The causal agent of a case of disease in man occurring in the Democratic Republic of the Congo with a similar clinical picture to smallpox was isolated and studied. The agent was identified as monkeypox virus. A comparative study of the isolated strain (Congo-8) and of viruses isolated from similar cases of illness in Liberia (Liberia-1 and Liberia-2 strains) and Sierra Leone (V-70 1 266 strain) showed that they were identical. A number of local species of monkeys and apes were examined serologically in the Congo region to determine the probability of human infection with monkeypox virus. It was confirmed that the animals had had contact with an agent of the poxvirus group. In 2 of the 7 sera examined, antibodies of the variola—vaccinia group of poxviruses were discovered (virus-neutralizing antibodies, precipitins, and antihaemagglutinins). In a chimpanzee, antihaemagglutinins were found in a titre of 1: 1 280, and in the same animal a variola-like virus was isolated from the kidneys. In the course of the investigation, it was shown conclusively that monkeypox virus and the strains under investigation could be distinguished from o dinary variola and vaccinia viruses on the basis of their behaviour in pig embryo kidney continuous cell line culture
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