A comparative study of strains of Ebola virus isolated from southern Sudan and northern Zaire in 1976.

During the 1976 Ebola virus outbreak in Sudan, the investigations team gained the impression that fewer haemorrhagic manifestations and few fatalities occurred during the later stages of the epidemic after the virus had undergone several generations in man. This impression was also noted in guinea pigs experimentally infected with Sudanese and Zairean strains of Ebola virus. The virulence of the Sudanese isolates was less intense than isolates emanating from Zaire. Similar findings were seen in monkeys; a Zairean isolated produced fatal infections, whereas monkeys inoculated with a Sudan strain generally recovered. Two monkeys, which had recovered from Sudanese strain infections and had developed high levels of antibody detectable by immunofluorescence, were challenged with the Zairean strain. Both developed viraemias and died. The mechanisms of this "failed protection" are discussed.