The family, Filoviridae consists of two genera, Marburgvirus and Ebolavirus, which are likely to have evolved from a common ancestor (Suzuki et al., 1997). The family is a group of enveloped, non-segmented, negative-strand RNA viruses. The genus Ebolavirus comprises of five species, Zaire, Sudan, Reston, Bundibugyo, and Tai Forest. The genome is approximately 19,000 nucleotides long and bears linearly arranged genes that encode seven structural proteins and one non-structural protein (Feldmann et al., 1993). Viruses of each species have genomes that are at least 30-40% divergent from one another, a level of diversity that presumably reflects differences in the ecologic niche in their evolutionary history.

Outbreaks of haemorrhagic fever caused by the Ebola virus are associated with high mortality rates, which is a distinguishing feature of this human pathogen. The highest lethality is associated with the Zaire subtype. Its rapid progression allows little opportunity to develop natural immunity, and there is currently no effective anti-viral therapy (Sullivan et al., 2000).

EBO strains from Zaire (EBO-Z) and Sudan (EBO-S) are related but distinguishable in terms of serology, virulence potential, and biochemical properties. Neither strain has hitherto been isolated from non-human primates that had not been infected experimentally (Jahrling et al., 1990). Another strain of Ebola was isolated in 1989 during an outbreak of infection in cynomolgus monkeys (Macaca fascicularis) in quarantine in Reston, VA, USA. The monkeys originated from the Philippines. Four animal handlers were infected in this US facility. All of them developed antibodies without any clinical signs. Four animal handlers were also found seropositive in the originating Philippine facility, but none had a history of haemorrhagic disease. This Reston strain of Ebola appears to be highly pathogenic for some monkey species but not for man (Le Guenno et al., 1995).