Thursday, 5 September 2019

Understanding the wild ecology of the Ebola Virus.

Ebola Hemorrhagic Fever was first diagnosed in 1976 and since then has caused repeated (and increasingly common) outbreaks in Humans and Animals in Central and West Africa, killing hundreds of people and being associated with major population declines among Chimpanzees and Gorillas. The disease is zoonotic (i.e. it spreads to Human populations from an animal host), and is known to infect a variety of Mammal species; it is thought likely that the Virus has a wild reserve in Fruit Bats, though this has never been proved. Blood samples from several species of Fruit Bat have been shown to contain antibodies to the Virus, but no Fruitbat has ever been found with a live infection (this is important, as an animal able to quickly produce antibodies and shake off infection would not be a suitable wild host for the disease, since it would seldom remain infected long enough to pass on the disease).

Health workers during an Ebola outbreak in the Democratic Republic of Congo in March 2018. John Bompengo/AP.

In a paper published in the Philosophical Transactions of the Royal Society Series B Biological Sciences on 12 August 2019, John Paul Schmidt of the Odum School of Ecology and Center for the Ecology of Infectious Diseases at the University of Georgia, Sean Maher of the Department of Biology at Missouri State University, John Drake, also of the Odum School of Ecology and Center for the Ecology of Infectious Diseases at the University of Georgia, Tao Huang of the Cary Institute of Ecosystem Studies, Maxwell Farrell, again of the Odum School of Ecology and Center for the Ecology of Infectious Diseases at the University of Georgia, and Barbara Han, also of the Cary Institute of Ecosystem Studies, describe the results of a study which sought to understand the nature of the wild Ebola Virus host by examining ecological traits associated with animals that have been shown to become infected with the Virus.

Schmidt et al. carried out a data search of existing publications which described animals being tested for the Ebola Virus, identifying 119 species of wild, captive, and domestic animals upon which such tests have been carried out, with 23 having tested positive for either the Virus or antibodies to it. These species include one species of Duiker, fourteen species of Bat, five Primates, two Rodents and a Shrew. Although fourteen of the twenty three species shown to have been infected were Bats, Schmidt were cautious of concluding that this meant the wild reservoir of the species was likely to be a Bat since 43 of the 119 species tested were Bats, due to the existing suspicion that this group might be the main reservoir of the Virus.

African Straw-Coloured Fruit Bats, Eidolon helvum. Bauchi et al. (2018).

The ecological traits that were most prevalent in this collection of animals were having a distribution range centred latitudinally between 48 and 78 N latitude, small litter sizes, large adult body sizes (more than 3 kg), long gestations, frugivorous diets, narrow diet breadth, early weaning, solitary or living in small social groups and narrow habitat breath. Some of this probably related to the high number of Bats in the test group, for example small litter sizes are typical of Bats, which tend to only have one young at a time, something also found in other groups sampled, such as Duikers and Primates. Other traits are more indicative, but also relate to the high number of Bats in the group, as Bats can generally be divided into large frugivorous species (more than 3 kg) and smaller insectivorous species (less than 3 kg). Antibodies to Ebola were widespread in fruit eating Bats, but not in insectivorous Bats (although these were much less heavily tested). The disease also seems to infect other fruit eating groups, such as Primates and Duikers, suggesting there is a connection between frugivory and infection.

Frugivores form an important ecological guild in tropical Africa, particularly in the dry season, when other food sources are rare, but many Tree species produce fruit. This has advantages for the Trees, which are more likely to get their seeds distributed at this time of year, and the animals, who are able to obtain food when it is otherwise scarce. However, this also means that multiple species of frugivorous animals will gather round fruiting Trees at the same time, often coming into contact with one-another's saliva and feces, something which probably provides an ideal route of transmission for the Ebola Virus. Since most Ebola outbreaks are known to have begun in the dry season, this gives a plausible method of cross-species transmission for the Virus, which can provide a possible explanation of how outbreaks start without actually identifying the wild reservoir host of the Virus.

Chimpanzees feeding from a Fruit Tree in the Congo Basin. World Atlas.

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