Human infection with monkeypox virus: laboratory investigation of six cases in West Africa.

Between September 1970 and May 1971 six cases of human infection with monkeypox virus were identified in three West African countries-Liberia, Sierra Leone, and Nigeria. Four of the cases were confirmed by viral isolation, and two were diagnosed on epidemiological and serological evidence. Poxvirus strains isolated from the four cases were indistinguishable from reference monkeypox strains (Copenhagen and Utrecht), and all were easily differentiated from variola and vaccinia viruses. The isolated strains produced small necrotic haemorrhagic pocks on CAM, grew well at 39.0 degrees C, formed large plaques in Vero cell cultures, showed markedly more virulence for chick embryos and mice than do variola strains, and produced large necrotic haemorrhagic local lesions with generalized illness and florid secondary exanthem when inoculated into rabbit skin.The finding of smallpox-like illness in humans resulting from infection with a poxvirus of lower animal origin serves to emphasize the importance of thorough epidemiological and laboratory evaluation of all suspect smallpox cases occurring in areas where smallpox has been or is about to be eradicated.