations for the 2014 Ebola-related animal survey. (A) In southeastern Guinea (Sierra Leone, Guinea, and Liberia are visible), scale bar stands for 50 km. (B) In and around the index village, Meliandou, scale bar stands for 100 m. Marí Saéz et al. (2014).

Primate meat is occasionally consumed in Meliandou, but is not usually caught locally, typically arriving as smoked meat from the Fouta Djallon region of northwest Guinea. This makes it an unlikely source of infection, as the Virus would have been required to reach the village in treated meat without infecting the original hunters, food processors or traders in the supply route. Nor are wild Primates typically found close to the village. Wild Primates, including Chimpanzees (Pan troglodytes verus) are found in two reserves in southern Guinea, Massif du Ziama and Fôrét classée de Diéké, however neither of these are close to Meliandou, and in neither location do the Apes show any signs of a population crash, which is typical of Ebola infections in Great Ape populations. This is in contrast to the situation following previous Ebola epidemics in Equatorial Africa, when Apes have been as badly affected as Humans.

Bats, however, are occasionally hunted around Meliandou, with men from the village and neighbouring communities regularly targeting Fruit Bats as a source of food year round, and both local hunters and government officials in the area reporting large Bat colonies in the area. In addition large groups of migratory Fruit Bats visit the area each year, and a large colony of wide-ranging Fruit Bats is present in a cave system in the Ziama Biosphere Reserve. Insectivorous Bats are also common in the area, frequently roosting under the eves of houses, where they are hunted by children, who grill them over small fires (hunting of small game by children is common in Guinea and neighbouring countries).

The village of Meliandou. Marí Saéz et al. (2014).

Marí Saéz et al. trapped 169 Bats belonging to thirteen species at four sights in southern Guinea (Meliandou, Kagbadou, Kéléma, and Ziama, including members of three species previously recorded to carry the Virus (Eidolon helvum, Hypsignathus monstrosus, and Mops condylurus). However neither Ebola RNA nor antibodies to the Virus could be recovered from any of these.

The first reported case of Ebola in Meliandou was a two-year-old boy, unlikely to have participated in hunting of Bats of any kind. Furthermore the boy’s father did not live in the village, and did not participate in hunting, making it unlikely that he had brought bats into the home. In order to try understand how the initial infection occurred Marí Saéz et al. carried out a detailed survey of the village, which comprised 31 houses, and its surroundings, farmland with a few large trees.

A large burned hollow tree stump was found about 50 m to the south of the house where the initial infection occurred, on the route from the village to a river used by local women to wash laundry. Villagers reported that children had frequently played in this tree, and that the tree had caught fire in March 2014, which resulted in a ‘rain of Bats’ falling from the tree. The Bats were described as lolibelo, which does not correspond to an exact species in the Linnean sense, but which could imply a number of small, long-tailed Bat species. After the fire the villagers collected the dead Bats with the intention of consuming them, but received notice of a ban on bushmeat consumption as part of a range of environmental health measures intended to combat the disease outbreak the following day, and disposed of the animals. Screening ash and soil samples from beneath the tree recovered DNA assigned confidently to Mops condylurus, a small insectivorous Bat, known to be able to survive Ebola infection, which is considered a possible reservoir for the disease.

The burnt hollow tree; in (D), the arrow points at a stick, most probably left there by children. Marí Saéz et al. (2014).

Marí Saéz et al. conclude that Bats are very likely to have been the Virus reservoir for the 2013 December 2013 Guinean Ebola outbreak, but caution that care needs to be taken in how this data is interpreted. Previous attempts at arranging Bat culls to control zoonotic diseases have been unsuccessful and sometimes resulted in Bats scattering to avoid being killed and carrying the disease to new areas. Bats also perform a number vital ecosystem services, including distributing seeds, pollinating some plants and controlling Insect populations. They therefore council that future public health initiatives should concentrate on highlighting the dangers of Bat handling, but seek to avoid promoting conflicts between Human and Bat populations. In addition they feel that future discussion of the cause of the outbreak should stress that while the initial infection occurred in Meliandou, this was essentially down to bad luck and great care should be taken to avoid the stigmatization of people from the village or the wider south Guinea area.

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