Ebola between outbreaks: Intensified Ebola hemorrhagic fever surveillance in the Democratic Republic of the Congo, 1981-1985

Surveillance for Ebola hemorrhagic fever was conducted in the Democratic Re public of the Congo from 1981 to 1985 to estimate the incidence of human in fection. Persons who met the criteria of one of three different case defini tions were clinically evaluated, and blood was obtained for antibody confir mation by IFA. Contacts of each case and 4 age- and sex-matched controls we re also clinically examined and tested for immunofluorescent antibody. Twen ty-one cases of Ebola infection (persons with an antibody titer of greater than or equal to 1:64, or lower if they fit the clinical case definition) w ere identified, with a maximum 1-year incidence of 9 and a case fatality ra te of 43%. Cases occurred throughout the year, but most (48%) occurred earl y in the rainy season. Fifteen percent of contacts had antibody titers grea ter than or equal to 1:64 to Ebola virus, compared with 1% of controls (P < .0001). Results suggest that Ebola virus periodically emerges from nature to infect humans, that person-to-person transmission is relatively limited, and that amplification to large epidemics is unusual.