We present the clinical features and course of 282 patients with human monkeypox in Zaire during 1980-1985. The ages of the patients ranged from one month to 69 years; 90% were less than 15 years of age. The clinical picture was similar to that of the ordinary and modified forms of smallpox. Lymphadenopathy, occurring in the early stage of the illness, was the most important sign differentiating human monkeypox from smallpox and chickenpox. The symptoms, signs, and the course of the disease in patients who had been vaccinated against smallpox differed significantly from those in unvaccinated subjects. Pleomorphism and "cropping" similar to that in chickenpox occurred in 31% of vaccinated and 18% of unvaccinated patients. The prognosis depended largely on the presence of severe complications. No deaths occurred among vaccinated patients. In unvaccinated patients the crude case-fatality rate was 11% but was higher among the youngest children (15%).

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Human monkeypox was first identified in 1970 in the Democratic Republic of the Congo. Extensive studies of this zoonotic infection in the 1970s and 1980s indicated a largely sporadic disease with a minority of cases resulting from person-to-person transmission, rarely beyond two generations. In August 1996, an unusually large outbreak of human monkeypox was reported, and cases continued through 1997 with peak incidence in August 1996, March 1997 and August 1997. Preliminary results from the field investigations in 1997 suggest a new epidemiological pattern where a majority of secondary cases result from person-to-person transmission, and a clinically milder disease. But there is preliminary laboratory evidence of a simultaneous outbreak of varicella in the same geographic region which will undoubtedly modify these preliminary results. Since smallpox was eradicated and vaccinia vaccination terminated in this region, the population of susceptible individuals has grown. The use of vaccination to protect the population at risk, however, must take into account HIV prevalence and the risk of generalized vaccinia when using vaccinia vaccine in populations where HIV is known to be present.

A study of 2,510 contacts of 214 patients with human monkeypox was conducted in Zaire from 1980 to 1984. Among the contacts of 130 primary cases of human monkeypox, a further 22 co-primary and 62 secondary cases were detected, and an additional fourteen people who had no evidence of clinical disease had positive serological results. A majority of the clinical and subclinical cases of monkeypox occurred in children less than 10 years of age. Immunity in vaccinated persons now appears to be waning because 16 overt cases occurred in contacts who had been vaccinated. The overall attack rate for contacts without a vaccination scar (7.2%) differed significantly from the attack rate for those who had been vaccinated in the past (0.9%). The attack rate for household contacts was significantly higher than that for other contacts, among both unvaccinated (four times higher) and vaccinated (seven times higher) household contacts. Many unvaccinated contacts living in the same household as the index case under conditions of maximum exposure, however, escaped not only the disease but also infection.

Human monkeypox is a zoonotic smallpox-like disease caused by an orthopoxvirus of interhuman transmissibility too low to sustain spread in susceptible populations. In February 1997, 88 cases of febrile pustular rash were identified for the previous 12 months in 12 villages of the Katako-Kombe Health Zone, Democratic Republic of Congo (attack rate = 22 per 1,000; case-fatality rate = 3.7%). Seven were active cases confirmed by virus isolation. Orthopoxvirus- neutralizing antibodies were detected in 54% of 72 patients who provided serum and 25% of 59 wild-caught animals, mainly squirrels. Hemagglutination-inhibition assays and Western blotting detected antibodies in 68% and 73% of patients, respectively. Vaccinia vaccination, which protects against monkeypox, ceased by 1983 after global smallpox eradication, leading to an increase in the proportion of susceptible people.