Transmission, Human Population, and Pathogenicity: the Ebola Case in Point

Ebolaviruses are the causative agents responsible for several outbreaks of hemorrhagic fever. Ebolavirus is a genus within the family Filoviridae. The genus Ebolavirus contains five species: Zaire ebolavirus (EBOV), Sudan ebolavirus (SUDV), Reston ebolavirus (RESTV), Täi Forest ebolavirus (TAFV), and Bundibugyo ebolavirus (BDBV) (1, 2). Ebolaviruses were identified for the first time during two major outbreaks of hemorrhagic fever disease, which took place almost at the same time in Yambuku, Democratic Republic of the Congo (DRC, previously Zaire), and Nzara, Sudan, in 1976 (3, 4), and were demonstrated to be caused respectively by the agents now known as EBOV and SUDV. More than 500 cases were reported in those outbreaks, with a striking mortality rate of 88% in Zaire and of 53% in Sudan. The origin of the name Ebola corresponds to a river in nearby Yambuku, DRC, the first location known to be affected by EBOV (5). Within the Filoviridae there is also the genus Marburgvirus, with its unique species, Marburg marburgvirus (MARV), which shares many epidemiological and pathogenic aspects with EBOV. MARV was actually the first filovirus discovered, in 1967 during an outbreak in Germany and Belgrade, Serbia, that resulted in the infection of several laboratory technicians who were manipulating tissues from African monkeys (6). At that time, structures with thread-like morphology were visualized by electron microscopy in organs of infected individuals, and the term filovirus was coined (7) to describe these agents. All ebolaviruses except RESTV have been described in Africa and are highly pathogenic for humans: EBOV and SUDV in 1976, TAFV in 1994, and BDBV in 2007. RESTV comes from Asia (Philippines), and for unknown reasons it is unique in not causing disease in humans, an observation based on a number of documented zoonotic infections, since RESTV can asymptomatically infect swine and eventually be transmitted to humans (8, 9). Intriguingly, despite being nonpathogenic for humans and swine, RESTV is capable of being highly lethal in nonhuman primates (NHPs) (9–15). Recently a third genus of filovirus, Cuevavirus, has been described infecting cave bats in northern Spain. The single agent within Cuevavirus, named Lloviu virus, has not been replicated in culture although it has been fully characterized by sequencing analysis. Its pathogenic potential for humans is unknown (16, 17).