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Tracking the Transmission of Ebola Virus

Does a declining gorilla population correlate with human Ebola outbreaks?

Ebola virus infection in humans is highly lethal and tends to occur in limited outbreaks in Africa. Such outbreaks are initiated by virus introduction to humans from a still-unknown source and are perpetuated by human-to-human transmission. A multinational team recently investigated 5 human Ebola virus (Zaire subtype) outbreaks, encompassing at least 10 epidemic chains, in western central Africa from October 2001 to May 2003. Each outbreak began with introduction of the virus into humans, usually hunters, who had handled dead primates or duikers (an antelope).

Genome sequencing of the causative viruses showed each epidemic chain to be associated with a different viral strain. Noting many deaths among large mammals (particularly gorillas) in the region immediately before and during the human Ebola virus outbreaks, the researchers tested several recovered animal carcasses for Ebola virus infection using combinations of serology, PCR, viral isolation, and other techniques: All 10 gorillas, all 3 chimpanzees, and 1 of 3 duikers tested positive. Genetic sequencing showed that several Ebola strains were involved, which indicated multiple introductions of virus into the animal populations. Transmission might have required exposure to fresh carcasses; those >3 or 4 days old were not infectious.

Comment: Ebola virus appears to be introduced to humans and other animals through contact with animals that died recently from Ebola infection. The outbreaks studied here reflect introductions of multiple Ebola strains rather than a sustained epidemic with a single strain. The reservoir of the virus remains unknown. Regional monitoring of animal mortality could provide early warning of future human outbreaks.

— Bradley E. Britigan, MD

Published in Journal Watch Infectious Diseases February 13, 2004

Citation(s):

Leroy EM et al. Multiple Ebola virus transmission events and rapid decline of central African wildlife. Science 2004 Jan 16; 303:387-90.

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