S. S. Marennikova

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.

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.

When the research on the ecology of monkeypox virus entered the latest stage in 1984, three groups of animals were considered priority candidates for maintenance of virus circulations in nature because of their relatively higher population density: terrestrial rodents, squirrels, and gregarious bats. Following the isolation of the virus from a wild squirrel in 1985 another survey was carried out in Bumba zone of Zaïre in January-February 1986, which included collection of animal samples, later on tested by WHO collaborating laboratories. No antibodies were found in 233 rodents tested. A high prevalence of monkeypox-specific antibodies (24.7%) found in 320 squirrels of Funisciurus anerythrus species suggested that these animals sustain virus transmission in the areas surrounding human settlements. A consistently high level of antibody prevalence found among squirrels Heliosciurus rufobrachium suggested that this species is also steadily involved in the transmission. The squirrels, the only mammals frequently infected by monkeypox in the areas of human activities, which are frequently trapped by the population, are obviously a major source of infection for human beings. Out of 39 primates collected during the survey from the primary forest, three were found seropositive for monkeypox. At the moment, however, it is difficult to say whether the primates play an important role in sustaining virus transmission in nature, or (like human beings) that they are an occasional host of the virus. Possibilities are discussed of monkeypox virus transmission being sustained by squirrels outside the areas of major human activities.

An outbreak of pox disease in Carnivora of the family Felidae occurred in the Moscow Zoo. Two forms of the disease were found: (1) fatal, fulminant pulmonary without skin lesions and (2) dermal with rash. The severity of the dermal form varied from subclinical to lethal. The pulmonary form was characterized by pneumonia and exudative pleuritis, and large concentrations of virus were observed in the lungs and exudate. In addition to Carnivora of the family Felidae, two giant anteaters had a severe form of the disease (dermal with hemorrhages) and died. The agent of the outbreak appeared to be very closely related to cowpox virus; however, pocks developed at a lower temperature than do those that result from infection with cowpox virus. Strains isolated from sick animals were identical to the virus previously isolated from an outbreak of pox among elephants and okapi. The most probable sources of infection were rats that were fed to some of the animals. During the outbreak, a female attendant at the zoo became infected.