Two large studies conducted on thousands of animals at various sites in Africa provide new information on the respective roles of monkeys and bats in the transmission of the Ebola virus and the outbreak of human epidemics.

Since the first Ebola outbreak in the Democratic Republic of Congo (DRC) in 1976, Africa has experienced 28 episodes of the disease. Between May 2017 and July 2018, the DRC even experienced three emerging issues. An increase in frequency that underscores the importance of understanding the origin of these epidemics, whose mortality rates range from 25 to 90%. Especially since they now also affect large cities, and therefore an even larger population.

Research has identified three species of the virus that are infecting humans in Africa, developped rapid diagnostic tests and is considering vaccine production, but many unknowns remain about the origin of infection in humans. In particular, what roles do bats and monkeys play? The potential reservoir of small flying mammals was mentioned in 2005 ¹. In other words, bats would harbour the virus without being sick, and could transmit it to humans or other animals.

Other animals such as chimpanzees or gorillas - apes - which, on the other hand, develop the disease. "Contact with apes has also been the starting point for several epidemics in Gabon and the Republic of Congo," says Martine Peeters. To confirm the roles of these animals, and thus better fight future epidemics, the researcher and Ahidjo Ayouba, both virologists at IRD, have embarked on a vast quest to detect signs of presence, or contact with the virus, in the blood or stool of non-human primates and chiroptera. ^{2 3}

Over 8,000 animals sampled

Between 2015 and 2017, they collected the blood of 4,022 bats, spread over 21 sites, in three countries: Guinea, DRC and Cameroon. "The latter country has not yet experienced an epidemic, but it has the same environment and wildlife as the DRC, Gabon or the Republic of Congo. And some Congolese and Gabonese epidemic centres are only 300 kilometres as the crow flies," says Martine Peeters.

In order to ensure the multiplicity of bat species tested - there are hundreds of them in Africa - the traps have been placed in different ecological environments: fields, villages, cities.... "The blood samples, collected on site before releasing the animals, were then tested with different antigens⁴ from several species of the Ebola virus." If there was reactivity, it indicated the presence of antibodies, and therefore that the animal had been in contact with the virus or a virus of the same family.

The same antibody detection technique was applied to 4,649 non-human primates from 36 species, including 2,327 ape samples (gorillas, chimpanzees and bonobos). Scientists performed reactivity tests on blood or stool samples for apes from Cameroon, the DRC or Côte d'Ivoire.

Bats in the spotlight

The strength of these two studies is based on several elements: the enlargement of the geographical areas considered, the multitude of animals tested (doubling the number of bats and tripling the number of non-human primates tested so far in a single study), the uniqueness of the technique used allowing homogeneity of results, and the plurality of virus species tested!

Result? In the end, eight bat species revealed the presence of reactive antibodies. "Some, widespread on the continent-like Eidelon helvum, Hypsygnatus monstruosus, - have even shown evidence of contact with two species of the Ebola virus, Zaire and Sudan," the researchers said. On the other hand, they found no sign of contact in only one monkey, from the Cercopithecus family. "And no reactive antibodies in apes," says Martine Peeters. This therefore supports the hypothesis that the latter are only intermediate hosts of the virus, and not the reservoirs. The evidence is thus multiplying to assign a role to bats.

To understand how the virus harbours itself in flying mammals, researchers are now working on an even more ambitious project: to monitor, over time, the prevalence of the virus in bat colonies. With the hope of confirming the presence of the virus, beyond those of antibodies.

===

Notes :

1. Eric M. Leroy, Brice Kumulungui, Xavier Pourrut, Pierre Rouquet, Alexandre Hassanin, Philippe Yaba, André Délicat, Janusz T. Paweska, Jean-Paul Gonzalez & Robert Swanepoel, Fruit bats as reservoirs of Ebola virus, Nature  ; 2005; 438 : 575–6

  2. Helene M. De Nys, Placide Mbala Kingebeni, Alpha K. Keita, Christelle Butel, Guillaume Thaurignac, Christian-Julian Villabona-Arenas, Thomas Lemarcis, Mare Geraerts, Nicole Vidal, Amandine Esteban, Mathieu Bourgarel, François Roger, Fabian Leendertz, Ramadan Diallo, Simon-Pierre Ndimbo-Kumugo, Justus Nsio-Mbeta, Nikki Tagg, Lamine Koivogui, Abdoulaye Toure, Eric Delaporte, Steve Ahuka-Mundeke, Jean-Jacques Muyembe Tamfum, Eitel Mpoudi-Ngole, Ahidjo Ayouba, Martine Peeters, Survey of Ebola Viruses in Frugivorous and Insectivorous Bats in Guinea, Cameroon, and the Democratic Republic of the Congo, 2015–2017, Emerging Infectious Disease ; décembre 2018 

  3. Ahidjo Ayouba, Steve Ahuka-Mundeke, Christelle Butel, Placide Mbala, Severin Loul, Nikki Tagg, Christian-Julian Villabona Arenas, Audrey Lacroix , Simon-Pierre Ndimbo-Kumugo, Alpha K. Keita7, Abdoulaye Toure, Emmanuel Couacy-Hymann, Sebastien Calvignac-Spencer, Fabian H. Leendertz, Pierre Formenty, Eric Delaporte, Jean-Jacques Muyembe-Tamfum, Eitel Mpoudi Ngole, Martine Peeters, Extensive serological survey of multiple African non-human primate species reveals low prevalence of IgG antibodies to four Ebola virus species,The Journal of Infectious Diseases  ; 18 janvier 2019

  4. Antigen: A substance that the body's immune system recognizes as foreign

This article was first published by the IRD and translated in English by Afriscitech.