Fruit Bats a reservoir for Ebola virus

Ebola virus (of the Filoviridae family) was first identified in 1976 in the Democratic Republic of Congo (ex- Zaire). It has been the source of several lethal epidemics in central Africa. Four subtypes exist, three of which rage on the African continent. The zaire subtype, the most dangerous for humans, was responsible for eight epidemics which have hit Gabon and the Republic of Congo since 1995. Infection by this subtype in humans is expressed by a violent haemorrhagic fever which in 80 % of cases kills the victim in a few days. There has been a succession of 14 epidemics of Ebola in Africa since 1976. Ten of which were caused by the zaire sub-type, generating 1850 cases resulting in 1300 deaths.

Viral transmission to humans occurs by way of direct contact with infected primate carcasses (1). However, although they are the source of human infection, these animals are not the reservoir for the virus. The large primates develop the disease and die only days after themselves being infected, following contamination events provoked by contact with the reservoir. Numerous investigations, conducted since 1976 and aiming to identify this reservoir, have been unsuccessful. Eric Leroy of the IRD in Gabon and his co-workers from the CIRMF have now identified some tropical bat species as a potential Ebola virus reservoir, the fruit of studies they undertook between 2001 and 2003 in the border region between Gabon and the Republic of Congo. They publish their findings today.

The human epidemics that have flared up since 2001 were linked to multiple viral outbreaks in several animal species including chimpanzees, gorillas and duiker. During these epidemic episodes, the researchers captured about 1000 small vertebrates in good health (rodents, shrews, bats, birds and squirrels) from the vicinity of carcasses of infected primates. They performed a range of analyses: a search for specific Ebola virus antibodies in the serum, and for viral genome in certain organs; isolation of the virus on sensitive cell lines; immunohistochemistry of organ sections.

These techniques helped detect specific zaire subtype antibodies in the serum of three species of bat: Hypsignasthus monstrosus, Epomops franqueti and Myonycteris torquata. Also demonstrated was the presence of Ebola RNA sequences in the liver and spleen of these same animals. These bats are therefore Ebola virus carriers without developing the disease, which indicates them to be a potential natural reservoir for that virus.

Epidemiological observations, moreover, showed the peak of mortality in the large primates to coincide with the dry season, a period during which food resources decline considerably. The bats and the primates hence enter into competition for feeding, which would increase occasions when they would come closer together and lead to more frequent contact. A further element is that the dry season is a time when bats give birth. Under the effect of several factors (food deficiencies, competition between males and parturition), the level and nature of immune responses in the bats probably change substantially during this period. A consequence of such conditions would be a resumption of viral replication, even the emergence of infectious virus in the blood of these animals. The large primates would be contaminated when the two species gather in close proximity to eat the fruit from the same tree, by way of direct contact with blood and placental fluids of bats released when female bats gave birth. A further array of investigations are under way: first seeking to isolate the virus in these tissues and subsequently to confirm the contamination and transmission routes described.

These results shed new light on the episodic nature of Ebola epidemics in primates and humans. In addition, clues as to the development of prevention strategies in order to protect the primates from Ebola virus could be provided by improved knowledge of the ecological distribution of these species of bats. The latter, especially Hypsignathus monstrosus, are indeed often caught and eaten by people living in epidemic-prone regions. Consequently, public awareness programmes and an input of food supplies essential for the needs of remote villages during the dry season should help avoid Ebola virus transmission from the bats to humans.