10:14pm Friday 18 August 2017

Wildlife origins of West African Ebola virus disease outbreak narrowed down

The team wanted to know whether there was a larger Ebola virus outbreak happening in wildlife in the region and how the index case, a two-year-old boy in Meliandou, Guinea, might have gotten infected and sparked the epidemic that has since spread into other areas of Guinea and then Sierra Leone, Liberia, Nigeria, Senegal, USA, Spain and Mali, representing the largest ever recorded outbreak.

To address these questions, Fabian Leendertz, head of the group “Epidemiology of Highly Pathogenic Microorganisms” at the Robert Koch Institute, Berlin, assembled a large international interdisciplinary team consisting of virologists, veterinarians, ecologists, epidemiologists and an anthropologist stemming from various institutions; the Robert Koch-Institute, Berlin, Germany, the Max-Planck-Institute for Evolutionary Anthropology, Leipzig, Germany, the Wild Chimpanzee Foundation, Guinea, the Phillips University of Marburg, Germany, the Institute of Tropical Medicine and International Health, Charité, Berlin, Germany, and the McGill University, Montreal, Canada. The manuscript reporting their findings is published Dec 30th 2014 in EMBO Molecular Medicine (Issue January 2015).

The team first investigated whether there were signs of a recent decline in wildlife populations in the two largest protected areas in southeastern Guinea. During past Ebola virus disease outbreaks, particularly in Central Africa, the virus has caused major die offs in wildlife, particularly in great apes and duikers, where up to 90% population declines have been reported and this sometimes led to hunters contracting the virus by handling and consuming carcasses found in the forest. The monitoring team did not encounter any wildlife carcasses in their surveys, nor did they document a recent decline in wildlife densities in the region. This suggests that there was likely not an “amplifying epidemic” in wildlife in the region, which could have enabled the virus to jump into the human population.

While the team was monitoring wildlife densities, it was announced by local health authorities that the first human case was a two-year-old boy from the small village of Meliandou. The landscape in that region is heavily human-modified, with Meliandou surrounded by plantations and bushland rather than tropical rainforests, as was the case for index villages in many previous Zaire Ebolavirus outbreaks. The team then set about to investigate how that two-year-old boy might have come into contact with the virus.

People, including children, were questioned about their hunting practices and contact to bushmeat. As in many other parts of Africa, both adults and children in the region hunt bats rather opportunistically. Children however tend to focus on small insectivorous bats, which they hunt under the roofs of houses or in other hides around the village. Once caught and killed, the bats are put on a stick and roasted over a fire. Even though this hunting practice is common throughout the region, special features in Meliandou might have been involved in enabling the transmission of the Ebola virus to a two-year-old boy. The team discovered a large burnt tree about 50 meters from the home of the index case, near a path used by women to reach their washing place at a small river. Children reported that they used to play regularly in and around this hollow tree, but that it had burned accidentally on March 24th, shortly before the research team arrived. Luckily the team was able to identify the species of bats that had inhabited this tree by identifying its DNA in soil and ash samples and deep-sequencing them. The bat species was identified as Mops condylurus, a species that has been discussed as a possible reservoir in early Ebola virus disease outbreaks in central Africa, it has then been shown to survive experimental infections and has been found with antibodies against Ebola viruses in the wild. Since the tree was burned, this species of bat was not found any more in Meliandou, but other fruit- and insect-eating bats were caught and subsequently analyzed for Ebola virus infection. As in many outbreaks before, no virus could be found.

The work presented by Leendertz and colleagues likely provides the best insight we will get into the origins of the current epidemic, but further in-depth ecological and experimental studies are urgently needed to understand the role of these and other bat species in Ebola viruses transmission and the circulation of these viruses in nature, to ultimately allow us to prevent future outbreaks.

In line with the “One Health” concept, (a worldwide strategy for expanding interdisciplinary collaborations and communications in all aspects of health care for humans, animals and the environment) the researchers emphasize that culling or eviction attempts targeting bat colonies aimed at mitigating human-wildlife conflicts or reducing disease transmission risk are frequently unsuccessful; in some cases even producing effects opposite to those desired by the initiative. Health education initiatives should inform the public about potential disease risks posed by bats, recommend minimizing contact with bats, and advice against the consumption of bats. Education programs should further teach about the crucial roles bats play in the ecosystem, including pollination, seed dispersal and insect reduction. These ecosystem services provide invaluable benefits to people directly and indirectly, thereby supporting local livelihoods and healthy ecosystems. Similarly, while the village of Meliandou had the misfortune to be where this epidemic began, care needs to be taken to avoid retribution attacks and stigmatization of the region.


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