Ebola virus is back, this time in West Africa, with over 350 cases and a 69% case fatality ratio at the time of this writing (Baize). The culprit is the Zaire ebola virus species, the most lethal Ebola virus known, with case fatality ratios up to 90%. The epicenter and site of first introduction is the region of Gueckedou in Guinea’s remote southeastern forest region, spilling over into various other regions of Guinea as well as to neighboring Liberia and Sierra Leone. News of this outbreak engenders three basic questions: (1) What in the world is Zaire ebolavirus doing in West Africa, far from its
usual haunts in Central Africa? (2) Why Guinea, where no Ebola virus has ever been seen before? (3) Why now? We’ll have to wait for the outbreak to conclude and more data analysis to occur to answer these questions in detail, and even then we may never know, but some educated speculation may be illustrative. The Ebola virus genus is comprised of five species, Zaire, Sudan, Tai Forest, Bundibugyo, and Reston, each associated with a consistent case fatality and more or less well-identified endemic area. Zaire ebola virus had been previously found only in three
Central African countries—the Democratic Republic of the Congo, Republic of the Congo, and Gabon. Thus, the logical assumption when Ebola virus turned up in Guinea was that this would be the Tai Forest species previously noted in Guinea’s neighbor, Cote d’Ivoire. How did Zaire ebolavirus get all the way over to West Africa? The two possibilities appear to be that the virus has always been present the region, but we just never noticed, or that it was recently introduced.
The initial report and phylogenetic analyses on the Guinea outbreak suggested that the Zaire ebola virus found in Guinea is a distinct strain from that noted in Central Africa (Baize), thus suggesting that the virus may not be a newcomer to the region. However, subsequent reworking and interpretations of the limited genetic data have cast some doubt on this conclusion (Dudas). If Zaire ebola virus had been circulating for some time in Guinea, one might expect greater sequence variation than the 97% homogeneity noted relative to that isolated from Central Africa (Baize). Phylogenetic arguments aside, if Ebola virus was present in Guinea, wouldn’t we have seen cases before? Not necessarily.
Many pathogens may be maintained in animals with which humans normally have little contact, thus providing limited opportunity for infection. Furthermore, the proportion of infected animals may often be very low, so even frequent contact may not result in pathogen transmission. Even if human Ebola virus infection has occurred, it may not be recognized; contrary to popular concept, the clinical presentation of viral hemorrhagic fever is often very nonspecific, with frank bleeding seen in a minority of cases, so cases may be mistaken for other, more common diseases or, in the case of Guinea, Lassa fever, which is endemic in the area of the outbreak (Bausch).
Nor are laboratory diagnostics routinely available in West Africa for most viral hemorrhagic fevers (Khan). Ebola virus testing of human serum samples collected as far back as 1996 as part of surveillance for Lassa fever in the same region as the current outbreak could help reveal whether humans had exposure to Ebola virus prior to this outbreak (Bausch). We are presently organizing with collaborators to conduct ELISA antigen testing, PCR, and cell culture for Ebola virus on samples from persons who met.