Morbidity and mortality of wild animals in relation to outbreaks of Ebola haemorrhagic fever in Gabon, 1994-2003

Antibody to Ebola virus was found in 14 (1.2%) of 1147 human sera collected in Gabon in 1981-1997. Six seropositive subjects were bled in the northeast in 1991, more than 3 years prior to recognition of the first known outbreak of Ebola haemorrhagic fever (EHF), whilst eight came from the southwest where the disease has not been recognised. It has been reported elsewhere that 98 carcasses of wild animals were found in systematic studies in northeastern Gabon and adjoining northwestern Republic of the Congo (RoC) during five EHF epidemics in August 2001 to June 2003, with Ebola virus infection being confirmed in 14 carcasses. During the present opportunistic observations, reports were investigated of a further 397 carcasses, mainly gorillas, chimpanzees, mandrills and bush pigs, found by rural residents in 35 incidents in Gabon and RoC during 1994-2003. Sixteen incidents had temporal and/or spatial coincidence with confirmed EHF outbreaks, and the remaining 19 appeared to represent extension of disease from such sites. There appeared to be sustained Ebola virus activity in the northeast in 1994-1999, with sequential spread from 1996 onwards, first westwards, then southerly, and then northeastwards, reaching the Gabon-RoC border in 2001. This implies that there was transmission of infection between wild mammals, but the species involved are highly susceptible and unlikely to be natural hosts of the virus.